11
LT1571 Series
R
RV
BAT
3
4 2 465
2 465
=
()
−
()
.
.
Lithium-ion batteries typically require float voltage accu-
racy of 1% to 2%. Accuracy of the LT1571-1 V
FB
voltage
is ±0.5% at 25°C and ±1% over full temperature. This
leads to the possibility that very accurate (0.1%) resistors
might be needed for R3 and R4. Actually, the temperature
of the LT1571-1 rarely exceeds 50°C in float mode
because charge currents have tapered off to a low level, so
0.25% resistors normally provide the required level of
overall accuracy.
External Shutdown
The LT1571 can be externally shut down by pulling the V
C
pin low with an open-drain N-FET, such as 2N7002. The V
C
pin should be pulled below 0.6V to stop switching. When
V
C
is pulled below 40mV, LT1571 supply current drops to
typical 150µA.
Removing input power to the charger puts the LT1571 into
a sleep mode and draws only 5µA from the battery.
Nickel-Cadmium and Nickel-Metal-Hydride Charging
The circuit in Figure 7 uses the LT1571-1 to charge NiCd
or NiMH batteries up to 20V with charge currents of 0.5A
when Q1 is on and 50mA when Q1 is off.
For a 2-level charger, R1 and R2 are found from:
I
R
R
I
R
II
BAT
PROG
LOW HI LOW
=
()( )
=
()()
=
()()
−
2000 2 465
1
2 465 2000
2
2 465 2000
.
..
All battery chargers with fast-charge rates require some
means to detect full charge in the battery and terminate the
high charge current. NiCd batteries are typically charged at
high current until the battery temperature begins to in-
crease or until the battery voltage reaches a peak and
begins to decrease (–dV/dt). This is an indication of near
full charge. The charge current is then reduced to a much
For example, if 10% threshold is needed for the 1A charger
(see Figure 4), then with R
PROG
= 4.93k:
R
k
k
CAP
==
1 331 4 93
020 010
65 6
.•.
.–.
.
Because of low level errors, as the threshold level is
reduced, the accuracy is also reduced. It is not recom-
mended to program a level less than 7.5%.
Preset Battery Voltage Settings
The LT1571-2 operates at 200kHz and is preset for 8.2V
battery voltage with SELECT pin floating and 8.4V with
SELECT pin grounded.
The LT1571-5 operates at 500kHz and is preset for 4.1V
battery voltage with SELECT pin floating and 4.2V with
SELECT pin grounded.
BAT2 pin is for Kelvin sensing the battery voltage and
should be connected to the battery.
Other Battery Voltage Settings
For battery voltages other than the preset voltages, the
LT1571-1 should be used. It operates at 200kHz and the
battery voltage is programmed with R3 and R4 divider at
V
FB
pin (Figure 6).
Figure 6. Programming Other Battery Voltages
R3
R4
V
FB
V
BAT
LT1571-1
1571 F06
Current through the R3/R4 divider is set at a compromise
value of 25µA to minimize battery drain when the charger
is off. The V
FB
pin input current of 3nA contributes very
little output voltage error and can be neglected.
With divider current set at 25µA, R4 = 2.465/25µA = 100k
and,
APPLICATIO S I FOR ATIO
WUUU
