U.S. patent application number 16/160789 was filed with the patent office on 2019-05-23 for light emitting device driver apparatus with multiple dimming modes and conversion control circuit thereof.
The applicant listed for this patent is RICHTEK TECHNOLOGY CORPORATION. Invention is credited to Wei-Ming Chiu, Chi-Hsiu Lin, Meng-Hsun Yang.
Application Number | 20190159310 16/160789 |
Document ID | / |
Family ID | 66532716 |
Filed Date | 2019-05-23 |
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United States Patent
Application |
20190159310 |
Kind Code |
A1 |
Yang; Meng-Hsun ; et
al. |
May 23, 2019 |
LIGHT EMITTING DEVICE DRIVER APPARATUS WITH MULTIPLE DIMMING MODES
AND CONVERSION CONTROL CIRCUIT THEREOF
Abstract
A light emitting device driver apparatus includes: an inductor;
a power switch which switches the inductor to convert an input
power to an output current for driving a light emitting device
circuit; and a conversion control circuit. When a dimming signal
exceeds a first dimming threshold, a reference current signal is
generated according to the dimming signal, and a switch control
signal controls the power switch according to a first PWM signal,
such that the output current corresponds to the level of the
dimming signal. When the dimming signal does not exceed the first
dimming threshold, the reference current signal is clamped to a
level corresponding to the first dimming threshold, and a second
PWM signal is generated according to the dimming signal, wherein
the second PWM signal enables the first PWM signal to generate the
switch control signal to control the power switch.
Inventors: |
Yang; Meng-Hsun; (Tainan,
TW) ; Lin; Chi-Hsiu; (Yunlin, TW) ; Chiu;
Wei-Ming; (Hsinchu, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RICHTEK TECHNOLOGY CORPORATION |
Zhubei City |
|
TW |
|
|
Family ID: |
66532716 |
Appl. No.: |
16/160789 |
Filed: |
October 15, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62590331 |
Nov 23, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M 3/157 20130101;
H02M 3/156 20130101; H02M 1/08 20130101; H05B 45/37 20200101; H02M
1/143 20130101; H05B 45/10 20200101; H02M 1/14 20130101 |
International
Class: |
H05B 33/08 20060101
H05B033/08; H02M 3/157 20060101 H02M003/157; H02M 1/14 20060101
H02M001/14; H02M 1/08 20060101 H02M001/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 7, 2018 |
CN |
201810425438.2 |
Claims
1. A light emitting device driver apparatus, comprising: an
inductor; a power switch, coupled to the inductor and configured to
operably switch the inductor to convert an input power to an output
current to drive a light emitting device circuit; and a conversion
control circuit, configured to operably control the power switch;
the conversion control circuit including: a modulation circuit,
configured to operably perform a pulse width modulation according
to a current related signal and a reference current signal to
generate a first PWM signal, wherein the modulation circuit
controls the current related signal such that the current related
signal corresponds to the reference current signal, wherein the
current related signal relates to the output current; a logic
driving circuit, configured to operably generate a switch control
signal according to the first PWM signal and a second PWM signal to
control the power switch to generate the output current; and a
dimming control circuit, configured to operably generate the
reference current signal according to a dimming signal, wherein:
when the dimming signal exceeds a first dimming threshold, the
reference current signal is generated according to a level of the
dimming signal, and the switch control signal controls the power
switch according to the first PWM signal, such that the output
current corresponds to the level of the dimming signal, whereby an
analog dimming control is performed; and when the dimming signal
does not exceed the first dimming threshold, the reference current
signal is clamped to a level which corresponds to the first dimming
threshold, and the second PWM signal is generated according to the
level of the dimming signal, wherein a frequency of the second PWM
signal is lower than a frequency of the first PWM signal; wherein
the second PWM signal enables the first PWM signal to generate the
switch control signal to control the power switch, wherein, during
an enable period of the second PWM signal, the output current
corresponds to the first dimming threshold, and during a disable
period of the second PWM signal, the power switch is controlled to
be OFF, whereby a pulse width modulation dimming control is
performed.
2. The light emitting device driver apparatus of claim 1, wherein
the modulation circuit includes: an error amplifier circuit,
configured to operably generate an error amplified signal according
to a difference between the current related signal and the
reference current signal; and a first comparator circuit,
configured to operably compare a first ramp signal and the error
amplified signal to generate the first PWM signal.
3. The light emitting device driver apparatus of claim 1, wherein
the dimming control circuit includes: a reference current generator
circuit, configured to operably convert a dimming reference signal
to generate a reference current signal; a signal selection circuit,
configured to operably compare the dimming signal and the first
dimming threshold, wherein when the dimming signal exceeds the
first dimming threshold, the dimming signal is selected as the
dimming reference signal, and when the dimming signal does not
exceed the first dimming threshold, the first dimming threshold is
selected as the dimming reference signal; and a second comparator
circuit, configured to operably compare the dimming signal and a
second ramp signal to generate the second PWM signal.
4. The light emitting device driver apparatus of claim 1, wherein
when the dimming signal exceeds the first dimming threshold, the
reference current signal is proportional to the dimming signal by a
predetermined ratio.
5. The light emitting device driver apparatus of claim 3, wherein a
peak level of the second ramp signal is equal to the first dimming
threshold.
