LT1510/LT1510-5
U
W U U
APPLICATIONS INFORMATION
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.
where RPROG is the total resistance from PROG pin to
ground.
For example, 1A charging current is needed.
With divider current set at 25µA, R4 = 2.465/25µA = 100k
and,
2.465V 2000
(
)(
)
RPROG
=
= 4.93k
1A
R4 V − 2.465
100k 8.4 − 2.465
( )(
)
(
)
BAT
R3 =
=
Charging current can also be programmed by pulse width
modulatingIPROG withaswitchQ1toRPROG atafrequency
higher than a few kHz (Figure 4). Charging current will be
proportionaltothedutycycleoftheswitchwithfullcurrent
at 100% duty cycle.
2.465 +R4 0.05µA 2.465 +100k 0.05µA
(
)
(
)
= 240k
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
tothepossibilitythatveryaccurate(0.1%)resistorsmight
be needed for R3 and R4. Actually, the temperature of the
LT1510 will rarely exceed 50°C in float mode because
chargingcurrentshavetaperedofftoalowlevel, so0.25%
resistors will normally provide the required level of overall
accuracy.
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.
LT1510
PROG
300Ω
External Shutdown
R
C
PROG
PROG
4.64k
1µF
The LT1510 can be externally shut down by pulling the VC
pin low with an open drain MOSFET, such as VN2222. The
VC 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 VC pin will still ensure the shutdown state over all
temperatures, but it results in slightly different conditions
as outlined below.
Q1
5V
0V
VN2222
PWM
I
= (DC)(1A)
BAT
1510 F04
Figure 4. PWM Current Programming
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-
rentdecreasingtozeroovertimeasthebatteryreachesfull
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.
If the VC pin is held below threshold, but above ≈ 0.4V, the
current flowing into the BAT pin will remain at about
700µA.PullingtheVC pinbelow0.4Vwillcausethecurrent
todropto ≈200µAandreverse, flowingout oftheBATpin.
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.
Current through the R3/R4 divider is set at a compromise
value of 25µA to minimize battery drain when the charger
isoffandtoavoidlargeerrorsduetothe50nAbiascurrent
of the OVP pin. Q3 can be added if it is desired to eliminate
If it is acceptable to have 200µA flowing into the battery
while the charger is in shutdown, simply pull the VC pin
directly to ground with the external MOSFET. The resistor
divider used to sense battery voltage will pull current out
10
Linear [ Linear ]
