Electronic Design

Micropower Circuit Offers Automatic Shutdown And Low-Battery Lockout

This micropower circuit provides shutdown, power-up, and low-battery lockout functions automatically, without the need for software or operator control (Fig. 1). Featuring 2.3-V hysteresis, the circuit eliminates the use of microprocessor I/O pins for charger detection, battery-threshold monitoring, and shutdown control. Under full load, the complete power-management circuit draws less than 200 µA of supply current.

After the cutoff point for battery discharge is reached, the circuit locks out (disconnects) the battery from the load (Load1). This action prevents circuit "chatter" and deep discharge until the battery is in its charger cradle. A common and annoying problem with battery-voltage monitors, chatter is the circuit's response to fluctuations in battery-terminal voltage. These fluctuations take place when the recurring load connections toggle the battery between its discharge and relaxed-open-circuit conditions.

For a three-cell NiCd battery, the typical terminal voltage is 4.9 V fully charged, 3.6 V under load, and 2.5 V at discharge. The load should be disconnected at 2.5 V, but not reconnected as the resulting terminal voltage (VBATT) floats to the open-circuit condition. Figure 1's circuit disconnects the load at 2.5 V and reconnects it while the battery is in its charger cradle (4.68 V).

An ultra-low-power micropro-cessor(µP)-reset device (U1) generates an active-low output (—RSTA) whenever VBATT equals 4.63 V. The output of a micropower latching comparator (U2) causes —RSTB to transition low if VBATT is less than 2.5 V. These outputs from U1 and U2 are ORed by diodes D1 and D2, generating a system-shutdown control (—RSTC). Figure 2 demonstrates the relationship between the various reset states and the battery-discharge profile.

When VBATT is less than 2.5 V, —RSTC disconnects Load1 from the battery by shutting down U3. U3 is a low-noise, low-dropout, linear regulator in an SOT-23 package. It has a preset output of 2.5 V, a maximum ground-pin current of 180 µA (when supplying 150 mA), and a supply current of only 10 nA during shutdown.

If VBATT is greater than 4.63 V, the circuit releases —RSTC. As a result, U3 comes out of shutdown and delivers power to the µP. Another µP-supervisor (U4) holds the —RSTD signal low until the microprocessor's VCC exceeds 2.2 V. Once —RSTD is released, the µP begins operating and clears U2 by pulsing its CLR input high for 1 µs.

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