Design Ideas: March 2, 1995

Designing a single-rate battery charger by using a current-mode switching-regulator controller is usually not a challenge. If operating at its current limit, the controller provides a constant output current that you can easily adapt to battery charging. However, the design gets more complicated if you must vary the charging current by using a dc control signal. This task is especially difficult with high battery voltages. Providing high-side charging-current sensing to eliminate sense resistors in series with battery ground is desirable.

The circuit in Fig 1 solves this problem by providing a simple means to control accurately a battery's charging rate. The circuit converts a high-voltage input source to constant current for charging battery stacks of five to 12 cells. The circuit's high efficiency, which varies with the charging current and voltage across the battery, minimizes the power dissipated in surface-mount components. Efficiency measures 92% for a 10V output, the middle of the operating range, and a full 1.5A of load current.

The circuit is based around IC₁, a step-down controller IC featuring fully synchronous rectification, current-mode control, and a constant off-time architecture. During IC₁'s on time, Q₁ conducts, and current builds in L₁. When this current reaches the value preset by IC₁'s on-chip error amplifier, Q₁ turns off and the current flows through Q₂. C₁ determines the off time, which is fixed. L₁ integrates current pulses from Q₁ to provide essentially constant output current. D₁ prevents the batteries from discharging through the feedback divider network when the charger is shut down or the input power is removed.

IC₁'s sense pins usually sense the current in L₁ as a voltage across current-sense resistor R₁. However, the current-sensing pins have a common-mode range limited to 13V. To accommodate higher output voltages, Fig 1 uses a special current-sensing circuit. A precision high-frequency amplifier, IC_2A, forces the voltage across R₂ to equal that across R₁. Neglecting Q₁'s base current, the same voltage appears across the now ground-referenced R₃. The common-mode range of the amplifier is not exceeded because the input powers IC₁. To improve high-frequency noise immunity, C₂ and C₄ filter out any high-frequency common-mode signals.

IC_2B senses the average output current using the R₄/C₃ lowpass network and servos this current versus the control input. When you remove the battery, IC_2B's output goes to ground, and the feedback resistors R₅ and R₆ clamp the output at a fixed level. If you omit these resistors, the output rises close to the input when the battery is disconnected. This circuit preserves output-current regulation with battery voltages within 1V of the input voltage.