Using digital potentiometer to realize key hold control

Overview

Using a microcontroller , you can write a program to suppress the jitter of the input control signal and extend the delay before the switch


is closed to prevent misoperation. However, using a microprocessor to implement a good user control interface requires a lengthy programming and verification process. This article uses a digital potentiometer to design a simple hardware solution that can ensure that the system can be adjusted when necessary. Figure 1. This design uses an extended delay manual reset device and a 32-tap nonvolatile digital potentiometer to implement a key-hold switch for system adjustment operations.

Implementation of key hold control

Figure 1 is the schematic diagram of the design in this article. Users can achieve increase/decrease control through a user-friendly interface. In the design, an extended delay manual reset device (U1, MAX6343) is selected to avoid the influence of key operation errors, and a 32-tap nonvolatile digital potentiometer (U2, MAX5471) is selected to increase/decrease VADJ. VADJ can be used for feedback loop control of power supply or backlight converter, and the data can be read directly through the A/D converter.

For the user interface, switch S2 first sets the increase or decrease control command, and the key holding switch S1 starts the control process. The

MAX6343's /MR input requires a 6.7s setup time to confirm that a valid /MR signal is detected. Therefore, after the user presses switch S1 and holds it for 6.7s, /RESET goes low.

The change from high to low on the MAX5471 /INC pin causes the digital potentiometer variable resistance output value to increase or decrease by 1/32 (because the MAX5471 has nonvolatile memory, it can retain the setting data in its memory even when there is no power).

To prevent the user from accidentally touching S1, transistor Q1 is added to reset the MAX6343's 6.7s timer after each reset or increase/decrease control command. The user can control S1 in this way until the required setting voltage (VADJ) is reached. This design can perform increase/decrease control every 6.7s. Because the MAX6343's /MR input has a 50kΩ pull-up resistor, Q1 can choose a general-purpose NPN tube similar to MMBT3904, and select a 200kΩ R1 resistor to ensure that the transistor is in saturation when /RESET is high.

Figure 1 shows a typical feedback network, with resistors R3 and R4 connected in series with the variable resistor MAX5471 (RADJ). The choice of value depends on the actual application; if R3 = R4 = 200kΩ, VADJ can be calculated as follows:

VADJ = ((RADJ + R4)/(R3 + R4 + RADJ)) x 3.3V (Equation 1)

When RADJ = 0Ω (minimum setting): VADJ = 1.65V

When RADJ = 50kΩ (maximum setting): VADJ = 1.83V

Therefore, (1.83 - 1.65)/32 (steps) = 5.7mV/step

in conclusion

Using the circuit provided in this application note , the system control can be prevented from being misoperated through a simple hardware solution. The user only needs to select the increase/decrease control, press and hold switch S1, and can slowly make system adjustments with a good user interface.