Design Ideas: June 23, 1994

The adjustable switching-regulator LM2575T-ADJ, IC₁, in Fig 1 replaces a power-wasting pnp transistor in the data-book design for the NiCd battery charger IC₂, a MAX713. (Its sister device, the MAX712, handles NiMH cells.) IC₂ "fast-charges" cells by monitoring the cell's voltage and charging current until they exhibit subtle anomalies that signal a fully charged cell. Because the circuit charges at only a 1.5C rate, the circuit does not need to sense the cell's temperature. (A 1C, or "standard," rate signifies charging the battery to its full capacity in 1 hour.)

The circuit in Fig 1 fast-charges 500-mAhr AA NiCd cells. Depending on the setting of SW₁ and SW₂, the circuit handles two to eight cells, providing the circuit's input voltage is at least 15V; at higher voltages, power IC₃ from IC₂'s pin 15. Charging starts when you plug in the battery. When the battery is fully charged, LED1 extinguishes, and the charger switches to trickle-charge mode.

The circuit employs IC₂ purely as a charge controller; IC₂ supplies the actual charging current to the battery. Experiments show that IC₁'s control loop is several orders of magnitude faster than IC₂'s. Therefore, a closed-loop system using IC₂'s DRV pin to control IC₁'s regulation proves unstable. Instead, IC₁'s feedback loop tries to maintain a voltage across C₁ that results in a 1.25V signal at IC₁'s FB pin.

R₁ sets the charging current. Select sense resistor R₂, which is in series with the battery, so that it develops 0.25V at the proper charging current (corresponding to 750 mA in this implementation). IC_3A scales this sense voltage for IC₁, and IC_3B buffers the sense voltage for IC₂. (IC_3B must supply IC₂'s expected -BATT voltage).

Because IC₁ is a switcher, it's efficiency is about 85%, and it needs no heat sink to pass charging currents under 1A. (DI #1447)