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LT1571 Series

GND: Ground Pin.

SW: NPN Power Switch Emitter. The Schottky catch diode

must be placed with very short lead length in close

proximity to SW pin and GND.

VCC1, VCC2: Input Supply. For good bypass, a low ESR

capacitor of 10

µForhigherisrequired,withtheleadlength

kept to a minimum. VCC should be between 8V and 26V

and at least 2V higher than VBAT for VBAT less than 10V, and

2.5V higher than VBAT for VBAT greater than 10V. Under-

voltage lockout starts and switching stops when VCC goes

below 7V (typical). Note that there is an internal parasitic

diode from SW pin to VCC pin. Do not force VCC below SW

by more than 0.7V with battery present. All VCC pins

should be shorted together close to the pins.

BOOST: This pin is used to bootstrap and drive the NPN

switch to a low on-voltage for low power dissipation. VBOOST

= VCC + VBAT when switch is on. For less power dissipation

use VBOOST = 3V to 6V (see Applications Information).

SENSE: Current Amplifier CA1 Input. Sensing can be at

either terminal of the battery. Note that current sense

resistor RS1 (0.08Ω) is between SENSE and BAT pins.

BAT: Current Amplifier CA1 Input.

BAT2 (LT1571-2, LT1571-5): This pin is used to connect

the battery to the internal preset voltage setting resistor.

An internal switch disconnects the internal divider from

the battery when the device is in shutdown or when input

power is disconnected. This disconnect function

eliminates current drain due to the resistor divider. This

pin should be connected to the positive node of the

battery if the internal preset divider is used. Otherwise

this pin should be grounded. Maximum voltage on this

pin is 20V.

PROG: This pin is for programming the charge current

and for system loop compensation. Charge current is

regulated to 2000

× the current drawn from the PROG

pin. During normal operation, VPROG stays close to

2.465V. If it is shorted to GND, switching will stop. When

a microprocessor-controlled DAC is used to program

charge current, it must be capable of sinking current at a

compliance up to 2.465V.

VC: This is the inner loop control signal of the current mode

PWM. Switching starts at 0.9V. In normal operation, a

higher VC corresponds to a higher charge current. A

capacitor of at least 0.1

µF to GND filters out noise and

controls the rate of soft-start. To shut down switching, pull

this pin below 0.6V. Typical current out of this pin is 60

µA.

When VC is pulled below 40mV, LT1571 supply current

drops to typical 150

µA.

SELECT (LT1571-2, LT1571-5): This pin is used to select

the preset battery voltage. For the LT1571-2, leave this pin

open for 8.2V and ground it for 8.4V. For the LT1571-5,

leave this pin open for 4.1V and ground it for 4.2V. For

other battery voltages, use the adjustable LT1571-1.

VFB (LT1571-1): This is the input to the amplifier VA (see

Block Diagram) with a threshold of 2.465V. Typical input

current is about 3nA. When charging batteries, VA moni-

tors the battery voltage and reduces charging current

when battery voltage reaches the preset value. If it is not

used (constant-current only mode), the VFB pin should be

grounded.

CAP: A 0.1

µF capacitor from CAP to ground is needed to

filter the sampled charge current signal. This filtered

signal is used to set the FLAG pin when the charge current

drops to 20% of the programmed maximum charge cur-

rent. This threshold level can be set as low as 7.5% of the

programmed maximum charge current by adding a resis-

tor on the CAP pin.

FLAG: This pin is an open-collector output that is used to

indicate end of charge. The FLAG pin is driven low when

the charge current drops below a certain percentage of the

programmed charge current as explained in the CAP pin

function. A pull-up resistor is required if this function is

used. This pin is capable of sinking at least 1mA. Maxi-

mum voltage on this pin is VCC.

PI FU CTIO S

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