6. The light emitting device driver apparatus of claim 2, wherein
the error amplifier circuit includes: a transconductance circuit,
configured to operably generate an error amplified current on a
transconductance output terminal according to the difference of the
current related signal and the reference current signal; a
compensation capacitor, configured to operably integrate the error
amplified current to generate the error amplified signal; and an
integrator control switch, coupled between the transconductance
output terminal and the compensation capacitor, wherein, when the
dimming signal exceeds the first dimming threshold, the integrator
control switch is controlled to conduct a current path from the
error amplified current to the compensation capacitor, and when the
dimming signal does not exceed the first dimming threshold, the
integrator control switch is controlled to conduct the current path
from the error amplified current to the compensation capacitor
during the enable period of the second PWM signal, and is
controlled to cut off the current path from the error amplified
current to the compensation capacitor during the disable period of
the second PWM signal.
7. The light emitting device driver apparatus of claim 1, wherein
the light emitting device circuit includes: a light emitting device
string, including a least one light emitting device; and an output
capacitor, coupled to the light emitting device string in parallel,
for reducing a current ripple flowing through the light emitting
device string.
8. The light emitting device driver apparatus of claim 1, wherein
when the dimming signal exceeds a second dimming threshold, the
reference current signal is clamped to a level which corresponds to
the second dimming threshold, whereby the output current is clamped
to an upper current limit.
9. The light emitting device driver apparatus of claim 1, wherein
the conversion control circuit receives the dimming signal through
a single pin to achieve the analog dimming control and the pulse
width modulation dimming control according to the level of the
dimming signal.
10. The light emitting device driver apparatus of claim 1, wherein
the conversion control circuit determines the first dimming
threshold according to a deviation characteristic obtained from
data of plural diming control circuits.
11. A conversion control circuit, configured to operably control
alight emitting device driver apparatus, the light emitting device
driver apparatus including an inductor and a power switch which is
coupled to the inductor, the power switch being configured to
operably switch the inductor to convert an input power to generate
an output current for driving a light emitting device circuit; the
conversion control circuit, configured to operably control the
power switch so as to control the light emitting device driver
apparatus, comprising: a modulation circuit, configured to operably
perform a pulse width modulation according to a current related
signal and a reference current signal to generate a first PWM
signal, wherein the modulation circuit controls the current related
signal such that the current related signal corresponds to the
reference current signal, wherein the current related signal
relates to the output current; a logic driving circuit, configured
to operably generate a switch control signal according to the first
PWM signal and a second PWM signal to control the power switch to
generate the output current; and a dimming control circuit,
configured to operably generate the reference current signal
according to a dimming signal, wherein: when the dimming signal
exceeds a first dimming threshold, the reference current signal is
generated according to a level of the dimming signal, and the
switch control signal controls the power switch according to the
first PWM signal, such that the output current corresponds to the
level of the dimming signal, whereby an analog dimming control is
performed; and when the dimming signal does not exceed the first
dimming threshold, the reference current signal is clamped to a
level which corresponds to the first dimming threshold, and the
second PWM signal is generated according to the level of the
dimming signal, wherein a frequency of the second PWM signal is
lower than a frequency of the first PWM signal; wherein the second
PWM signal enables the first PWM signal to generate the switch
control signal to control the power switch, wherein, during an
enable period of the second PWM signal, the output current
corresponds to the first dimming threshold, and during a disable
period of the second PWM signal, the power switch is controlled to
be OFF, whereby a pulse width modulation dimming control is
performed.
12. The conversion control circuit of claim 11, wherein the
modulation circuit includes: an error amplifier circuit, configured
to operably generate an error amplified signal according to a
difference between the current related signal and the reference
current signal; and a first comparator circuit, configured to
operably compare a first ramp signal and the error amplified signal
to generate the first PWM signal.
13. The conversion control circuit of claim 11, wherein the dimming
control circuit includes: a reference current generator circuit,
configured to operably convert a dimming reference signal to
generate a reference current signal; a signal selection circuit,
configured to operably compare the dimming signal and the first
dimming threshold, wherein when the dimming signal exceeds the
first dimming threshold, the dimming signal is selected as the
dimming reference signal, and when the dimming signal does not
exceed the first dimming threshold, the first dimming threshold is
selected as the dimming reference signal; and a second comparator
circuit, configured to operably compare the dimming signal and a
second ramp signal to generate the second PWM signal.
14. The conversion control circuit of claim 11, wherein when the
dimming signal exceeds the first dimming threshold, the reference
current signal is proportional to the dimming signal by a
predetermined ratio.
15. The conversion control circuit of claim 13, wherein a peak
level of the second ramp signal is equal to the first dimming
threshold.
16. The conversion control circuit of claim 11, wherein the error
amplifier circuit includes: a transconductance circuit, configured
to operably generate an error amplified current on a
transconductance output terminal according to the difference of the
current related signal and the reference current signal; a
compensation capacitor, configured to operably integrate the error
amplified current to generate the error amplified signal; and an
integrator control switch, coupled between the transconductance
output terminal and the compensation capacitor, wherein, when the
dimming signal exceeds the first dimming threshold, the integrator
control switch is controlled to conduct a current path from the
error amplified current to the compensation capacitor, and when the
dimming signal does not exceed the first dimming threshold, the
integrator control switch is controlled to conduct the current path
from the error amplified current to the compensation capacitor
during the enable period of the second PWM signal, and is
controlled to cut off the current path from the error amplified
current to the compensation capacitor during the disable period of
the second PWM signal.
