The UC3909 family of Switchmode Lead-Acid Battery Chargers accurately controls lead acid battery charging with a highly efficient average current mode control loop. This chip combines charge state logic with average current PWM control circuitry. Charge state logic commands current or voltage control depending on the charge state. The chip includes undervoltage lockout circuitry to insure sufficient supply voltage is present before output switching starts. Additional circuit blocks include a differential current sense amplifier, a 1.5% voltage reference, a 3.9mV/C thermistor linearization circuit, voltage and current error amplifiers, a PWM oscillator, a PWM comparator, a PWM latch, charge state decode bits, and a 100mA open collector output driver.