This article introduces a complete solution for simple and efficient control of voltages up to 20 V using a push-button digital potentiometer. This complete solution provides an adjustable power supply that can be used in various applications that require an adjustable voltage output. Figure 1 shows the corresponding switching regulator with variable output power, using the AD5116 digital potentiometer and ADCMP371 comparator with an integrated push-pull output stage. By adding a switch instead of a button, a microcontroller can be used to adjust the voltage.

This article introduces a complete solution for simple and efficient control of voltages up to 20 V using a push-button digital potentiometer. This complete solution provides an adjustable power supply that can be used in various applications that require an adjustable voltage output. Figure 1 shows the corresponding switching regulator with variable output power, using the AD5116 digital potentiometer and ADCMP371 comparator with an integrated push-pull output stage. By adding a switch instead of a button, a microcontroller can be used to adjust the voltage.

AD5116 has 64 usable cursor positions, and the tolerance of end-to-end resistance is ±8%. In addition, AD5116 contains an EEPROM to store the cursor position, which can be set manually by pressing the button. This function is very useful for applications that require a fixed standard power-up voltage.

The circuit is powered by the voltage VIN, which can reach 20 V. The power supply voltage VDD of AD5116 and ADCMP371 can also be generated by VIN, for example, through a voltage regulator such as ADP121.

Use digital potentiometer to generate adjustable voltage output

Figure 1. A high-voltage switching regulator with variable output and push-button control.

The working principle of the circuit

The output voltage VOUT is controlled by the switching frequency of the feedback network. It is fed back to the comparator through the voltage divider, and then compared with the reference voltage set by the digital potentiometer. If the voltage obtained from VOUT is higher than the reference voltage, the comparator output switches to a low level to block the NMOS Transistor T1 and the PMOS transistor T2, thereby reducing VOUT. If the voltage taken from VOUT is lower than the reference voltage, the comparator output switches to a high level, and the two transistors switch to the on state (saturation), thereby increasing VOUT. Through this comparison-based function, the transistors operate in short pulses in the on/off mode, keeping the loss of each transistor low. In addition to the output voltage of the potentiometer, the switching frequency is also affected by the load of VOUT.

As the output voltage of the digital-to-analog converter (DAC) increases, the turn-off time of T2 becomes longer, and the output of the comparator increases accordingly. The comparator output provides a series of higher frequency and faster positive power output pulses. If the DAC output voltage drops, the situation is reversed.

The filtered VOUT is determined by Equation 1.

VW is the DAC output voltage at the tap W of the potentiometer.

The nominal value of the resistance between tap A and tap B of AD5116 is 5 kΩ, which is divided into 64 steps. At the lower end of the range, the typical wiper resistance RW drops to between 45 Ω and 70 Ω. The VW output voltage relative to GND is:

Where RWB is:

RWB is the resistance value between the wiper W and the lower end GND.

RAB is the total resistance of the potentiometer.

VA is the voltage at the top of the voltage divider string; in this example, it is equal to VDD.

D is the decimal equivalent of the binary code in the RDAC register of AD5116.

The RDAC register of AD5116 is controlled by the buttons PD and PU. The default power-on position (for example, VOUT = 0 V) ​​can be stored in the EEPROM of the potentiometer through the ASE pin.

Filter output: reduce ripple

In order to obtain a smooth output voltage VOUT and reduce the ripple caused by the switches T1 and T2, an additional filter circuit is required (see Figure 2). When designing this filter, need to consider AD5116 and switching frequency, as well as its operating voltage range.

For the circuit shown in Figure 2, the switching frequency range is approximately 1.8 Hz to 500 Hz. Because this value is quite low, it is usually necessary to use larger R, L, and C values ​​when determining the cutoff frequency of the filter. However, the series resistance of the filter and the output load form a voltage divider, which reduces the output voltage. Therefore, when choosing the R value, you should choose a relatively low value.

Figure 2. Filter circuit for smoothing the output voltage.

AD5116

Nominal resistance tolerance error: ±8% (value)

Cursor current: ±6 mA

Temperature coefficient in variable resistor mode: 35 ppm/°C

Low power consumption: 2.5 μA (value, 2.7 V, 125°C)

Wide bandwidth: 4 MHz (5 kΩ option)

Power-on EEPROM refresh time:

Typical data retention period at 125°C: 50 years

1 million write cycles

2.3 V to 5.5 V power supply

Built-in adaptive debouncer

Wide operating temperature range: -40°C to +125°C

2 mm × 2 mm × 0.55 mm, 8-pin ultra-thin LFCSP package

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