17. The conversion control circuit of claim 11, wherein when the
dimming signal exceeds a second dimming threshold, the reference
current signal is clamped to a level which corresponds to the
second dimming threshold, whereby the output current is clamped to
an upper current limit.
18. The conversion control circuit of claim 11, wherein the
conversion control circuit receives the dimming signal through a
single pin to achieve the analog dimming control and the pulse
width modulation dimming control according to the level of the
dimming signal.
19. The conversion control circuit of claim 11, wherein the
conversion control circuit determines the first dimming threshold
according to a deviation characteristic obtained from data of
plural diming control circuits.
Description
CROSS REFERENCE
[0001] The present invention claims priority to U.S. 62/590,331,
filed on Nov. 23, 2017, and to CN 201810425438.2, May 7, 2018.
BACKGROUND OF THE INVENTION
Field of Invention
[0002] The present invention relates to a light emitting device
driver apparatus; particularly, it relates to a light emitting
device driver apparatus with multiple dimming modes. The present
invention also relates to a conversion control circuit for use in
the light emitting device driver apparatus with multiple dimming
modes.
Description of Related Art
[0003] FIGS. 1A and 1B show a prior art light emitting device
driver apparatus (light emitting device driver apparatus 1) and a
conversion control circuit (conversion control circuit 15) thereof.
The light emitting device driver apparatus 1 controls the power
switch SWN by the conversion control circuit 15, so as to switch
the inductor L to convert an input power VDD to an output current
IOUT to drive a light emitting device circuit 60. In FIG. 1B, the
dimming control circuit 50 receives an analog (continuous) dimming
signal DIM to generate a reference current signal IREF. The error
amplifier circuit 20 generates an error amplified signal EAO
according to a current related signal ISN and the reference current
signal IREF. The comparator circuit 30 compares a ramp signal RMP
and the error amplified signal EAO to generate a PWM signal PP to
control the power switch SWN, such that the output current IOUT
corresponds to the reference current signal IREF. From one
perspective, the output current IOUT is controlled to correspond to
the level of the dimming signal DIM. In other words, the output
current IOUT can be controlled by adjusting the level of the
dimming signal DIM, and the brightness of the light emitting device
circuit 60 is correspondingly adjusted. This kind of dimming
control is referred to as analog dimming control herein. Note that
the ramp signal RMP may be for example another current related
signal.
[0004] FIG. 2 shows characteristic curves corresponding to the
prior art shown in FIG. 1B. The prior art shown in FIG. 1B has a
drawback that: when the dimming signal DIM is at a lower level, the
conversion curve of different dimming control circuits may vary one
from another due to deviations in manufacture or due to mismatches
among components in the conversion control circuit 50. Taking FIG.
2 as an example, when the dimming signal DIM is at a lower level
such as V1, the output current IOUT generated by different dimming
control circuits (cases 1-3 as shown in FIG. 2) may have different
values respectively, such as ILED1, ILED and ILED3 as shown in FIG.
2. This kind of deviations is more perceivable when the dimming
signal DIM is at a lower level, while, when the dimming signal DIM
is at a higher level such as V2, the variation is smaller.
[0005] FIG. 3 shows a schematic diagram of a conversion control
circuit (conversion control circuit 3) of another prior art light
emitting device driver apparatus. The conversion control circuit 3
performs pulse width modulation to generate a PWM signal PP
according to a fixed reference current signal IREF'. A dimming
signal DIM' in PWM form modulates the PWM signal PP such that the
output current IOUT (or ILED) is related to (for example
proportional to) the duty ratio of the dimming signal for dimming
control. In this prior art, the frequency of the PWM signal PP is
higher than the frequency of the dimming signal DIM'. This kind of
dimming control is referred to as digital dimming control or pulse
width modulation (PWM) dimming control.
[0006] FIG. 4 shows characteristic waveforms corresponding to FIG.
3. The prior art in FIG. 3 has a drawback that the ripple of the
current flowing through the light emitting string 61 is relatively
larger (as compared to analog dimming), especially when the duty
ratio of the dimming signal DIM' is low.
[0007] Compared to the prior arts in FIGS. 1 and 3, the present
invention is advantageous in reducing the brightness deviation at
lower brightness level in analog dimming control, and also in
reducing the current ripple of the light emitting string in PWM
dimming control.
SUMMARY OF THE INVENTION
[0008] From one perspective, the present invention provides a light
emitting device driver apparatus, comprising: an inductor; a power
switch, coupled to the inductor and configured to operably switch
the inductor to convert an input power to an output current to
drive a light emitting device circuit; and a conversion control
circuit, configured to operably control the power switch; the
conversion control circuit including: a modulation circuit,
configured to operably perform a pulse width modulation according
to a current related signal and a reference current signal to
generate a first PWM signal, wherein the modulation circuit
controls the current related signal such that the current related
signal corresponds to the reference current signal, wherein the
current related signal relates to the output current; a logic
driving circuit, configured to operably generate a switch control
signal according to the first PWM signal and a second PWM signal to
control the power switch to generate the output current; and a
dimming control circuit, configured to operably generate the
reference current signal according to a dimming signal, wherein:
when the dimming signal exceeds a first dimming threshold, the
reference current signal is generated according to a level of the
dimming signal, and the switch control signal controls the power
switch according to the first PWM signal, such that the output
current corresponds to the level of the dimming signal, whereby an
analog dimming control is performed; and when the dimming signal
does not exceed the first dimming threshold, the reference current
signal is clamped to a level which corresponds to the first dimming
threshold, and the second PWM signal is generated according to the
level of the dimming signal, wherein a frequency of the second PWM
signal is lower than a frequency of the first PWM signal; wherein
the second PWM signal enables the first PWM signal to generate the
switch control signal to control the power switch, wherein, during
an enable period of the second PWM signal, the output current
corresponds to the first dimming threshold, and during a disable
period of the second PWM signal, the power switch is controlled to
be OFF, whereby a pulse width modulation dimming control is
performed.
