The LTC4009 is a constant-current/constant-voltage battery charger controller. It uses a synchronous quasi-constant frequency PWM control architecture that will not gener- ate audible noise with ceramic bulk capacitors. Charge current is set by the combination of external sense, input and programming resistors. Current being delivered by the charger can be monitored as a voltage across the programming resistor. The LTC4009 features fully adjustable output voltage. An external resistor divider and precision internal reference set the fi nal output voltage at which charge current will be reduced to zero. The device includes AC adapter input current limiting, which maximizes charge rate for a fi xed input power level. An external sense resistor programs the input current limit, and the ICL status pin indicates when the battery charge current is being reduced as a result of AC adapter current limiting. The CHRG status pin is active during all charging modes, including special indication when the charge current has decreased below 10% of the programmed current.