U.S. patent application number 11/623326 was filed with the patent office on 2007-05-24 for pwm illumination control circuit with low visual noise for driving led.
This patent application is currently assigned to BEYOND INNOVATION TECHNOLOGY CO., LTD.. Invention is credited to Bor-Yuh Chang, Shih-Chung Huang, Chung-Che Yu.
Application Number | 20070114949 11/623326 |
Document ID | / |
Family ID | 34632338 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070114949 |
Kind Code |
A1 |
Yu; Chung-Che ; et
al. |
May 24, 2007 |
PWM ILLUMINATION CONTROL CIRCUIT WITH LOW VISUAL NOISE FOR DRIVING
LED
Abstract
A pulse width modulation (PWM) illumination control circuit with
low visual noise for driving a light-emitting diode (LED) is
provided. An illumination control pulse-generating unit is used to
generate an illumination control pulse signal according to an
illumination-adjusting signal. The duty cycle or frequency of the
illumination control pulse signal varies within a predetermined
scope for controlling a DC/DC converter to drive the light-emitting
diode so that overall visual noise level of the PWM illumination
control circuit is improved.
Inventors: |
Yu; Chung-Che; (Taipei City,
TW) ; Chang; Bor-Yuh; (Taipei City, TW) ;
Huang; Shih-Chung; (Taipei City, TW) |
Correspondence
Address: |
JIANQ CHYUN INTELLECTUAL PROPERTY OFFICE
7 FLOOR-1, NO. 100
ROOSEVELT ROAD, SECTION 2
TAIPEI
100
TW
|
Assignee: |
BEYOND INNOVATION TECHNOLOGY CO.,
LTD.
Taipei City
TW
|
Family ID: |
34632338 |
Appl. No.: |
11/623326 |
Filed: |
January 16, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10708212 |
Feb 17, 2004 |
7183723 |
|
|
11623326 |
Jan 16, 2007 |
|
|
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Current U.S.
Class: |
315/247 |
Current CPC
Class: |
Y02B 20/30 20130101;
H05B 45/10 20200101; H05B 45/3725 20200101 |
Class at
Publication: |
315/247 |
International
Class: |
H05B 41/24 20060101
H05B041/24 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 2003 |
TW |
92134517 |
Claims
1. A pulse width modulation illumination control circuit for
controlling the illumination of light-emitting diodes inside a
liquid crystal display, comprising: an illumination control
pulse-generating unit, for receiving an illumination-adjusting
signal and generating an illumination control pulse signal
according to the illumination-adjusting signal, wherein a frequency
of the illumination control pulse signal varies with time within a
predetermined range; and a DC/DC converter, coupled to the
illumination control pulse-generating unit for driving the
light-emitting diodes according to the illumination control pulse
signal.
2. The control circuit of claim 1, wherein the illumination control
pulse-generating unit is implemented using a microprocessor.
3. A pulse width modulation illumination control circuit for
controlling the illumination of light-emitting diodes inside a
liquid crystal display, comprising: an illumination control
pulse-generating unit, for receiving an illumination-adjusting
signal and generating an illumination control pulse signal
according to the illumination-adjusting signal, wherein a phase
shift of the illumination control pulse signal varies with time
within a predetermined range; and a DC/DC converter, coupled to the
illumination control pulse-generating unit for driving the
light-emitting diodes according to the illumination control pulse
signal.
4. An illumination control pulse generating device for receiving an
illumination adjusting signal, the illumination control pulse
generating device comprising: a random signal generating unit, for
generating a random signal; a combine unit, coupled to the random
signal generating unit for combining the random signal with the
illumination adjusting signal to form a random signal combining
illumination adjusting signal; and a comparator, coupled to the
combine unit for comparing the random signal combining illumination
adjusting signal with a comparing signal to produce a illumination
control pulse signal, wherein a frequency of the illumination
control pulse signal varies with time within a predetermined
range.
5. The illumination control pulse generating device of claim 4,
wherein the random signal generating unit is a noise generator.
6. The illumination control pulse generating device of claim 4,
wherein the combine unit is an analogue adder.
7. The illumination control pulse generating device of claim 4,
wherein the comparing signal is a triangular wave.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional application of U.S.
application Ser. No. 10/708,212 filed on Feb. 17, 2003 which claims
the priority benefit of Taiwan application serial no. 92134517,
filed on Dec. 8, 2003. The entirety of each of the above-mentioned
patent applications is hereby incorporated by reference herein and
made a part of this specification.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an illumination control
circuit. More particularly, the present invention relates to a
pulse width modulation (PWM) illumination control circuit with low
visual noise for driving a light-emitting diode (LED).