[0009] In one embodiment, the modulation circuit includes: an error
amplifier circuit, configured to operably generate an error
amplified signal according to a difference between the current
related signal and the reference current signal; and a first
comparator circuit, configured to operably compare a first ramp
signal and the error amplified signal to generate the first PWM
signal.
[0010] In one embodiment, the dimming control circuit includes: a
reference current generator circuit, configured to operably convert
a dimming reference signal to generate a reference current signal;
a signal selection circuit, configured to operably compare the
dimming signal and the first dimming threshold, wherein when the
dimming signal exceeds the first dimming threshold, the dimming
signal is selected as the dimming reference signal, and when the
dimming signal does not exceed the first dimming threshold, the
first dimming threshold is selected as the dimming reference
signal; and a second comparator circuit, configured to operably
compare the dimming signal and a second ramp signal to generate the
second PWM signal.
[0011] In one embodiment, when the dimming signal exceeds the first
dimming threshold, the reference current signal is proportional to
the dimming signal by a predetermined ratio.
[0012] In one embodiment, a peak level of the second ramp signal is
equal to the first dimming threshold.
[0013] In one embodiment, the error amplifier circuit includes: a
transconductance circuit, configured to operably generate an error
amplified current on a transconductance output terminal according
to the difference of the current related signal and the reference
current signal; a compensation capacitor, configured to operably
integrate the error amplified current to generate the error
amplified signal; and an integrator control switch, coupled between
the transconductance output terminal and the compensation
capacitor, wherein, when the dimming signal exceeds the first
dimming threshold, the integrator control switch is controlled to
conduct a current path from the error amplified current to the
compensation capacitor, and when the dimming signal does not exceed
the first dimming threshold, the integrator control switch is
controlled to conduct the current path from the error amplified
current to the compensation capacitor during the enable period of
the second PWM signal, and is controlled to cut off the current
path from the error amplified current to the compensation capacitor
during the disable period of the second PWM signal.
[0014] In one embodiment, the light emitting device circuit
includes: a light emitting device string, including a least one
light emitting device; and an output capacitor, coupled to the
light emitting device string in parallel, for reducing a current
ripple flowing through the light emitting device string.
[0015] In one embodiment, when the dimming signal exceeds a second
dimming threshold, the reference current signal is clamped to a
level which corresponds to the second dimming threshold, whereby
the output current is clamped to an upper current limit.
[0016] In one embodiment, the conversion control circuit receives
the dimming signal through a single pin to achieve the analog
dimming control and the pulse width modulation dimming control
according to the level of the dimming signal.
[0017] In one embodiment, the conversion control circuit determines
the first dimming threshold according to a deviation characteristic
obtained from data of plural diming control circuits.
[0018] From another perspective, the present invention provides a
conversion control circuit, configured to operably control a light
emitting device driver apparatus, the light emitting device driver
apparatus including an inductor and a power switch which is coupled
to the inductor, the power switch being configured to operably
switch the inductor to convert an input power to generate an output
current for driving a light emitting device circuit; the conversion
control circuit, configured to operably control the power switch so
as to control the light emitting device driver apparatus,
comprising: a modulation circuit, configured to operably perform a
pulse width modulation according to a current related signal and a
reference current signal to generate a first PWM signal, wherein
the modulation circuit controls the current related signal such
that the current related signal is corresponded to the reference
current signal, wherein the current related signal relates to the
output current; a logic driving circuit, configured to operably
generate a switch control signal according to the first PWM signal
and a second PWM signal to control the power switch to generate the
output current; and a dimming control circuit, configured to
operably generate the reference current signal according to a
dimming signal, wherein when the dimming signal exceeds a first
dimming threshold, the reference current signal is generated
according to a level of the dimming signal, and the switch control
signal reflects the first PWM signal to control the power switch,
such that the output current corresponds to the level of the
dimming signal, whereby an analog dimming control is performed;
when the dimming signal does not exceed the first dimming
threshold, the reference current signal is clamped to a level which
corresponds to the first dimming threshold, and the second PWM
signal is generated according to the level of the dimming signal,
wherein a frequency of the second PWM signal is lower than a
frequency of the first PWM signal; wherein the second PWM signal
enables the first PWM signal to generate the switch control signal
to control the power switch, wherein, during the enable period of
the second PWM signal, the output current corresponds to the first
dimming threshold, and during the disable period of the second PWM
signal, the power switch is controlled to be OFF, whereby a pulse
width modulation dimming control is performed.
[0019] The objectives, technical details, features, and effects of
the present invention will be better understood with regard to the
detailed description of the embodiments below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1A shows a block diagram of a prior art light emitting
device driver apparatus.
[0021] FIG. 1B shows a schematic diagram of a conversion control
circuit of a prior art light emitting device driver apparatus.
