LT1571-1 Ver la hoja de datos (PDF) - Linear Technology
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LT1571-1
Linear Technology
LT1571-1 Datasheet PDF : 16 Pages
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LT1571 Series
PI FU CTIO S
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 10µF or higher is required, with the lead length
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 0.1µF 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 60µA.
When VC is pulled below 40mV, LT1571 supply current
drops to typical 150µA.
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 0.1µF 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.
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