10
LT1510/LT1510-5
where R
PROG
is the total resistance from PROG pin to
ground.
For example, 1A charging current is needed.
R
V
A
k
PROG
=
()()
=
2 465 2000
1
493
.
.
Charging current can also be programmed by pulse width
modulating I
PROG
with a switch Q1 to R
PROG
at a frequency
higher than a few kHz (Figure 4). Charging current will be
proportional to the duty cycle of the switch with full current
at 100% duty cycle.
When a microprocessor DAC output is used to control
charging current, it must be capable of sinking current
at a compliance up to 2.5V if connected directly to the
PROG pin.
APPLICATIONS INFORMATION
WUU
U
even this low current drain. A 47k resistor from adapter
output to ground should be added if Q3 is used to ensure
that the gate is pulled to ground.
With divider current set at 25µA, R4 = 2.465/25µA = 100k
and,
R
RV
RA
k
kA
k
BAT
3
4 2 465
2 465 4 0 05
100 8 4 2 465
2 465 100 0 05
240
=
()
−
()
+
()
=
−
()
+
()
=
.
..
..
..µµ
Lithium-ion batteries typically require float voltage accu-
racy of 1% to 2%. Accuracy of the LT1510 OVP 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
LT1510 will rarely exceed 50°C in float mode because
charging currents have tapered off to a low level, so 0.25%
resistors will normally provide the required level of overall
accuracy.
External Shutdown
The LT1510 can be externally shut down by pulling the V
C
pin low with an open drain MOSFET, such as VN2222. The
V
C
pin should be pulled below 0.8V at room temperature
to ensure shutdown. This threshold decreases at about
2mV/°C. A diode connected between the MOSFET drain
and the V
C
pin will still ensure the shutdown state over all
temperatures, but it results in slightly different conditions
as outlined below.
If the V
C
pin is held below threshold, but above ≈ 0.4V, the
current flowing
into
the BAT pin will remain at about
700µA. Pulling the V
C
pin below 0.4V will cause the current
to drop to ≈ 200µA and reverse, flowing
out
of the BAT pin.
Although these currents are low, the long term effect may
need to be considered if the charger is held in a shutdown
state for very long periods of time, with the charger input
voltage remaining. Removing the charger input voltage
causes all currents to drop to near zero.
If it is acceptable to have 200µA flowing into the battery
while the charger is in shutdown, simply pull the V
C
pin
directly to ground with the external MOSFET. The resistor
divider used to sense battery voltage will pull current out
Figure 4. PWM Current Programming
PWM
R
PROG
4.64k
300Ω
PROG
C
PROG
1µF
Q1
VN2222
5V
0V
LT1510
1510 F04
I
BAT
= (DC)(1A)
Lithium-Ion Charging
The circuit in Figure 2 uses the 16-pin LT1510 to charge
lithium-ion batteries at a constant 1.3A until battery volt-
age reaches a limit set by R3 and R4. The charger will then
automatically go into a constant-voltage mode with cur-
rent decreasing to zero over time as the battery reaches full
charge. This is the normal regimen for lithium-ion charg-
ing, with the charger holding the battery at “float” voltage
indefinitely. In this case no external sensing of full charge
is needed.
Current through the R3/R4 divider is set at a compromise
value of 25µA to minimize battery drain when the charger
is off and to avoid large errors due to the 50nA bias current
of the OVP pin. Q3 can be added if it is desired to eliminate