[0022] FIG. 2 shows characteristic curves corresponding to the
circuit shown in FIG. 1B.
[0023] FIG. 3 shows a schematic diagram of a conversion control
circuit of another prior art light emitting device driver
apparatus.
[0024] FIG. 4 shows characteristic waveforms of corresponding to
the circuit shown in FIG. 3.
[0025] FIG. 5A shows a schematic diagram of an embodiment of the
light emitting device driver apparatus according to the present
invention.
[0026] FIG. 5B shows a schematic diagram of an embodiment of a
conversion control circuit of the light emitting device driver
apparatus according to the present invention.
[0027] FIG. 5C shows a schematic diagram of an embodiment of a
modulation circuit of the light emitting device driver apparatus
according to the present invention.
[0028] FIG. 6 shows characteristic curves corresponding to
embodiments according to the present invention.
[0029] FIGS. 7A-7B show schematic waveforms corresponding to
embodiments in FIGS. 5B, 5C and 6 according to the present
invention.
[0030] FIG. 7C shows schematic waveforms of a prior art light
emitting device driver apparatus.
[0031] FIG. 8A shows a schematic diagram of a specific embodiment
of the dimming control circuit of the light emitting device driver
apparatus according to the present invention.
[0032] FIG. 8B shows a schematic diagram of another specific
embodiment of the dimming control circuit of the light emitting
device driver apparatus according to the present invention.
[0033] FIG. 8C shows a schematic diagram of a more specific
embodiment of the dimming control circuit of the light emitting
device driver apparatus according to the present invention.
[0034] FIG. 9 shows a schematic diagram of a specific embodiment of
the error amplifier circuit of the light emitting device driver
apparatus according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] The drawings as referred to throughout the description of
the present invention are for illustration only, to show the
interrelations between the circuits and the signal waveforms, but
not drawn according to actual scale.
[0036] FIG. 5A shows a schematic diagram of an embodiment of the
light emitting device driver apparatus (light emitting device
driver apparatus 5) according to the present invention. FIG. 5B
shows a schematic diagram of an embodiment of the conversion
control circuit (conversion control circuit 10) of the light
emitting device driver apparatus according to the present
invention. The light emitting device driver apparatus 5 comprises
an inductor L, a power switch SWN, and a conversion control circuit
10. The power switch SWN is coupled to the inductor L and is
configured to operably switch the inductor L to convert an input
power VDD to an output current IOUT to drive a light emitting
device circuit 60. The conversion control circuit 10 is configured
to operably control the power switch SWN for generating the output
current IOUT. In one embodiment, the light emitting device circuit
60 includes a light emitting device string 61 which includes at
least one light emitting device (for example but not limited to the
light emitting devices 611-61N, wherein N is a natural number). In
one embodiment, as shown in FIG. 5A, the light emitting device
circuit 60 includes an output capacitor CO which is coupled to the
light emitting device string 61 in parallel and is configured to
filter the output current IOUT to reduce the current ripple flowing
through the light emitting device string 61. In one embodiment, the
power switch SWN and the inductor L can be configured as a buck
converter as shown in FIG. 5A. In other embodiments, the power
switch SWN and the inductor L can be configured as a boost
converter, a buck-boost converter or a flyback converter.
[0037] Still referring to FIG. 5B, the conversion control circuit
10 includes a modulation circuit 80, a logic driving circuit 40,
and a dimming control circuit 50. The modulation circuit 80 is
configured to operably perform a pulse width modulation according
to a current related signal ISN and a reference current signal IREF
to generate a first PWM signal PWM1, wherein the current related
signal ISN relates to the output current IOUT. The modulation
circuit 80 controls the current related signal ISN such that the
current related signal ISN is corresponding to the reference
current signal IREF. In one embodiment, the modulation circuit 80
controls the current related signal ISN such that the level of the
current related signal ISN is substantially equal to the level of
the reference current signal IREF. The logic driving circuit 40 is
configured to operably generate a switch control signal GT
according to the first PWM signal PWM1 and a second PWM signal
PWM2, for controlling the power switch SWN to generate the output
current IOUT. How the second PWM signal PWM2 is generated will be
described in detail later.
[0038] FIG. 5C shows a schematic diagram of an embodiment of the
modulation circuit (modulation circuit 80) of the light emitting
device driver apparatus according to the present invention. In this
embodiment, the modulation circuit 80 includes an error amplifier
circuit 20 and a first comparator circuit 30. The error amplifier
circuit 20 is configured to operably generate an error amplified
signal EAO according to a difference between the current related
signal ISN and the reference current signal IREF. The first
comparator circuit 30 is configured to operably compare a first
ramp signal RMP1 and the error amplified signal EAO to generate the
first PWM signal PWM1. By feedback loop control, the two inputs of
the error amplifier circuit 20 will be substantially equal to one
the other, i.e., the level of the current related signal ISN will
be substantially equal to the level of the reference current signal
IREF.
[0039] In one embodiment, the first ramp signal RMP1 can be a ramp
signal related to an inductor current, or a ramp signal irrelevant
to current. The first ramp signal RMP1 can be of fixed frequency or
non-fixed frequency. The current related signal ISN can be for
example a continuous or discontinuous current related signal
obtained by sensing the inductor current or sensing a current
flowing through the power switch SWN. The current related signal
ISN can be utilized as an input of the error amplifier circuit 20
and/or as the first ramp signal RAP1.
