July 3, 1997

Charger serves load while charging battery

Bernard Konrad, Maxim Integrated Products, Graefelfing, Germany

The battery-charger circuit in Figure 1 monitors the battery voltage and automatically initiates a recharge if required. Its output-current capacity is sufficient to simultaneously charge the battery and supply the load. It thus provides continuous power to the load during recharge operations. The circuitry in IC₂ provides "smart" charge terminations for either NiCd or NiMH batteries. Unlike chargers that use a pulsed trickle charge during power standby, this circuit does not disconnect the load from the power source. The battery and load remain connected in parallel, so the battery is always fully charged when you remove power. Other advantages include high efficiency, small circuit area, and the use of inexpensive standard components.

IC₁ and its associated components form a low-dropout, step-down dc-dc controller configured as a switch-mode current source. IC₂ is a fast-charge controller for batteries. Connecting IC₂'s DRV output, which normally drives the base of a pnp transistor's linear current source, to IC₁'s feedback terminal causes IC₂'s current-regulator circuitry to control the external switch-mode current source. Connecting pullup resistor R₂ to the battery voltage via D₂ provides negative voltage feedback that improves the overall dynamic performance. R₅'s value is a function of the desired charge rate.

To enable the circuit to deliver a controlled fast charge as well as a regulated low trickle charge, IC₂'s CC input (compensation for the chip's constant-current regulation loop) terminates in the R₄-C₆ series RC network. The network integrates the internal error signal for the loop. Load current and battery-charging current come from the same source, with the battery acting as a buffer. If a load surge, such as the high turn-on current for a printer motor, depresses the battery voltage and causes the charger to fall out of regulation, the circuit applies a fast charge until IC₂ senses the full-charge condition. The circuit then resumes its regulated trickle-charge mode. (DI #2044)

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