[0004] 2. Description of the Related Art
[0005] In recent years, conventional cathode ray tubes (CRT) are
gradually being replaced by liquid crystal displays (LCD) due to
big improvements in the semiconductor manufacturing techniques. LCD
has many advantages over CRT including lower power consumption, a
lighter weight, a higher resolution, higher degree of color
saturation and a longer service life. For these advantages, LCD is
being widely used in many electronic products including digital
cameras, notebook computers, desktop monitors, mobile phones,
personal digital assistants (PDA), car television, global
positioning systems (GPS), palm-top game player, electronic
translators and even digital watches and so on.
[0006] In general, a liquid crystal display uses an array of
light-emitting diodes (LED) driven by a simple driving circuit to
serve as the light source. However, due to the special properties
of an LED, brightness of the LED is not linearly related to the
driving current. Furthermore, color of the LED may also vary
according to the driving current. Hence, for a liquid crystal
display that uses LED as a back light or illumination system,
difficulties are often encountered when the illumination is varied
by directly adjusting the driving current.
[0007] To avoid the difficulties of illumination adjustment through
an amplitude variation of the driving current, a constant amplitude
driving current is used with illumination adjustment achieved
through a pulse width modulation (PWM) of the driving current.
Ultimately, the LED is able to produce consistent emitting
efficiency within a broad range.
[0008] FIG. 1 is a block diagram of a conventional pulse width
modulation illumination control circuit. FIG. 2 is a diagram
showing the relationships between illumination control pulse
signals and light-emitting diode driving current signals for the
circuit in FIG. 1. In FIG. 1, an illumination control pulse signal
Con that set the illumination of the light-emitting diode is sent
to a DC/DC converter 110 to produce a light-emitting diode driving
current signal Id for driving a light-emitting diode. The waveform
diagrams (a), (b) and (c) in FIG. 2 represent three different pulse
width settings of the light-emitting diode driving current signals
Id. For example, the light-emitting diode is at full illumination
(100%) in FIG. 2(a), at 20% of the full illumination in FIG. 2(b)
and at 50% of the full illumination in FIG. 2(c).
[0009] To prevent any perceived flickering in the light-emitting
diode by the human eyes, the frequency of the illumination control
pulse signal Con cannot be too low, typically above 200 Hz. In
other words, the illumination control pulse signal Con must operate
at a sufficiently high frequency so that the human eyes can retain
a visual image and yet perceive a steady change of illumination
without flickering. Obviously, these control signals may be
implemented using a simple switching circuit that controls the
on/off states of the entire DC/DC converter.
[0010] Because the frequency and duty cycle of the aforementioned
illumination control pulse signal Con is set to be fixed according
to the required illumination, interference with the vertical,
horizontal scanning signals may occur when used as back light in a
liquid crystal display. The difference in frequency between the
back light and the video signals often leads to the so-called
`fanning effect`, a watery wave pattern of the image on a display
screen. In addition, the switching on or off of the DC/DC converter
also leads to a significant loading on the power supply that
provides power to the DC/DC converter. In other words, a ripple
waveform synchronized with the illumination control pulse signal
Con is also produced in the power supply. Once again, the ripple
waveform may affect the video display signals leading to a
flickering screen.
[0011] To prevent interference between the illumination control
pulse signal Con and the vertical, horizontal scanning signal due
to the frequency difference, the illumination control pulse signal
Con and the horizontal scanning signals are synchronized to a
frequency an integral multiple of each other. However, this
arrangement will increase the production cost. To reduce the ripple
waveform in the power supply, the frequency of the illumination
control pulse signal Con can be increased. Yet, increasing the
frequency of pulse signal Con leads to higher power consumption.
With the demand of a larger display screen and a lesser visual
noise, fabricating a light-emitting diode illuminated liquid
crystal display with low noise and broad adjustable range of
illumination is increasingly difficult.
SUMMARY OF THE INVENTION
[0012] Accordingly, one objective of the present invention is to
provide a pulse width modulation (PWM) illumination control circuit
with low visual noise for driving a light-emitting diode (LED). By
varying the duty cycle or frequency of an illumination control
pulse signal and maintaining an average duty cycle and frequency,
visual noise interference due to pulse width modulation is
reduced.
[0013] To achieve these and other advantages and in accordance with
the purpose of the invention, as embodied and broadly described
herein, the invention provides a low visual noise pulse width
modulation illumination control circuit for controlling the
illumination of light-emitting diodes inside a liquid crystal
display. The low visual noise pulse width modulation illumination
control circuit comprises an illumination control pulse generating
unit and a DC/DC converter. The illumination control
pulse-generating unit receives an illumination-adjusting signal.