[0040] Note that the embodiment of the modulation circuit 80 in
FIG. 5C is not for limiting the scope of the present invention.
According to the present invention, in other embodiments, the
modulation circuit can be other modulation circuit having a
structure configured for pulse width modulation control, pulse
frequency modulation control, constant ON time control, constant
OFF time control or bang bang control, with fixed frequency or
non-fixed frequency.
[0041] Referring to the FIGS. 5A-5C together with FIG. 6 which
shows characteristic curves corresponding to embodiments according
to the present invention, the dimming control circuit 50 is
configured to operably generate the reference current signal IREF
according to a dimming signal DIM, wherein when the dimming signal
DIM exceeds a first dimming threshold DIML, the reference current
signal IREF is generated according to a level of the dimming signal
DIM, and the switch control signal GT controls the power switch SWN
according to the first PWM signal PWM1, such that the output
current IOUT corresponds to the level of the dimming signal DIM,
whereby an analog dimming control is performed. More specifically,
in one embodiment, when the dimming signal DIM exceeds the first
dimming threshold DIML (for example in the analog dimming control
region as shown in FIG. 6), the reference current signal IREF is
proportional to the dimming signal DIM with a predetermined ratio
K, wherein K is a real number. In one preferred embodiment, K is a
positive real number. The error amplifier 20 together with other
related circuits regulate the output current IOUT by PWM modulation
and feedback control, such that the output current IOUT corresponds
(for example but not limited to being equal to or with another
predetermined ratio) to the reference current IREF, whereby the
output current IOUT corresponds to the level of the dimming signal
DIM.
[0042] In one embodiment, the first dimming threshold DIML can be a
predetermined fixed value or an adjustable variable. In one
embodiment, the first dimming threshold DIML can be determined
according to for example the deviation characteristics shown in the
dimming curves in FIG. 2. In one embodiment, the first dimming
threshold DIML can be a boundary value where the deviation starts
to cause the dimming effect of different circuits to vary
significantly (for example out of a tolerance range). In one
embodiment, the conversion control circuit 10 determines the first
dimming threshold DIML according to a deviation parameter of the
dimming control circuit 50.
[0043] Still referring to FIGS. 5B and 6, when the dimming signal
DIM does not exceed the first dimming threshold DIML (i.e. the
pulse modulation dimming control region as shown in FIG. 6), the
dimming control circuit 50 generates the reference current signal
IREF according to the first dimming threshold DIML. In this
embodiment, the first dimming threshold is a predetermined value,
and when the dimming signal DIM does not exceed the first dimming
threshold DIML, the current signal IREF is at a first current level
IRL (wherein, for example, the ratio of the reference current
signal IREF to the first dimming threshold DIML is the
predetermined ratio K). From one perspective, the dimming control
circuit 50 controls the reference current signal IREF to be clamped
to a level which corresponds to the first dimming threshold DIML
when the dimming signal DIM does not exceed the first dimming
threshold DIML.
[0044] The aforementioned "pulse modulation dimming control region"
indicates that within this region, the output current is modulated
by "pulses" of the second pulse modulation signal PWM2. The
modulation scheme can be for example but not limited to pulse width
modulation (PWM), pulse amplitude modulation (PAM), other types of
modulation schemes, or the combination thereof. The aforementioned
"pulse modulation dimming control region" of the present invention
will be described in detail later.
[0045] FIGS. 7A-7B show schematic waveforms corresponding to
embodiments in FIGS. 5B, 5C and 6 according to the present
invention. When the dimming signal DIM does not exceed the first
dimming threshold DIML (i.e. within the "pulse modulation dimming
control region"), the dimming control circuit 50 compares the
dimming signal DIM with the second ramp signal RMP2 to generate the
second PWM signal PWM2. In one embodiment, the frequency F2 of the
second PWM signal PWM2 is lower than the frequency F1 of the first
PWM signal PWM1. In one embodiment, the frequency F2 of the second
PWM signal PWM2 can be for example 1 kHz, and the frequency F1 of
the first PWM signal PWM1 can be for example 100 kHz. As shown in
FIG. 7A, the second PWM signal PWM2 enables the first PWM signal
PWM1 to generate the switch control signal GT to control the power
switch SWN for generating the output current IOUT. During the
enable period of the second PWM signal PWM2 (for example when PWM
is at high state during t5-t6 as shown in FIG. 7A), the output
current IOUT corresponds to the first dimming threshold DIML. Since
IRL=DIML*K, from another perspective, during the enable period of
the second PWM signal PWM2, the output current IOUT corresponds to
the first current level IRL of the reference current signal IREF.
In one embodiment, during the disable period of the second PWM
signal PWM2 (for example when PWM is at low state during t6-t7 as
shown in FIG. 7A), the power switch SWN is controlled to be OFF by
the switch control signal GT. From one perspective, when the
dimming signal DIM does not exceed the first dimming threshold
DIML, the average of the output current IOUT corresponds to the
first dimming threshold DIML multiplied by the duty ratio D2 of the
second pulse modulation signal PWM2, whereby a pulse width
modulation dimming control is performed. In other words, the
average of the output current IOUT corresponds to the first current
level IRL multiplied by the duty ratio D2 of the second pulse
modulation signal PWM2. Note that, from one perspective, the light
emitting device driver apparatus of the present invention has
characteristics of both PWM and PAM, and is different from the
prior art in FIG. 3 which employs PWM scheme only. However, this is
not to limit the scope of the present invention. In one embodiment,
the light emitting device driver apparatus of the present invention
can only perform PWM scheme for dimming during the "pulse
modulation dimming control region". In other words, in this
embodiment, when the dimming signal DIM does not exceed the first
dimming threshold DIML, the average of the output current IOUT can
be configured to correspond to the second dimming threshold DIMH
multiplied by the duty ratio D2 of the second pulse modulation
signal PWM2, as shown in FIG. 6.
