This design idea appeared in the October 30, 2006 issue of Portable Design
The design of a handheld device today requires that you simplify and reduce the controls to a minimum. A circuit that enables a single pushbutton to turn power on and off can be very useful. Figure 1
shows a single-pushbutton power-control circuit that consists of only a few small components, and consumes little power.
The normally open (NO) single-contact pushbutton connects to a debouncer (the MAX6816
, IC1) that guarantees a single-output edge (rise or fall) each time the button is pushed or released. The MAX6816's output drives the clock input of a flip-flop (IC2) wired as a T-type (toggle) flip-flop. The flip-flop output, in turn, controls a low-dropout (LDO) linear regulator (the MAX6484
, IC3). The MAX6484 powers a handheld gadget, which turns on or off each time that the pushbutton is operated. The same circuit can also drive other types of power regulator, if the regulator features a logic-level power-management input.
Figure 1. This normally open pushbutton connects to a debouncer, the MAX6816 (IC1) and IC2. This single-pushbutton ON/OFF control circuit lets you toggle the PWR OUT voltage by controlling the MAX6484 LDO (IC3).
The 1MΩ resistor and 100nF capacitor connected to the flip-flop's CLR input ensure that the circuit always powers up in the same state (OFF) when PWR_IN is connected first. The ON/OFF control circuit (pushbutton, debouncer, and flip-flop) operates between 2V and 5.5V; it draws about 3.5µA when the supply voltage is 3.5V. The MAX6816 is available in a 4-pin SOT143 package, IC2 in an MO-187 package, and the MAX6484 in a 6-ball UCSP™ package.