LCM Power Supply Design:
In the liquid crystal display module (LCM), VLED+, VLED-, VCC, AVDD, VGL, and VGH are key power pins that provide different voltages of power support for different parts of the display panel. Their specific meanings are as follows:
VCC:
Meaning: Logic supply voltage / interface supply voltage / digital core voltage.
Function: Power the digital logic circuits on the LCM, including the core logic of the display controller (Driver IC), the timing controller (which may be integrated in the Driver IC), and the interface circuits (such as SPI, I2C, RGB, MIPI DSI, etc.) that communicate with the external host controller (such as MCU, CPU).
AVDD (or VDD, VDA, VDDA, AVCC, Source Driver Power):
Meaning: Analog supply voltage / source driver supply voltage / Gamma reference voltage power supply.
Function: Provide power for the source driver that drives the liquid crystal pixels. The source driver is responsible for generating accurate grayscale voltages (through the Gamma circuit) based on the image data and applying them to the column lines of the panel. AVDD is the reference for these grayscale voltages, and its voltage range directly affects the maximum voltage difference across the liquid crystal unit, thereby affecting the contrast and display effect.
VLED+ (or LED+, LEDA, LED Anode):
Meaning: LED backlight positive supply voltage.
Function: This is the positive power input of the LED backlight. It provides a high potential end for the current path of the LED string in series or parallel/series-parallel combination.
VLED- (or LED-, LEDK, LED Cathode):
Meaning: LED backlight cathode.
Function: This is the current return or cathode of the LED backlight. In most designs, it is connected to the current output of the LED backlight driver chip (usually the current detection resistor or the source/emitter of the power switch tube). The driver chip achieves precise control of the LED current by controlling the current path of the VLED- terminal.
VGL :
Meaning: Gate Off Voltage (negative voltage).
Function: In TFT-LCD, it is used to turn off the TFT switching transistor for each row of pixels. It is a negative voltage (relative to VCOM - common electrode voltage). Applying enough negative voltage to the gate line (through the gate driver) ensures that the TFT is fully turned off, preventing charge leakage on the pixel capacitor, thereby keeping the pixel voltage stable and avoiding image retention or crosstalk.
VGH (or VON, VGHP, Gate On Voltage):
Meaning: Gate On Voltage (positive voltage).
Function: In TFT-LCD, it is used to turn on the TFT switching transistor for each row of pixels. It is a relatively high positive voltage (much higher than VCC and AVDD). Applying this high voltage to the gate line (through the gate driver) ensures that the TFT is fully turned on, so that the pixel voltage generated by the source driver can be quickly and accurately written to the pixel capacitor.
The VGH and VGL signals play an important role in the driving process of the LCD display. By accurately controlling these two voltages, high-quality image display effects can be achieved.
In the power supply of the TFT liquid crystal screen driving circuit, the VGH voltage and VGL voltage are responsible for opening the TFT (thin film field effect transistor) to charge the capacitor (correct the voltage across the capacitor) and closing the TFT to keep the capacitor voltage (one cycle time). If there is a problem with the VGH and VGL voltages, the voltage is lost or the voltage amplitude changes, it will cause image failure and many failure phenomena. Because the circuit that generates the VGH and VGL voltages is relatively special, there are many components and the voltages restrain each other.
Taking the input voltage of 5V and the screen: TM070RDH10 as an example, the LCM power supply is designed:
VLED: LCM backlight power supply. The LCD itself does not emit light. If you want it to display the picture, you must use a white light backlight source. The common white light backlight source is generally composed of several white LED lamps.
As shown in Figure 1, you can see that the backlight of the LCM is a multi-level LED series and parallel connection.
figure 1
There are two main parameters to pay attention to when designing a backlight power supply (Figure 2)：
figure 2
Forward voltage Vf and forward current If, when designing the circuit, the circuit should be designed according to the description in the manual. VLED is the power pin that provides the operating voltage for the LED backlight string, and its voltage value depends on the number of LEDs in series and the forward voltage drop (Vf) of a single LED.
When designing the backlight power supply, a constant current drive method should be used, which is mainly determined by the volt-ampere characteristics of the diode.
Backlight power supplies usually use switching LED drivers.
Taking AP3031 as an example, the design of LED driver circuits is introduced. AP3031 is an inductor-based DC/DC boost converter designed to drive LED arrays. The 1.4A switching current allows AP3031 to be used for different 7' to 10' LCD panel backlights (usually 3*13 LED arrays). The IC uses a constant frequency 1MHz PWM control scheme. The fixed frequency is about 1MHz to adjust the LED current, and it is fed back to the chip through an external current detection resistor to adjust the output current; the low feedback voltage is 200mV, which is also the biggest difference between LED drivers and traditional BOOST boost chips. The low feedback voltage can minimize power consumption (not explained here).
The six pins of AP3031 are shown in the figure (Figure 3):
figure3
The following figure is a circuit diagram with a current of 160ma (Figure 4):
figure 4
It can be seen that the use of this chip is similar to that of the boost chip. Among them, VCC=5V, connected to a 0R resistor, input capacitor 22uf, R5 is the enable pin, and the 4th pin of AP3031 is connected to the PWM pin at the same time, which is convenient for using PWM to adjust the backlight. R6 is the feedback resistor, and the maximum operating current is calculated through R6. The calculation formula is I=Vfb/R, where I is the output current and Vfb is the feedback voltage. From the datasheet (Figure 5), we can see that Vfb=200mv and R is the feedback resistor.
figure 5
AVDD: Used to provide a stable power supply voltage for the analog circuit part of the LCD screen, such as the reference voltage of the source driver and the gamma correction circuit.
According to the sample screen datasheet (Figure 6), we can see that the typical value of AVDD is 10.4V.
figure 6
Use a boost circuit and MP1541 as the boost chip. The six pins of MP1541 are shown in Figure 7:
figure 7
The circuit diagram is as follows (Figure 8):
figure 8
Among them, VCC=5V, connected to 0R resistor, input capacitor 22uf, R6 is the enable pin, R3, R4 are feedback resistors, and the output voltage calculation formula is shown in Figure 9:
figure 9
Calculation shows that AVDD = 10.3V.
VGL and VGH: VGH voltage is responsible for charging the TFT gate capacitor and keeping the capacitor voltage for one field cycle, while VGL voltage is responsible for closing the TFT gate. From the datasheet, VGL is required to be -7V and VGH is required to be +16V. The circuit principle is as follows (Figure 10):
figure 10
Among them, the SW pin can be regarded as a square wave signal. When SW is turned on to the ground, the level on the side of C2 connected to the diode is AVDD, SW=GND, and the voltage difference on both sides is AVDD. When SW increases, it can be seen from the characteristic that the voltage across the capacitor cannot change suddenly that the level on the side of C2 connected to the diode is AVDD+Vsw. At this time, the voltage difference across C3 is AVDD+Vsw, and VGH is obtained through the voltage regulator diode, and the same is true for VGL.