The ADP3810 and ADP3811 combine a programmable current limit with a battery voltage limit to provide a constant current, constant voltage battery charger controller. In secondary side, off-line applications, the output directly drives the diode side of an optocoupler to give isolated feedback control of a primary side PWM. The circuitry includes two gain (gm) stages, a precision 2.0 V reference, a control input buffer, an Undervoltage Lock Out (UVLO) comparator, an output buffer and an over-voltage comparator.
The current limit amplifier senses the voltage drop across an external sense resistor to control the average current for charging a battery. The voltage drop can be adjusted from 25 mV to 300 mV, giving a charging current limit from 100 mA to 1.2 amps with a 0.25 (ohm) sense resistor. An external dc voltage on the VCTRL input sets the voltage drop. Because this input is high impedance, a filtered PWM output can be used to set the voltage.
As the battery voltage approaches its voltage limit, the voltage sense amplifier takes over to maintain a constant battery voltage. The two amplifiers essentially operate in an “OR” fashion. Either the current is limited, or the voltage is limited.
The ADP3810 has internal thin-film resistors that are trimmed to provide a precise final voltage for LiIon batteries. Four voltage options are available, corresponding to 1-4 LiIon cells as follows: 4.2 V, 8.4 V, 12.6 V and 16.8 V.
The ADP3811 omits these resistors allowing any battery voltage to be programmed with external resistors.
