TWI400679B - Circuit and method for driving backlight unit - Google Patents

Description

Driving circuit and method of backlight unit

The present invention relates to a driving circuit for a Light Emitting Diode (LED), and more particularly to a driving circuit for driving an LED with a digital signal and a driving method thereof.

As shown in FIG. 1, FIG. 1 is a circuit diagram of an LED backlight driving circuit according to the prior art. The backlight driving circuit portion is mainly composed of a voltage converting circuit 110 and a driving unit 120. The backlight unit 130 is composed of a plurality of LED strings (not shown), and each LED string includes a plurality of LED strings. LEDs. The brightness of the backlight unit 130 can be adjusted via the magnitude of the current through the LED string. The resistor R _EXT shown in FIG. 1 is used to set a constant current value through the LED string, and the dimming control signal V _CTRL is used to control the brightness of the backlight unit 130. At present, the main LED driving circuit dimming mode is mainly divided into two types, namely analog mode and frequency pulse wave modulation (PPWM) mode. The analog method mainly controls the current through the LED by external voltage V _CTRL to control The brightness of the LED is as shown in Figure 1 above. However, this type of dimming may result in different optical current values (such as color temperature, chromaticity, etc.) due to different driving current values of individual LEDs.

In addition, the method of performing LED dimming by frequency pulse modulation (PWM) signal is a commonly used conventional technique, mainly using a duty cycle of a PWM signal to adjust a driving current through a LED string. The length of time is dimmed by controlling the illumination time of the LED string. 2 is an LED backlight driving power according to a frequency pulse modulation method according to a prior art. Road map. The main difference between FIG. 2 and FIG. 1 is that the driving unit 120 controls the lighting time of the LED string according to the received PWM signal (this dimming mode is generally called LED Dimming Control). As shown in FIG. 3, FIG. 3 is a comparison diagram of a PWM signal and an LED enable according to the prior art. The period T of the PWM signal can be divided into a period T1 and a period T2. When the PWM signal is enabled (period T1), the LED causes Can (bright), indicated by ON; when the PWM is disabled (period T2), the LED is disabled (dark) and is indicated by OFF. Since the LED needs to continuously switch between light and dark to form the required average brightness, such a driving method makes the equivalent load of the LED unstable, thereby affecting the conversion efficiency of the voltage conversion circuit 110. 4 is a current waveform diagram of a voltage conversion circuit according to the prior art. As shown in FIG. 4, when the PWM signal is switched between potentials, the voltage conversion circuit enters a Discontinuous Conduction Mode (DCM mode) to cause power consumption of the voltage conversion circuit.

The invention provides a driving circuit for replacing the analog signal or the PWM signal with a digital signal to achieve the dimming purpose. The invention determines the enabling ratio of the LED string and its corresponding position according to the digital signal to obtain the desired brightness change. Since the LED string of the present invention does not need to continuously perform the switching between light and dark, the equivalent load is relatively stable, so the power consumption of the voltage conversion circuit can be reduced.

The invention further provides a driving method of the LED backlight unit, which can use the digital signal to perform dimming of the backlight unit, and can also switch different driving modes to set the on-current value of the individual LEDs, thereby increasing the backlight unit in the dimming function. Convenience.

In view of the above, the present invention provides a driving circuit suitable for driving a backlight unit, the backlight unit comprising a plurality of LED strings, the driving circuit comprising a voltage conversion circuit, a driving unit and a digital dimming unit. The voltage conversion circuit is configured to provide a driving voltage to the backlight unit, and the driving unit is coupled to the other end of the backlight unit for controlling the conduction current of the LED string. The digital dimming unit outputs a digital signal to the driving circuit, wherein the driving circuit correspondingly enables the LED strings according to the digital signal, and adjusts the brightness of the backlight unit according to the enabling ratio of the LED strings.

In an embodiment of the invention, the voltage conversion circuit is a booster circuit.

In an embodiment of the invention, the driving unit further includes a temporary register for temporarily storing the digital signal.

In an embodiment of the invention, the digital dimming unit further outputs a displacement clock signal and a shackle signal to the driving unit, and the driving unit temporarily stores the digital signal according to the displacement clock signal, and locks the shackle according to the shackle signal. Received digital signal.

In an embodiment of the invention, the digital dimming unit further outputs a control signal to the driving unit. When the control signal is enabled, the driving unit respectively adjusts the on current value of the LED string according to the digital signal.

From another point of view, the present invention further provides a driving method of a backlight unit. The backlight unit includes a plurality of LED strings. The driving method includes the following steps: first, temporarily storing a digital signal according to a displacement clock signal; And latching the received digital signal according to a latch signal; and enabling the LED string according to the digital signal, and adjusting the brightness of the backlight unit according to the enabling ratio of the LED strings.

