LT1571-1 LINER [Linear Technology], LT1571-1 Datasheet - Page 8

• Manufacturer Part Number: LT1571-1
• Description: Constant-Current/ Constant-Voltage Battery Charger with Preset Voltage and Termination Flag
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT1571 Series
is achieved with V
time to reach full charge current is about 9ms and it is
assumed that input voltage to the charger will reach full
value in less than 3ms. Capacitance can be increased up to
1 F if longer input start-up times are needed.
In any switching regulator, conventional time-based soft
starting can be defeated if the input voltage rises much
slower than the time-out period. This happens because the
switching regulators in the battery charger and the com-
puter power supply are typically supplying a fixed amount
of power to the load. If the input voltage comes up slowly
compared to the soft-start time, the regulators will try to
deliver full power to the load when the input voltage is still
well below its final value. If the adapter is current limited,
it cannot deliver full power at reduced output voltages and
the possibility exists for a quasi “latch” state where the
adapter output stays in a current limited state at reduced
output voltage. For instance, if maximum charger plus
computer load power is 20W, a 24V adapter might be
current limited at 1A. If adapter voltage is less than (20W/1A
= 20V) when full power is drawn, the adapter voltage will be
pulled down by the constant 20W load until it reaches a lower
stable state where the switching regulators can no longer
supply full load. This situation can be prevented by utilizing
undervoltage lockout , set higher than the minimum adapter
voltage where full power can be achieved.
A fixed undervoltage lockout of 7V is built into the LT1571.
A higher lockout voltage can be implemented with a Zener
diode D2 (see Figure 2).
8
V
IN
Figure 2. Undervoltage Lockout
C
2k
U
D2
V
at 1.1V. With a 0.3 F capacitor, the
Z
1N4148
U
D1
D3
V
W
C
LT1571
GND
V
CC
1571 F02
U
The lockout voltage will be V
For example, for a 24V adapter to start charging at 22V
choose V
low and charger off.
Charge Current Programming
The basic formula for charge current is (see Block
Diagram):
where R
ground.
For example, 1A charge current is needed.
Charge current can also be programmed by pulse width
modulating I
higher than a few kHz (Figure 3). Charge current will be
proportional to the duty cycle of Q1 with full current at
100% duty cycle.
When a microprocessor DAC output is used to control
charge current, it must be capable of sinking current
at a compliance up to 2.5V if connected directly to the
PROG pin.
I
R
BAT
PROG
PROG
Z
= 21V. When V
I
PROG
Figure 3. PWM Current Programming
PROG
5V
0V
I
2 465
BAT
is the total resistance from PROG pin to
.
= (DC)(1A)
PWM
with a switch Q1 to R
2000
V
1
A
2000
IN
LT1571
is less than 22V, D1 keeps V
IN
2 465
R
300
R
4.64k
Q1
VN2222
PROG
PROG
.
PROG
= V
4 93
.
Z
V
+ 1V.
k
PROG
2000
C
1 F
1571 F03
PROG
at a frequency
IN
C
,