Design Ideas: October 12, 1995

IC₁ is a buck-mode switching regulator that controls the external power switch, Q₁, and the synchronous rectifier, Q₂. These n-channel MOSFETs are more efficient than equivalent p-channel types, because the n-channel devices' on-resistance is lower, and they thus drop less voltage when conducting a given amount of current. IC₁ includes a charge pump for generating the positive gate-drive voltage that Q₁ requires.

The battery-charging current develops a voltage across R₂, a 25-m ohm resistor. Op-amp IC₂ amplifies this voltage and provides positive feedback to IC₁. This feedback enables IC₁ to maintain the charging current at 2.5A. While charging, the circuit can also supply current to a separate load, up to a limit set by the current transformer, T₁, and the sense resistor, R₁.

T₁ improves efficiency by lowering the power dissipation in R₁. The transformer turns ratio of 1:70 routes only ¹/₇₀ of the total battery-plus-load current through R₁, creating a feedback voltage that enables IC₁ to limit the overall current to a level compatible with the external components.

Efficiency exceeds 96% at high output voltages of approximately 9V and greater. Lower output voltages produce less output power, so the relatively fixed amount of dissipation associated with Q₁, Q₂, and IC₁ represents a larger percentage of the total. Depending on the input voltage, efficiencies for outputs of 4V range from approximately 87 to 92%. At 6V outputs, the efficiency ranges from approximately 91 to 94%.

If you inadvertently disconnect the battery during a charge, V_OUT can't rise to a dangerous level, as it can in a boost-mode topology, because the charger's buck-mode topology limits the maximum output voltage to V_IN. Be sure the 100-µF output capacitors have a voltage rating of at least V_IN. (DI #1764)