Using MAX5438 and MAX749 to adjust LCD contrast

AdjusTIng the Contrast of LCD Display with MAX5438 and M AX749

WAN Li, YU Dan, CHENG Yuan, CHEN Guangdong

(Department of Control Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China)

Key words: negaTIve voltages; LCD; adjusTIng of contrast

1 Digital potentiometer adjustment This is a method that uses a digital potentiometer to change the fixed negative bias to an adjustable negative bias to adjust the contrast. The following uses the widely used MAX5438 as an example to introduce a practical contrast negative bias adjustment scheme.
1.1 MAX5438 chip introduction MAX5438 is a 7-bit 128-step digital potentiometer produced by MAXIM. The internal structure is shown in Figure 1. It consists of a 7-bit shift register, a 7-bit data latch, a decoding module, a potentiometer, and control clock logic. The control signal of MAX5438 includes 3 input signals: chip select ( ), Data input (DIN) and serial clock (SCLK), digital logic is driven by a voltage of + 2.7V ~ + 5.25V. The voltage across its internal resistance is provided externally by VDD and VSS. The voltage between VDD and VSS must be greater than 9V and less than 31.5V. This makes the MAX5838 have great flexibility in the choice of input voltage. At the same time, the external circuit of the MAX5438 is very simple, and almost no external components are required to realize the output regulation of the input voltage.

Fall, and then simulate the clock signal on the SCLK pin. On the rising edge of each clock, a bit of data is read into the serial shift register from the DIN pin. 8-bit data requires 8 clock pulses to complete the input. It should be noted that the data bit D7 is meaningless, so the real MSB is read in at the rising edge of the second pulse. After all data bits are entered, Should change from low to high, at this time the data is latched into the potentiometer control register, the position of the resistance slider W changes with the input data through the decoding circuit. The timing is shown in Figure 2.

Use MAX5438S to realize the hardware circuit with adjustable negative pressure. The fixed negative bias voltage is provided by the MAX202 chip, and the three I / O pins of the microcontroller are connected to the three control signals of the MAX5438.
Since there is no EPROM inside the MAX5438, the position of the slider cannot be recorded, so at the beginning of the program, you must write a data to it as the starting value for later adjustment. This can be selected according to the actual situation of the circuit as a suitable value for the liquid crystal used as the initial value, which can greatly reduce the number of contrast adjustments. The starting value ranges from 00 to 7F, and these two values ​​correspond to the minimum and maximum output voltage, respectively. In the actual circuit shown in Figure 2, the input voltage of the MAX5438 is -12V provided by the MAX202, and the output voltage range is -9V to 0V. Due to the 128-step adjustment capability, the minimum value of the output voltage change is 9/128 or about 0.1 V. For most liquid crystals, the negative bias voltage displays better at -8V to -9V, so the initial value is between 02 and 06. After entering the contrast adjustment menu, increase or decrease the initial value by pressing the key. Each time the key is pressed, the new value is sent to the MAX5438 to change its output voltage.

This is a method of generating an adjustable negative bias with a DC-DC converter to adjust the contrast. This method is applicable to the case where there is no available negative bias voltage in the system circuit or the negative bias voltage cannot meet the needs. The following uses MAX749 as an example to introduce its general use method.
2.1 Functional Description of MAX749 MAX749 is an inverted PFM switching regulator. Input voltage from + 2V to + 6V can generate negative LCD bias voltage. The output voltage can reach above -100V, which can be adjusted by internal digital-to-analog converter, or by a PWM signal or potentiometer. The MAX749 uses a unique current control scheme that reduces quiescent current and improves efficiency. In shutdown mode, the quiescent current is only 15mA. The MAX749 maintains the set value of the DAC in the shutdown mode, simplifying software control. When the MAX749 generates a negative voltage, the external circuit can be driven by a P-channel MOSFET or a PNP transistor. The input voltage can be adjusted digitally or by a potentiometer. Its internal structure is shown in Figure 4.

Driving with MOSFET can output larger current and voltage, and the conversion efficiency is also higher. The circuit is shown in Figure 5.

After the initial power-on or reset, the counter is reset to an intermediate value of 32, and the output VOUT = RFB × 13.3μA. After that, every time ADJ comes with a rising pulse, the counter will increase by 1, and the output voltage will increase by RFB × 0.208μA. When the pulse is reset, the counter is reset to zero, and the output voltage becomes the minimum, VOUT = RFB × 6.67μA, and then the output voltage of the ADJ is increased by RFB × 0.208μA. Since the counter can only increase by itself, that is, the value of the output voltage can only be gradually reduced with the pulse of the ADJ, which brings inconvenience to the contrast adjustment. In order to solve this problem, we need to design from both hardware and software.
On the hardware, an RFB with an appropriate resistance value should be selected according to the liquid crystal used, so that the counter can provide a negative pressure greater than that required by the liquid crystal at the intermediate value of 32, but the RFB cannot be too large, otherwise RFB × 0.20 8μA will increase the accuracy of contrast adjustment accordingly. On the software, increase or decrease the number of pulses to be sent to the MAX 74 9 by pressing the button. Each time the key is pressed, a signal is sent to reset the MAX749, and then a corresponding pulse is sent to the ADJ through the pin of the microcontroller. In this way, in actual adjustment, the output voltage of the counter at 32 can be used as the starting point. The more the number of pulses, the lower the output negative pressure value, and the less the number of pulses, the more the output negative pressure value. High, to adjust the contrast of the LCD.