In an embodiment of the invention, the driving method further includes the following steps: when a control signal is enabled, respectively adjusting the on-current value of the LED string according to the digital signal.

The invention adopts a digital method to perform dimming of the LED backlight unit, and adjusts the average brightness of the backlight unit as a whole by using the enabling ratio of the LED string. Therefore, the LED in the backlight unit does not need to perform continuous light-dark switching according to the PWM signal, so that the equivalent load of the LED is relatively stable, and the voltage conversion circuit is prevented from entering the DCM mode to reduce the power consumption of the voltage conversion circuit. At the same time, the invention can cooperate with the switching of the driving mode, and set the driving current of the individual LED strings by the digital signal, so that the dimming function of the backlight unit is more flexible.

The above described features and advantages of the present invention will be more apparent from the following description.

FIG. 5A is a circuit diagram of an LED driving circuit according to an embodiment of the invention. The driving circuit 500 is used to drive the backlight unit 540, which includes a voltage conversion circuit 510, a driving unit 520, and a digital dimming unit 530. The backlight unit 540 includes a plurality of LED strings (not shown), and each of the LED strings includes at least one LED. The voltage conversion unit 510 is coupled to the backlight unit 540 and is mainly used for converting the level of the DC voltage, such as a DC-DC converter such as a boost circuit or a Buck circuit. The input voltage V _{IN is} converted to a driving voltage required by the backlight unit 540. The driving unit 520 is coupled between the other end of the backlight unit 540 and the digital dimming unit 530. In general, the driving unit 520 is mainly used to adjust the on-current of the LED and perform functions such as dimming, such as Dimming, which can be replaced by the driver chip at present, so the internal structure is not described here.

In this embodiment, the digital dimming unit 530 outputs a digital signal to the driving unit 520 according to the required backlight brightness. The driving unit 520 temporarily stores the received digital signal in a register (not shown), and then controls whether the LED string is enabled or not according to the digital signal to adjust the brightness of the backlight unit, and the brightness of the backlight unit 540 is adjusted. Corresponds to the enabling ratio of the LED string. The driving unit 520 generally enables the LED string by controlling the current conduction path of the LED string. When the current path of the LED string is turned on, the brightness of the LED string depends on the current value of the LED string. In the present embodiment, the brightness of the backlight unit 540 is adjusted by the enabling ratio of the LED string. In other words, when the required brightness is 50% of the maximum brightness, then half of the LED strings are enabled in turn, and the more the number of enabled LED strings, the stronger the brightness of the backlight unit 540.

In order to make the brightness of the backlight unit 540 uniform, the LED string can be enabled in a positional interval according to the position of the LED string. For example, the odd string can be enabled first, then the even string or the first half of the LED string can be enabled. Re-enable the LED string in the second half. In addition to setting the enabling order according to the set position of the LED string, the brightness of the backlight unit 540 can also be adjusted in a time-interval manner to make it more uniform (for example, changing the energy every time a predetermined time interval is changed) The LED strings, and the enabled LED strings, are evenly distributed throughout the backlight unit 540). In addition to the above-described adjustment methods, those skilled in the art should be able to easily infer the remaining enabling modes after the disclosure of the present invention, and will not be described herein.

When the brightness of the backlight unit 540 does not need to be changed, the LED string does not need to be changed. The brightness and dark state is changed, so that the PWM signal is used as the dimming mode in the prior art, and the equivalent load of the LED string of the present invention can be relatively stable. Therefore, the output waveform of the voltage conversion circuit 510 is not affected by the change in the LED string load and enters the DCM mode. Fig. 5B is an output waveform diagram of the voltage conversion circuit according to the embodiment. As can be seen from FIG. 5B, the output waveform of the voltage conversion circuit 510 of the present embodiment can fall in a continuous conduction state (CCM mode), which effectively saves power consumption.

Figure 6 is a diagram of a drive circuit in accordance with another embodiment of the present invention. FIG. 6 is a circuit diagram showing the details of the embodiment of FIG. 5. Wherein the backlight unit 540 includes an LED string L ₁ ~ L _n, digital dimming unit 530 outputs the digital signal SIN, displacement clock signal SCLK, XLAT latch signal and a control signal to the driving unit 520 MODE. The digital signal SIN is transmitted to the driving unit 520 in conjunction with the displacement clock signal SCLK, as shown in FIG. 7, which is a signal waveform diagram according to the present embodiment. The digital signal SIN transmits the bit data bits (0) to bit(n) to the register in the driving unit 520 along with the clocks 1~n of the shift clock signal SCLK. Where bit(0) is the most significant bit (MSB), bit(n) is the least significant bit (LSB), and n is a positive integer, which is used to indicate the total number of LED strings. It can also be used as an index value for LED strings, clocks, and bit data.

