Datasheet LTC4001 (Analog Devices) - 10

LTC4001
U U W U APPLICATIO S I FOR ATIO Soft-Start and Compensation Capacitor Selection
The IDET threshold (a charge current threshold used to The LTC4001 has a low current trickle charger and a determine when the battery is nearly fully charged) is PWM-based high current charger. Soft-start is used when- programmed in much the same way as the PROG pin, ever the high rate charger is initially turned on, preventing except that the IDET threshold is 91.5 times the current high start-up current. Soft-start ramp rate is set by the delivered by the IDET pin. This current is usually set with internal 12.8µA pull-up current and an external capacitor. an external resistor from IDET to ground, but it may also The control range on the SS pin is approximately 0.3V to be set with a current output DAC. The voltage on the PROG 1.6V. With a 0.1µF capacitor, the time to ramp up to pin is nominally 1.213V. maximum duty cycle is approximately 10ms. For 200mA IDET current (corresponding to C/10 for a The external capacitor on the SS pin also sets the compen- 2AHr battery): sation for the current control loop and the float voltage 91 5 . • 1.213V control loop. A minimum capacitance of 10nF is required. R = ≅ 554. Ω IDET 9 0 2 . A
Charge Current and IDET Programming
1.10kΩ programs approximately 100mA and 274Ω ap- The LTC4001 has two different charge modes. If the proximately 400mA. battery is severely depleted (battery voltage less than For applications where IDET is set to one tenth of the high 2.9V) a 50mA trickle current is initially used. If the battery rate charge current, and slightly poorer charger current voltage is greater than the trickle charge threshold, high and IDET threshold accuracy is acceptable, the PROG and rate charging is used. IDET pins may be tied together and a single resistor, R1, This higher charge current is programmable and is ap- can program both (Figure 1). proximately 915 times the current delivered by the PROG 457 5 1 213 pin. This current is usually set with an external resistor R1= . • . I from PROG to GNDSENS, but it may also be set with a CHARGE current output DAC connected to the PROG pin. The and voltage on the PROG pin is nominally 1.213V. For 2A charge current: I IDET CHARGE = 10 915 • 1 213 . V R = ≅ 554. Ω PROG 9 2A LTC4001 PROG IDET R1 274Ω FOR 2A GNDSENS 4001 F01
Figure 1. Programming Charge Current and IDET Threshold with a Single Resistor
4001f 10