[0046] From another perspective, according to the present
invention, when the dimming signal DIM does not exceed the first
dimming threshold DIML, the average of the current ILED of the
light emitting device string 61 (corresponding to the average of
the output current IOUT) is adjusted by controlling the duty ratio
of the second pulse modulation signal PWM2 for pulse modulation
dimming. According to the present invention, when the dimming
signal DIM exceeds the first dimming threshold DIML, the output
current IOUT is controlled to correspond to the level of the
dimming signal DIM, and when the dimming signal DIM does not exceed
the first dimming threshold DIML, the dimming signal DIM is
converted in a PWM form to control the output current IOUT to
correspond to the first dimming threshold DIML multiplied by the
duty ratio of the second pulse modulation signal PWM2. And since
the second pulse modulation signal PWM2 is obtained by comparing
the dimming signal DIM with the second ramp signal PWM2, in one
embodiment, the analog dimming control region and the pulse
modulation dimming control region in the dimming curve (i.e. IOUT
vs. DIM as shown in FIG. 6) can be combined seamlessly (connected
continuously) if the levels of the peak and valley of the second
ramp signal RMP2 are properly selected. In one embodiment, when the
peak level of the second ramp signal RMP2 is set to be the first
dimming threshold DIML (as shown in FIG. 7A), the dimming curve is
continuous and the two regions (the analog dimming control region
and the pulse modulation dimming control region) are connected with
each other at the first dimming threshold DIML (i.e. the joint of
the two regions, of as shown FIG. 6). In one preferred embodiment,
when the valley level of the second ramp signal RMP2 is set to be
corresponding to the extension line of the dimming curve in the
analog dimming control region, the slopes of the dimming curve in
the analog dimming control region and the pulse modulation dimming
control region are substantially the same (FIG. 6). From one
perspective, when the peak level of the second ramp signal RMP2 is
set to be the first dimming threshold DIML and when the dimming
signal DIM exceeds the first dimming threshold DIML (for example
after t10 in FIG. 7A), the switch control signal GT can still be
regarded as being generated by the second pulse modulation signal
PWM enabling the first pulse modulation signal PWM1, wherein the
duty ratio of the second pulse modulation signal PWM2 is 100%.
[0047] Note that the selection of the peak and valley of the second
ramp signal RMP2 described as above is for illustration and not for
limiting the scope of the present invention. In other embodiments,
the slopes of the dimming curve sections in the analog dimming
control region and the pulse modulation dimming control region can
be configured to be different, or the joint of the two regions can
be discontinuous (in either axis or both axes) by selecting the
peak and valley of the second ramp signal RMP2 different from the
embodiment described above.
[0048] It is also worth noting that, according to the present
invention, in the pulse modulation dimming control region, the
current ILED flowing through the light emitting device string 61
has a smaller current ripple. FIGS. 7A-7B show schematic waveforms
corresponding to the embodiments in FIGS. 5B, 5C and 6 according to
the present invention. FIG. 7C shows schematic waveforms of a prior
art (FIG. 3) light emitting device driver apparatus. According to
the present invention, as previously described, during the enable
period of the second PWM signal PWM2 and when the dimming signal
DIM does not exceed the first dimming threshold DIML, the output
current IOUT corresponds to the first dimming threshold DIML (i.e.
corresponding to the first current level IRL of the reference
current signal IREF as in FIG. 7B). In other words, with the same
target output current level, during the enable period of the second
pulse modulation signal PWM2, the light emitting device driver
apparatus of the present invention performs pulse width modulation
with a lower current level (e.g. corresponding to IRL as in FIG.
7B), which leads to a lower current amplitude (in PWM form) of the
output current IOUT. Consequently, the current ripple of the
current ILED flowing through the light emitting device string 61
can be effectively reduced (FIG. 7B). As a comparison, the prior
art performs the pulse width modulation (e.g. PWM2' in FIG. 7C)
with a higher current level (corresponding to IRH as shown in FIG.
7C), which leads to a higher current amplitude (in PWM form) of the
output current IOUT'. Consequently, the current ripple of the
current ILED flowing through the light emitting device string of
the prior art is larger (FIG. 7C).
[0049] Also note that, according to the present invention, during
the pulse modulation dimming control region, the dimming is still
controlled by adjusting the level the dimming signal DIM, which is
consistent with the analog dimming control and hence simplifies the
application circuitry for generating the dimming signal DIM. In one
embodiment, the conversion control circuit (e.g. conversion control
circuit 10) can be an integrated circuit. In one embodiment, the
conversion control circuit 10 receives the dimming signal DIM
through a single pin (of the integrated circuit) to achieve both
the analog dimming control and the pulse width modulation dimming
control according to the level of the dimming signal DIM.