Please refer to FIG. 8 at the same time. FIG. 8 is a schematic diagram of the data temporary storage sequence according to the embodiment. As shown in FIG. 8, the bit data bit(0)~bit(n) of the digital signal SIN is temporarily stored in the register in the driving unit 520 with the clocks 1~n of the shift clock signal SCLK. When all the bit data has been transferred, the shackle signal XLAT will be enabled (logic high), and the drive unit 520 will lock all the bit data bit(0)~bit(n) according to the bit data bit ( 0) ~bit(n) "0" and "1" signal selective enable LED strings L ₁ ~ L _n .

For example, the bit data bit(0)~bit(n) corresponds to the LED string L ₁ ~L _{n respectively} , and when the corresponding bit data bit(0)~bit(n) is "1", the corresponding LED string L ₁ L _N will be enabled (bright). When the bit data bit(0)~bit(n) is "0", the corresponding LED string L ₁ ~L _n will be disabled (dark). Next, a 50% luminance places an example will be described, referring to FIG 9, FIG. 9 is a bit of the information bit according to the present embodiment in Example (0) ~ bit (n) with the LED string L ₁ ~ L _n according to the control of FIG. Figure 9 shows the data types of the four bit data bits (0)~bit(n) to indicate four different dimming arrangements, all of which indicate that half of the LED strings L ₁ ~L _n are bright, half of the LEDs string L ₁ ~ L _n is dark only difference that can be caused by LED string L ₁ ~ L _n different positions. In the first type, the first half of the LED strings L ₁ to L _n are bright, the second half of the LED strings L ₁ to L _n are dark, and the second half is reversed. The third type is the way of light and dark interlacing, and the fourth type is the opposite. Those having ordinary knowledge in the art should be able to infer the remaining feasible bit data bit(0)~bit(n), which are not described here.

In addition, it should be noted that, in this embodiment, the driving unit 520 can not only individually enable the LED strings L ₁ ~L _n according to the digital signal SIN, but also individually adjust the current flowing through the LED string, and switch the mode thereof. It is then controlled by the control signal MODE. Referring to FIG. 7, when the control signal MODE is at a logic high level, the driving unit 520 controls whether the LED strings L ₁ L L _n are enabled or not according to the digital signal SIN. When the control signal MODE is enabled (logic low in this embodiment), the driving unit 520 adjusts the magnitude of the flow through the LED strings L ₁ to L _n according to the digital signal SIN. Accordingly, in this embodiment, not only the number of enable strings of the LED strings L ₁ to L _n can be used for dimming, but also the on-current of the individual LED strings L ₁ to L _n can be adjusted to perform fine dimming. The dimming function of the embodiment is more flexible.

The manner in which the ON currents of the LED strings L ₁ to L _n are adjusted by the digital signal SIN is, for example, as follows. In the present embodiment, the current of each LED string is divided into 64 levels by the bit data of 6 bits (0). ~63), its control method is as follows:

Wherein, I _OUT is the on current of the LED string, I _MAX is the maximum current set by the driving unit 520, and DC _n is the selection mode (corresponding to the current level 0 to 63), which is determined by the corresponding bit data, and n represents the corresponding LED string (or number of LED channels). The I _MAX can be determined by the resistance R _EXT and the parameters of the LED driving chip. The resistor R _{EXT is} coupled between the driving unit 520 and the ground GND, and is usually an adjusting resistor for driving the wafer to adjust the constant current of the LED string. Since I _MAX is related to the current flowing through the resistor R _EXT , I _MAX can be expressed as above, where _{VR_EXT} represents the voltage difference across the resistor R _EXT , and Factor is determined by the driving unit 520, and different driving The wafer will have different settings.

Through the switching of the control signal MODE, the digital signal SIN of the embodiment can be used not only to control the enable or disable of the LED string, but also to respectively use Adjust the on-current value that the LED string turns on. In combination with the above two driving modes, the embodiment can more conveniently adjust the required backlight brightness and effectively reduce the power consumption of the voltage conversion circuit.

From another point of view, the above embodiment can be summarized as a driving method of a backlight unit. Referring to FIG. 10, FIG. 10 is a flowchart of a driving method according to another embodiment of the present invention. The backlight unit includes a plurality of LED strings. The driving method includes the following steps: first, temporarily storing a digital signal according to a displacement clock signal (step S101); and then puncturing the received digital signal according to a shackle signal (step S102); then controlling whether the LED string is enabled or not according to the digital signal to adjust the brightness of the backlight unit (step S103).

In another embodiment of the present invention, the driving method further includes the step of: adjusting a turn-on current value of the LED string according to the digital signal when a control signal is enabled. Those skilled in the art will be able to easily infer the remaining details of the driving method of the present embodiment through the description of the above embodiments, and will not be described here.

