8
LT1571 Series
The LT1571 is a current mode PWM step-down (buck)
charger. The battery charge current is programmed by a
resistor R
PROG
(or a DAC output current) at the PROG pin
(see Block Diagram). Amplifier CA1 converts the charge
current through R
S1
to a much lower current I
PROG
(500µA/
A) fed into the PROG pin. Amplifier CA2 compares the
output of CA1 with the programmed current and drives the
PWM loop to force them to be equal. High DC accuracy is
achieved with averaging capacitor C
PROG
. Note that I
PROG
has both AC and DC components. I
PROG
goes through R1
and generates a ramp signal that is fed to the PWM control
comparator C1 through buffer B1 and level shift resistors
R2 and R3, forming the current mode inner loop. The
BOOST pin drives the NPN switch (Q
SW
) into saturation
and reduces power loss. For batteries like lithium-ion that
require both constant-current and constant-voltage charg-
ing, the 0.5%, 2.465V reference and the amplifier VA
reduce the charge current when battery voltage reaches
the preset level. For NiMH and NiCd, VA can be used for
overvoltage protection. When input voltage is removed,
the V
CC
pin drops to 0.7V below the battery voltage forcing
the charger into a low-battery drain (5µA typical) sleep
mode. To shut down the charger, simply pull the V
C
pin low
with a transistor.
Comparator E6 monitors the charge level and signals
through the FLAG pin when charging is in voltage mode
and the charge current has reduced to 20% or less. This
charge complete signal can be used to start a timer for
charging termination.
Input and Output Capacitors
In the charger circuits in Figures 1 and 2, the input
capacitor C
IN
is assumed to absorb all input switching
ripple current in the converter, so it must have adequate
ripple current rating. Worst-case RMS ripple current will
be equal to one half of the output charge current. Actual
capacitance value is not critical. Solid tantalum capacitors
such as the AVX TPS and Sprague 593D series have high
ripple current rating in a relatively small surface mount
package, but
caution must be used when tantalum capaci-
tors are used for input bypass
. High input surge currents
are possible when the adapter is hot-plugged to the
charger and solid tantalum capacitors have a known
failure mechanism when subjected to very high turn-on
surge currents. Selecting a high voltage rating on the
capacitor will minimize problems. Consult with the manufac-
turer before use. Alternatives include new high capacity
ceramic capacitors from Tokin or United Chemi-Con/
MARCON, et al. OS-CON can also be used.
The output capacitor C
OUT
is also assumed to absorb
output switching ripple current. The general formula for
capacitor ripple current is:
I
V
V
V
Lf
RMS
BAT
BAT
CC
=
()
−
()()
029 1
1
.
For example, with V
CC
= 16V, V
BAT
= 8.4V, L1 = 33µH and
f = 200kHz, I
RMS
= 0.18A.
EMI considerations usually make it desirable to minimize
ripple current in the battery leads. Beads or inductors can
be added to increase battery impedance at the 200kHz
switching frequency. Switching ripple current splits
between the battery and the output capacitor depending
on the ESR of the output capacitor and the battery imped-
ance. If the ESR of C
OUT
is 0.2Ω and the battery impedance
is raised to 4Ω with a bead of inductor, only 5% of the
ripple current will flow into the battery.
Soft-Start
The LT1571 is soft-started by the 0.33µF capacitor on V
C
pin. On start-up, the V
C
pin voltage will rise quickly to 0.5V,
then ramp at a rate set by the internal 45µA pull-up current
and the external capacitor. Charge current starts ramping
up when the V
C
pin voltage reaches 0.9V and full current
OPERATIO
U
APPLICATIO S I FOR ATIO
WUUU
