The circuit in Fig 1 supports portable applications in which a microprocessor controls battery charging. Step-up switching regulator IC₁ boosts V_IN (5V) to supply a combination of charging and load current. (Note that the 5V source must have short-circuit protection.) IC₂ is a high-side current-sense amplifier that monitors charging current. Commands from the processor are CHARGE ON /OFF and FAST/TRICKLE CHARGE.

IC₂ produces an output current (OUT) equal to 10^-4 of the current through sense-resistor R₉. During a fast charge, Q₃ and Q₄ are on, so this output current flows through the parallel combination of R₁₁ and R₄ (approximately). The resulting feedback to pin 3 of IC₁ maintains the fast-charge current through R₉ at 500 mA. This feedback also enables the regulator to supply up to 500 mA of load current, along with the fixed 500-mA charging current. Transistor Q₂ limits the battery voltage to 10V (at 2V/cell).

During the fast charge, the external processor and multichannel A/D converter monitor the battery-terminal voltage. When this converter senses a change in slope in this voltage, the processor terminates the fast charge by asserting a high on FAST/TRICKLE CHARGE. This turns Q₃ off, causing a rise in the feedback that lowers the charging current to the trickle-charge rate ([Greek];60 mA).

If IC₁ shuts down, or if the load current plus charging current exceed the capacity of IC₁, the R₉ current reverses as current flows out of the battery. IC₂ indicates a reversal by letting R₁₃ pull its open-collector SIGN output high, turning Q₄ off and turning Q₅ on. The current through R₁₂ then produces a voltage proportional to the battery's discharge current (5A through R₉ produces a full-scale response of 3V across R₁₂).

By sampling and integrating this voltage over time, the A/D-and-processor combination can monitor energy removed from the battery. Using this result and the terminal-voltage measurement, the processor can initiate a fast charge (by asserting FAST/TRICKLE CHARGE low) before the battery reaches the end of its life. (DI #1714) EDN