In summary, the present invention replaces the dimming mode of the conventional LED backlight in a digital manner to prevent the output waveform of the voltage conversion circuit from entering the DCM mode, thereby reducing its power consumption. In addition, the present invention utilizes the enabling ratio of the LED string to perform dimming, and reduces the current variation caused by the switching of the LED string due to light and dark switching, so that the color temperature and the chromaticity are more stable.

Although the present invention has been disclosed in the above preferred embodiments, it is not intended to limit the invention, and any one of ordinary skill in the art can make some modifications and refinements without departing from the spirit and scope of the invention. , Therefore, the scope of the invention is defined by the scope of the appended claims.

110‧‧‧Voltage conversion circuit

120‧‧‧ drive unit

130‧‧‧Backlight unit

500‧‧‧ drive circuit

510‧‧‧Voltage conversion circuit

520‧‧‧ drive unit

530‧‧‧Digital dimming unit

540‧‧‧Backlight unit

R _EXT ‧‧‧resistance

V _CTRL ‧‧‧ dimming control signal

T‧‧‧ PWM signal period

During T1, T2‧‧

ON‧‧‧LED enabled

OFF‧‧‧LED disability

V _IN ‧‧‧ input voltage

SIN‧‧‧ digital signal

SCLK‧‧‧displacement clock signal

XLAT‧‧ 拴 拴 lock signal

MODE‧‧‧ control signal

L ₁ ~L _n ‧‧‧LED string

Bit(0)~bit(n)‧‧‧ bit data

MSB‧‧‧ most significant bit

LSB‧‧‧ least significant bit

1~n‧‧‧Shift clock signal SCLK clock index value

GND‧‧‧ Grounding

S101~S103‧‧‧Steps

FIG. 1 is a circuit diagram of an LED backlight driving circuit according to the prior art.

2 is a circuit diagram of an LED backlight driving circuit according to a frequency pulse modulation method according to the prior art.

3 is a comparison diagram of PWM signal and LED enable according to the prior art.

4 is a current waveform diagram of a voltage conversion circuit according to the prior art.

FIG. 5A is a circuit diagram of an LED driving circuit according to an embodiment of the invention.

Fig. 5B is an output waveform diagram of the voltage conversion circuit according to the embodiment.

Figure 6 is a diagram of a drive circuit in accordance with another embodiment of the present invention.

Fig. 7 is a signal waveform diagram according to the present embodiment.

FIG. 8 is a schematic diagram showing the timing of data temporary storage according to the embodiment.

FIG. 9 is a comparison diagram of the bit material bits (0) to bit (n) and the LED strings L ₁ to L _n according to the present embodiment.

FIG. 10 is a flow chart of a driving method according to another embodiment of the present invention.

510‧‧‧Voltage conversion circuit

520‧‧‧ drive unit

530‧‧‧Digital dimming unit

540‧‧‧Backlight unit

R _EXT ‧‧‧resistance

V _IN ‧‧‧ input voltage

SIN‧‧‧ digital signal

SCLK‧‧‧displacement clock signal

XLAT‧‧ 拴 拴 lock signal

MODE‧‧‧ control signal

L ₁ ~L _n ‧‧‧LED string

GND‧‧‧ Grounding

Claims (6)

A driving circuit is adapted to drive a backlight unit, the backlight unit includes a plurality of LED strings, the driving circuit includes: a voltage conversion circuit for providing a driving voltage to the backlight unit; and a driving unit coupled The other end of the backlight unit is configured to control an on current of the LED strings; and a digital dimming unit outputs a digital signal to the driving circuit; wherein the driving circuit is correspondingly enabled according to the digital signal The light emitting diode strings are adjusted according to the enabling ratio of the light emitting diode strings, and the digital dimming unit further outputs a displacement clock signal and a latch signal to the driving unit. The driving unit temporarily stores the digital signal according to the displacement clock signal, and locks the received digital signal according to the shackle signal. The driving circuit of claim 1, wherein the voltage converting circuit is a boosting circuit. The driving circuit of claim 1, wherein the driving unit further comprises a temporary register for temporarily storing the digital signal. The driving circuit of claim 1, wherein the digital dimming unit further outputs a control signal to the driving unit, and when the control signal is enabled, the driving unit respectively adjusts the two light emitting signals according to the digital signal. The on current value of the polar body string. A driving method of a backlight unit, the backlight unit includes a plurality of LED strings, and the driving method includes: Temporarily storing a digital signal according to a displacement clock signal; latching the digital signal received according to a shackle signal; and correspondingly enabling the light emitting diode strings according to the digital signal, and according to the light emitting diodes The enabling ratio of the body string adjusts the brightness of the backlight unit. The driving method of claim 5, further comprising: adjusting a conduction current value of the LED strings according to the digital signal when a control signal is enabled.