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 The circuit in Fig 1 supports portable applications in which a microprocessor controls battery charging. Step-up switching regulator IC1 boosts VIN (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 end714) EDN of its life. (DI #1
Category: Electrical
Type: Glossaries and Dictionaries
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