Method of driving light-source with improved contrast ratio and display apparatus for performing the method
A method of driving a light-source module includes adjusting a frequency of a boosting switching signal based on a dimming signal which controls luminance of a light-emitting diode (“LED”) string of the light-source module, where the LED string comprises a plurality of LEDs connected to each other in series, and controlling a main transistor in response to the boosting switching signal to transfer a driving voltage to the LED string.
1. A method of driving a light-source module, the method comprising:
determining, by a light-source driving part, a duty ratio using a detection voltage detected from an end portion of a light-emitting diode (“LED”) string of the light-source module, the light-source driving part comprising a main transistor and a control switching element connected to the end portion of the LED string;
generating a pulse width modulation (“PWM”) control signal having a current control pulse to control the control switching element, wherein the LED string comprises a plurality of LEDs connected to each other in series; and
controlling the control switching element in response to the PWM control signal,
wherein the generating the PWM control signal comprises generating a chopping wave of a first driving frequency,
the light-source module comprises a frequency oscillating part comprising a first resistor, and
the chopping wave has a driving frequency using a following Equation:
f
op
=
K
R
R
T
[
k
Ω
]
,
where K is a constant, and R RT denotes a resistance in kiloohm of the first resistor,
wherein the light-source driving part comprises:
an image analyzing part configured to analyze an image signal and to determine a target luminance value of the LED string;
a dimming level determining part configured to determine a dimming signal of the LED string using the target luminance value; and
a driving signal generating part comprising the main transistor, configured to adjust a driving frequency of a boosting switching signal based on the dimming signal and configured to drive the main transistor in response to the boosting switching signal,
wherein the driving signal generating part comprises:
the frequency oscillating part configured to generate the chopping wave;
a frequency modulating part configured to modulate the driving frequency of the chopping wave to the first driving frequency, or a second driving frequency higher than the first driving frequency, based on the dimming signal which controls luminance of the LED string; and
a constant current control part configured to generate first or second PWM control signal including the current control pulse repeated with the first or second driving frequency in an enable period of the dimming signal based on the chopping wave of the first or second driving frequency, and
wherein the frequency modulating part comprises:
an input part configured to receive the dimming signal;
a voltage dividing part configured to divide a voltage of the dimming signal;
a diode part connected to the input part and the voltage dividing part;
an oscillating transistor configured to operate in response to a divided voltage received from the voltage dividing part; and
a resistor part comprising a second resistor connected in parallel to the first resistor of the frequency oscillating part, which determines the driving frequency of the chopping wave.
2. The method of claim 1 , wherein the generating the PWM control signal comprises:
generating the chopping wave of the first driving frequency when a level of the dimming signal is greater than a preset value; and
generating the first PWM control signal.
3. The method of claim 2 , wherein the generating the PWM control signal comprises:
generating the chopping wave of the second driving frequency when the level of the dimming signal is less than the preset value; and
generating the second PWM control signal.
4. The method of claim 2 , wherein the preset value corresponds to a duty ratio of 2 percent or less.
5. The method of claim 1 , wherein the generating the PWM control signal comprises:
generating the chopping wave of the first driving frequency when a level of the detection voltage is substantially equal to a preset value; and
generating the first PWM control signal.
6. The method of claim 5 , wherein the generating the PWM control signal comprises:
generating the chopping wave of the second driving frequency when the level of the detection voltage is different from the preset value; and
generating the second PWM control signal.
7. A display apparatus comprising:
a display panel configured to receive an image signal and to display an image corresponding to the image signal;
a light-source module comprising a light-emitting diode (“LED”) string, wherein the LED string comprises a plurality of LEDs connected to each other in series; and
a light-source driving part comprising a main transistor and a control switching element connected to an end portion of the LED string, and configured to generate a pulse width modulation (“PWM”) control signal having a current control pulse to control the control switching element,
wherein the light-source driving part is configured to determine a duty ratio using a detection voltage detected from the end portion of the LED string, to generate the PWM control signal having the current control pulse of the duty ratio, and to adjust a driving frequency of the PWM control signal,
the light-source module further comprises a frequency oscillating part which comprises a first resistor and generates a chopping wave, and
the chopping wave has a driving frequency using a following Equation:
f
op
=
K
R
R
T
[
k
Ω
]
,
where K is a constant, and R RT denotes a resistance in kiloohm of the first resistor,
wherein the light-source driving part comprises:
an image analyzing part configured to analyze the image signal and to determine a target luminance value of the LED string;
a dimming level determining part configured to determine a dimming signal of the LED string using the target luminance value; and
the a driving signal generating part comprising the main transistor, configured to adjust a driving frequency of a boosting switching signal based on the dimming signal and configured to drive the main transistor in response to the boosting switching signal,
wherein the driving signal generating part comprises:
the frequency oscillating part configured to generate the chopping wave;
a frequency modulating part configured to modulate the driving frequency of the chopping wave to the first driving frequency, or a second driving frequency higher than the first driving frequency, based on the dimming signal which controls luminance of the LED string; and
a constant current control part configured to generate first or second PWM control signal including the current control pulse repeated with the first or second driving frequency in an enable period of the dimming signal based on the chopping wave of the first or second driving frequency, and
wherein the frequency modulating part comprises:
an input part configured to receive the dimming signal;
a voltage dividing part configured to divide a voltage of the dimming signal;
a diode part connected to the input part and the voltage dividing part;
an oscillating transistor configured to operate in response to a divided voltage received from the voltage dividing part; and
a resistor part comprising a second resistor connected in parallel to the first resistor of the frequency oscillating part, which determines the driving frequency of the chopping wave.
8. The display apparatus of claim 7 , wherein a level of the dimming signal corresponds to a duty ratio of 2 percent or less.