Pushbuttons and digital potentiometer control boost converter

Digitally controlled potentiometers are useful for generating analog control voltages under the control of a microcontroller. In some applications, manual pushbutton switches could replace a microcontroller and simplify product design. Mechanical switches exhibit contact bounce, and, when a user actuates them, they may open and close many times before reaching a stable state. A digital potentiometer's control inputs lack switch-debouncing capabilities, and its up/down control is not suited for pushbutton operation. Figure 1 illustrates solutions to these issues and shows how to use a digital potentiometer to control a boost converter.

The potentiometer, IC₁, a MAX5160M, presents an end-to-end resistance of 100 kΩ. To increment the wiper's position, W, you press and hold the  pushbutton, S₂, to pull the  pin high and then press and release pushbutton S₁ to pulse the input. Similarly, you decrement the wiper position by releasing S₂ and pulsing S₁.

A time-delay network comprising R₁, R₂, and C₁ masks S₁'s switch bounce, which would otherwise toggle the wiper's position between V_DD and approximately 0V. When you press S₁, capacitor C₁ charges via R₂ and causes the pin to ramp slowly toward 0V, thereby removing the effects of S₁'s contact bounce. The R₁C₁ time constant requires that you depress S₁ for several milliseconds before the INC input takes effect.

In this application, switching converter IC_2,, a MAX1771, operates as a standard boost converter and increases its 5V input to a higher voltage positive output. You can use Equation 1 to set IC₂'s output to a nominal 12V output without the digital potentiometer:

 EQUATION 1

Connecting IC₁'s wiper via 10-kΩ resistor R₅ to IC₂'s FB (feedback) node sets IC₂'s voltage-feedback level. Although inclusion of feedback resistors R₃, R₄, and R₅ and digital potentiometer IC₁ complicates the precise calculation of IC₂'s output voltage, you can simplify the math by calculating the output voltages at the potentiometer's extreme settings. Thus, with IC₁'s wiper set to 0V, R; equals the paralleled resistance of R₄ and R₅, and IC₂'s maximum output voltage becomes Equation 2:

  EQUATION 2

or V_MAX=16.84V.

With IC₁'s wiper set to 5V, you can attempt to calculate the minimum output voltage by summing voltages into the feedback node:

  EQUATION 3

which simplifies to V_MIN=0.48V. However, Equation 3 provides an incorrect value for V_MIN because a boost converter's output voltage cannot go below its input voltage. You can approximate V_MIN by substituting a value of 10 kΩ for R₅ in Equation 3 and solving for V_MIN: V_MIN=4.93V. Refer to the manufacturer's application notes for additional component information.

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