[0050] FIG. 8A shows a schematic diagram of a specific embodiment
of the dimming control circuit (dimming control circuit 50) of the
light emitting device driver apparatus according to the present
invention. The dimming control circuit 50 includes: a reference
current generator circuit 51, a signal selection circuit 52, and a
second comparator circuit 53. The reference current generator
circuit 51 is configured to operably convert the dimming reference
signal VREF to generate the reference current signal IREF. In one
embodiment, the ratio of the reference current signal IREF to the
dimming reference signal VREF is the predetermined ratio K. In one
embodiment, the signal selection circuit 52 is configured to
operably compare the dimming signal DIM and the first dimming
threshold DIML. When the dimming signal DIM exceeds the first
dimming threshold DIML, the dimming signal DIM is selected as the
dimming reference signal VREF. When the dimming signal DIM does not
exceed the first dimming threshold DIML, the first dimming
threshold DIML is selected as the dimming reference signal VREF.
The second comparator circuit 53 is configured to operably compare
the dimming signal DIM and a second ramp signal RMP2 to generate
the second PWM signal PWM2.
[0051] FIG. 8B shows a schematic diagram of another specific
embodiment of the dimming control circuit (dimming control circuit
50') of the light emitting device driver apparatus according to the
present invention. Referring to FIGS. 8B and 6, in one embodiment,
when the dimming signal DIM exceeds a second dimming threshold
DIMH, the dimming control circuit 50' clamps the reference current
signal IREF a level which corresponds to the second dimming
threshold DIMH, whereby the output current IOUT is clamped to an
upper current limit. In one embodiment, as shown in FIG. 8B, when
the dimming signal DIM exceeds a second dimming threshold DIMH, the
signal selection circuit 52' selects the second dimming threshold
DIMH as the dimming reference signal VREF. In one embodiment, the
second dimming threshold DIMH is higher than the first dimming
threshold DIML.
[0052] FIG. 8C shows a schematic diagram of a more specific
embodiment of the dimming control circuit (dimming control circuit
50') of the light emitting device driver apparatus according to the
present invention. In one embodiment, the signal selection circuit
52 includes plural comparators and selection switches, which are
configured to select one of the dimming signal DIM, the first
dimming threshold DIML or the second dimming threshold DIMH as the
dimming reference signal VREF. In one embodiment, the reference
current generator circuit 51 is a linear amplifier circuit which is
configured to convert the dimming reference signal VREF to the
reference current signal IREF, wherein the ratio between the
reference current signal IREF and the dimming reference signal VREF
is determined by the resistors as shown in FIG. 8C.
[0053] FIG. 9 shows a schematic diagram of a specific embodiment of
the error amplifier circuit (error amplifier circuit 20) of the
light emitting device driver apparatus according to the present
invention. The error amplifier circuit 20 includes a
transconductance circuit 21, a compensation capacitor CC, and an
integrator control switch SWC. The transconductance circuit 21 is
configured to operably generate an error amplified current IGO on a
transconductance output terminal GO according to the difference
between the current related signal ISN and the reference current
signal IREF. The compensation capacitor CC is configured to
operably integrate the error amplified current IGO to generate the
error amplified signal EAO. The integrator control switch SWC is
coupled between the transconductance output terminal GO and the
compensation capacitor CC. When the dimming signal DIM exceeds the
first dimming threshold DIML, the integrator control switch SWC is
controlled to conduct a current path from the error amplified
current IGO to the compensation capacitor CC, and when the dimming
signal DIM does not exceed the first dimming threshold DIML, the
integrator control switch SWC is controlled to conduct the current
path from the error amplified current IGO to the compensation
capacitor CC during the enable period of the second PWM signal
PWM2, but is controlled to cut off the current path from the error
amplified current IGO to the compensation capacitor CC during the
disable period of the second PWM signal PWM2. Thus, the voltage
across the compensation capacitor CC is maintained during the
disable period of the second pulse modulation signal PWM2 by
cutting off the current path from the error amplified current IGO
to the compensation capacitor CC, so that when the second PWM
signal PWM2 is enabled for the next time, the light emitting device
driver apparatus of the present invention can re-start from the
steady state in the previous enable period of the second PWM signal
PWM2 and does not require a soft start as often required in the
power up stage of a conventional light emitting device driver
apparatus, whereby the dimming is better controlled.
[0054] The present invention has been described in considerable
detail with reference to certain preferred embodiments thereof. It
should be understood that the description is for illustrative
purpose, not for limiting the scope of the present invention. It is
not limited for each of the embodiments described hereinbefore to
be used alone; under the spirit of the present invention, two or
more of the embodiments described hereinbefore can be used in
combination. For example, two or more of the embodiments can be
used together, or, a part of one embodiment can be used to replace
a corresponding part of another embodiment. Furthermore, those
skilled in this art can readily conceive variations and
modifications within the spirit of the present invention. For
example, to perform an action "according to" a certain signal as
described in the context of the present invention is not limited to
performing an action strictly according to the signal itself, but
can be performing an action according to a converted form or a
scaled-up or down form of the signal, i.e., the signal can be
processed by a voltage-to-current conversion, a current-to-voltage
conversion, and/or a ratio conversion, etc. before an action is
performed. In addition, when it is described that a parameter "is"
or "is equal to" a number, it does not require that the parameter
"exactly is" or "is precisely equal to" the number; a certain
tolerable error is acceptable. The spirit of the present invention
should cover all such and other modifications and variations, which
should be interpreted to fall within the scope of the following
claims and their equivalents.
* * * * *