According to the illumination-adjusting signal, the illumination
control pulse-generating unit generates an illumination control
pulse signal having a duty cycle set to vary within a predetermined
range. The DC/DC converter is coupled to the illumination control
pulse-generating unit so that the illumination control
pulse-generating unit can drive the light-emitting diodes according
to the illumination control pulse signal.
[0014] In one embodiment of the invention, the illumination control
pulse-generating unit of the low visual noise PWM illumination
control circuit further comprises a noise generator, an analogue
adder and a comparator. The noise generator generates a noise
signal. The analogue adder is coupled to the noise generator for
receiving the illumination-adjusting signal and the noise signal to
produce a noise signal loaded illumination-adjusting signal. The
comparator is coupled to the analogue adder for comparing the noise
signal loaded illumination-adjusting signal with a triangular wave
and producing the illumination control pulse signal.
[0015] In one embodiment of the invention, the noise signal level
produced by the low visual noise PWM illumination control circuit
can be adjusted.
[0016] The present invention also provides an alternative low
visual noise pulse width modulation illumination control circuit
for controlling the illumination of light-emitting diodes inside a
liquid crystal display. The low visual noise pulse width modulation
illumination control circuit comprises an illumination control
pulse generating unit and a DC/DC converter. The illumination
control pulse-generating unit receives an illumination-adjusting
signal. According to the illumination-adjusting signal, the
illumination control pulse-generating unit generates an
illumination control pulse signal having a frequency set to vary
within a predetermined range. The DC/DC converter is coupled to the
illumination control pulse-generating unit so that the illumination
control pulse-generating unit can drive the light-emitting diodes
according to the illumination control pulse signal.
[0017] In one embodiment of the invention, the operations carried
out by the illumination control pulse-generating unit of the low
visual noise PWM illumination control circuit are implemented using
a microprocessor.
[0018] In one embodiment of the invention, the phase of the
illumination control pulse signal produced by the low visual noise
PWM illumination control circuit also varies within a predetermined
range.
[0019] Accordingly, the present invention provides a low visual
noise PWM illumination control circuit for driving light-emitting
diodes such that visual noise interference due to pulse width
modulation is reduced by varying the duty cycle or frequency of an
illumination control pulse signal and maintaining an average duty
cycle and frequency.
[0020] It is to be understood that both the foregoing general
description and the following detailed description are exemplary,
and are intended to provide further explanation of the invention as
claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification. The following drawings
illustrate embodiments of the invention and, together with the
description, serve to explain the principles of the invention.
[0022] FIG. 1 is a block diagram of a conventional pulse width
modulation illumination control circuit.
[0023] FIG. 2 is a diagram showing the relationships between
illumination control pulse signals and light-emitting diode driving
current signals for the circuit in FIG. 1.
[0024] FIG. 3 is a block diagram of a light-emitting diode low
visual noise PWM illumination control circuit according to one
preferred embodiment of the present invention.
[0025] FIG. 4 is a circuit diagram of an illumination control
pulse-generating unit according to the preferred embodiment of the
present invention.
[0026] FIG. 5 is a diagram showing the waveform of the illumination
control pulse signal produced by the illumination control
pulse-generating unit shown in FIG. 4.
[0027] FIG. 6 is a flow chart showing the steps for operating the
illumination control pulse-generating unit according to the
preferred embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Reference will now be made in detail to the present
preferred embodiments of the invention, examples of which are
illustrated in the accompanying drawings. Wherever possible, the
same reference numbers are used in the drawings and the description
to refer to the same or like parts.
[0029] FIG. 3 is a block diagram of a light-emitting diode low
visual noise PWM illumination control circuit according to one
preferred embodiment of the present invention. The low visual noise
pulse width modulation (PWM) illumination control circuit 300 in
FIG. 3 is adapted to control the illumination level of
light-emitting diodes (not shown) inside a liquid crystal display.
The low visual noise PWM illumination control circuit 300 comprises
an illumination control pulse-generating unit 310 and a DC/DC
converter 320. The illumination control pulse-generating unit 310
is used for receiving an illumination-adjusting signal Ref.
According to the illumination-adjusting signal Ref, the
illumination control pulse-generating unit 310 generates an
illumination control pulse signal Con. To improve the visual noise
interference of pulse width modulation, the duty cycle or frequency
of the illumination control pulse signal Con is permitted to vary
within a predetermined range. Hence, differential frequency
interference between a fixed illumination control pulse signal Con
and the vertical/horizontal scanning signal leading to the
so-called `fanning effect` with wavy lines on the display screen is
prevented. In addition, the DC/DC converter 320 drives the
light-emitting diodes according to the illumination control pulse
signal Con generated by the illumination control pulse-generating
unit 310.
