3
DEMO MANUAL DC086
BATTERY CHARGER
OPERATIO
U
lead-acid batteries, reprogram the OVP (overvoltage pro-
tection) feedback resistor divider for the correct battery
charging voltage using the following formula:
R
R
V
V
F
OUT
REF
=
3
1
–
where R
F
= total resistance between OVP pin and BAT pin,
V
REF
= 2.465V, select R3 = 4.99k, 1% or less.
Maximum charging current (up to 1.5A) is programmed
by R1 using the following formula:
R
I
CHARGE
1
2000 2 465
=
(
)(
)
.
where 2.465V = reference voltage present at PROG pin.
The maximum charging current (or current limit) is 2000
times the current out of the PROG pin. This current has both
AC and DC components present; therefore, to provide high
DC accuracy, averaging components R2 and C3 are required.
For nickel-cadmium and nickel-metal-hydride batteries,
the normal charging method is constant current. Fast
charging requires some method to detect full charge and
terminate the high charge current. Some methods often
used to indicate full charge include battery temperature
rise and observing battery voltage profile during charging.
LT1510 Step-Down Switching Regulator
The LT1510 is designed for constant-voltage and/or con-
stant-current operation with a 0.5% voltage accuracy and
a 5% current accuracy. An external resistor voltage divider
programs the output voltage, while a single resistor (or a
programming current from a DAC) sets the maximum
charging current .
An internal 2A NPN switch operating at 200kHz provides
high efficiency with low inductor values using a minimum
number of external components. The charging current
sense resistor is included on the die and can be wired for
sensing charging current at either the positive or negative
side of the battery.
Refer to the LT1510 data sheet for complete product
specifications and design notes DN111 and DN124 for
additional application information.
The DC086 demonstration board is intended for evaluat-
ing the LT1510 switching regulator battery charger IC.
Solder terminals are provided for easy hookup to a power
supply and to a lithium-ion battery to be charged. The
correct charging voltage for either 1, 2 or 3 cells is
selectable by a combination of jumpers (J1 and J2) on the
board. See the schematic diagram for jumper information.
Current limit is set for 1.29A by resistor R1.
With a suitable input power supply and a discharged
battery connected to the demo board, the battery will
begin charging at the programmed current limit of 1.29A.
As the battery charges, the voltage rises and approaches
the program voltage of either 4.2V, 8.4V or 12.6V. The
charger will then maintain a constant voltage across the
battery, with the charging current decreasing to zero over
time as the battery reaches a fully charged condition.
The IC goes into a sleep mode when the input voltage is
removed. In the sleep mode, the drain from the battery due
to the LT1510 is approximately 3
µ
A. An additional source
of battery drain is due to the leakage current of Schottky
diode D1. Selecting a low leakage Schottky diode such as
a Motorola MBRD340 or a low leakage 3A silicon diode
can minimize this current drain.
Some lithium-ion battery manufacturers recommend ter-
minating the constant-voltage float mode after the charge
current has dropped below a specific level (typically 50mA
to 100mA) and a specific amount of time has elapsed
(typically from 30 to 90 minutes). This may extend the life
of the battery, but check with the manufacturer of the
battery you are using for details. Included on the board is
an area for breadboarding a timing circuit.
Other Battery Types
Although the demo board was designed for charging
Li-Ion batteries, simple modifications will allow other
battery types to be charged.
Sealed lead-acid batteries are charged using a current-
limited constant voltage. Over a 0
°
C to 40
°
C temperature
range, a charge voltage of 2.35V/cell can be used with no
charge termination needed. The maximum charge current
is determined by the battery manufacturer and is typically
0.25C or less. To use the demo board for charging sealed
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
Note: C is the capacity rating of the battery in Ampere-Hours.
