Datasheet LT1571 (Analog Devices) - 6

LT1571 Series
U U U PI FU CTIO S GND:
Ground Pin. a microprocessor-controlled DAC is used to program
SW:
NPN Power Switch Emitter. The Schottky catch diode charge current, it must be capable of sinking current at a must be placed with very short lead length in close compliance up to 2.465V. proximity to SW pin and GND.
VC:
This is the inner loop control signal of the current mode
V
PWM. Switching starts at 0.9V. In normal operation, a
CC1, VCC2:
Input Supply. For good bypass, a low ESR capacitor of 10µF or higher is required, with the lead length higher VC corresponds to a higher charge current. A kept to a minimum. V capacitor of at least 0.1µF to GND filters out noise and CC should be between 8V and 26V and at least 2V higher than V controls the rate of soft-start. To shut down switching, pull BAT for VBAT less than 10V, and 2.5V higher than V this pin below 0.6V. Typical current out of this pin is 60µA. BAT for VBAT greater than 10V. Under- voltage lockout starts and switching stops when V When VC is pulled below 40mV, LT1571 supply current CC goes below 7V (typical). Note that there is an internal parasitic drops to typical 150µA. diode from SW pin to VCC pin. Do not force VCC below SW
SELECT (LT1571-2, LT1571-5):
This pin is used to select by more than 0.7V with battery present. All VCC pins the preset battery voltage. For the LT1571-2, leave this pin should be shorted together close to the pins. open for 8.2V and ground it for 8.4V. For the LT1571-5,
BOOST:
This pin is used to bootstrap and drive the NPN leave this pin open for 4.1V and ground it for 4.2V. For switch to a low on-voltage for low power dissipation. V other battery voltages, use the adjustable LT1571-1. BOOST = VCC + VBAT when switch is on. For less power dissipation
VFB (LT1571-1):
This is the input to the amplifier VA (see use VBOOST = 3V to 6V (see Applications Information). Block Diagram) with a threshold of 2.465V. Typical input
SENSE:
Current Amplifier CA1 Input. Sensing can be at current is about 3nA. When charging batteries, VA moni- either terminal of the battery. Note that current sense tors the battery voltage and reduces charging current resistor R when battery voltage reaches the preset value. If it is not S1 (0.08Ω) is between SENSE and BAT pins. used (constant-current only mode), the VFB pin should be
BAT:
Current Amplifier CA1 Input. grounded.
BAT2 (LT1571-2, LT1571-5):
This pin is used to connect
CAP:
A 0.1µF capacitor from CAP to ground is needed to the battery to the internal preset voltage setting resistor. filter the sampled charge current signal. This filtered An internal switch disconnects the internal divider from signal is used to set the FLAG pin when the charge current the battery when the device is in shutdown or when input drops to 20% of the programmed maximum charge cur- power is disconnected. This disconnect function rent. This threshold level can be set as low as 7.5% of the eliminates current drain due to the resistor divider. This programmed maximum charge current by adding a resis- pin should be connected to the positive node of the tor on the CAP pin. battery if the internal preset divider is used. Otherwise this pin should be grounded. Maximum voltage on this
FLAG:
This pin is an open-collector output that is used to pin is 20V. indicate end of charge. The FLAG pin is driven low when the charge current drops below a certain percentage of the
PROG:
This pin is for programming the charge current programmed charge current as explained in the CAP pin and for system loop compensation. Charge current is function. A pull-up resistor is required if this function is regulated to 2000× the current drawn from the PROG used. This pin is capable of sinking at least 1mA. Maxi- pin. During normal operation, VPROG stays close to mum voltage on this pin is VCC. 2.465V. If it is shorted to GND, switching will stop. When 6