[0030] FIG. 4 is a circuit diagram of an illumination control
pulse-generating unit according to the preferred embodiment of the
present invention. As shown in FIG. 4, the illumination control
pulse-generating unit 400 comprises a noise generator 410, an
analogue adder 420 and a comparator 430. The noise generator 410
further comprises a resistor 411 and an amplifier 421 electrically
connected together and the analogue adder 420 further comprises a
plurality of resistors 422, 423, 425 and an amplifier 421
electrically connected together. The noise generator 410 outputs a
noise signal Nos after the amplifier 412 inside the noise generator
410 amplifies the thermal noise produced by the resistor 411. The
noise signal Nos is transmitted to the analogue adder circuit 420
such that the noise signal Nos and an illumination-adjusting signal
Ref originally set to control the output duty cycle of the DC
voltage are summed together to produce a noise signal loaded
illumination-adjusting signal Ref. The resistor 422 is a variable
resistor so that the level of the noise signal Nos loaded on the
illumination-adjusting signal Ref can be adjusted. The noise signal
loaded illumination-adjusting signal Ref is transmitted to the
comparator 430 where the signal is compared with a triangular wave
Tri to produce an illumination control pulse signal Con having a
duty cycle that varies within the acceptable noise signal level as
shown in FIG. 5.
[0031] As shown in FIG. 5, although the duty cycle of the
illumination control pulse signal Con varies on each transient
moment of each cycle, the average power of the noise is zero.
Hence, the average duty cycle of the entire circuit after adding
the noise is identical to one without adding any noise. In other
words, the illumination of the light-emitting diodes after adding
noise to the circuit is identical to the illumination without
adding any noise to the circuit.
[0032] FIG. 6 is a flow chart showing the steps for operating the
illumination control pulse-generating unit according to the present
invention. When the illumination control pulse-generating unit 310
as shown in FIG. 3 is implemented using a microprocessor, the steps
in FIG. 6 can be carried out to produce an illumination control
pulse signal Con with a variable frequency so that visual noise
interference due to pulse width modulation is reduced.
[0033] Assuming that the illumination control pulse signal Con in
FIG. 3 has a frequency F=1/T, where T is the cycle of the
illumination control pulse signal Con, n illumination control pulse
signals Con with different cycle time such as T0, T1, T2, . . . ,
Tn-1 such that (T0+T1+T2+ . . . +Tn-1)/n=T can be designed.
Furthermore, the n illumination control pulse signals Con with
different cycle time can be permuted to form a queue before turning
each signal out sequentially. For example, if sequence 0 is {T0,
T1, T2, . . . , Tn-1}, sequence 1 is {T0, T2, . . . ,} and so on,
the n illumination control pulse signals Con with different cycle
time may be arranged to form a list of K different non-repeating
sequence including sequence 0, sequence 1, sequence 2, . . . ,
sequence K-1. Thereafter, the steps depicted in FIG. 6 can be
executed using the microprocessor so that illumination control
pulse signals Con each having a different frequency are
sequentially output. The operating steps of a digitally operated
illumination control pulse-generating unit with a low visual noise
level are explained as follows.
[0034] In step S610, variables I, J are set to 0. Thereafter, in
step S620, the I.sup.th illumination control pulse signal cycle in
sequence J and the received illumination-adjusting signal are
combined to produce an illumination control pulse signal. In step
S630, a 1 is added to the variable I in preparation for retrieving
the next illumination control pulse signal cycle in sequence J. In
step S640, the value of I is checked to determine whether it is
equal to n. When the value of I is not equal to n, the operation
returns to step S620. However, if the value of I is equal to n,
step S650 is executed to reset I to 0 and add 1 to the value of J
in preparation for retrieving the first illumination control pulse
signal cycle of the next sequence. Thereafter, step S660 is
executed to determine whether the value of J is equal to K. When
the value of J is not equal to K, the operation returns to step
S620. On the other hand, if the value of J is equal to K, step S670
is executed to reset the value of J to 0 and return the operation
to step S620.
[0035] The steps carried out in aforementioned description assumes
the existence of K sequences. However, anyone familiar with the
technique may understand that the operation is greatly simplified
when K is 1. In addition, the phase of the illumination control
pulse signal generated in step S620 can be set to vary within a
predetermined range so that an illumination control pulse signal
with a wider frequency range is produced.
[0036] It will be apparent to those skilled in the art that various
modifications and variations can be made to the structure of the
present invention without departing from the scope or spirit of the
invention. In view of the foregoing, it is intended that the
present invention cover modifications and variations of this
invention provided they fall within the scope of the following
claims and their equivalents.
* * * * *