LT1513/LT1513-2
U
W U U
APPLICATIONS INFORMATION
The LT1513 is an IC battery charger chip specifically opti-
mized to use the SEPIC converter topology. A complete
chargerschematicisshowninFigure1.TheSEPICtopology
has unique advantages for battery charging. It will operate
with input voltages above, equal to or below the battery
voltage, has no path for battery discharge when turned off,
andeliminatesthesnubberlossesofflybackdesigns. Italso
has a current sense point that is ground referred and need
not be connected directly to the battery. The two inductors
shown are actually just two identical windings on one
inductorcore,althoughtwoseparateinductorscanbeused.
Figure3.D2,C6andR6formapeakdetectortodrivethegate
of the FET to about the same as the battery voltage. If power
isturnedoff,thegatewilldropto0Vandtheonlydrainonthe
batterywillbethereverseleakageofthecatchdiodeD1. See
Diode Selection for a discussion of diode leakage.
C2
D1
L1A
ADAPTER
INPUT
R1
D2
V
IN
V
SW
L1B
R3
+
LT1513
GND
C1
V
FB
A current sense voltage is generated with respect to ground
across R3 in Figure 1. The average current through R3 is
always identical to the current delivered to the battery. The
LT1513 current limit loop will servo the voltage across R3
to –100mV when the battery voltage is below the voltage
limit set by the output divider R1/R2. Constant-current
charging is therefore set at 100mV/R3. R4 and C4 filter the
currentsignaltodeliverasmoothfeedbackvoltagetotheIFB
pin.R1andR2formadividerforbatteryvoltagesensingand
set the battery float voltage. The suggested value for R2 is
12.4k. R1 is calculated from:
R6
470k
C6
470pF
R2
1513 F03
SCHEMATIC SIMPLIFIED FOR CLARITY
D2 = 1N914, 1N4148 OR EQUIVALENT
Figure 3. Eliminating Divider Current
Maximum Input Voltage
Maximum input voltage for the LT1513 is partly determined
by battery voltage. A SEPIC converter has a maximum
switch voltage equal to input voltage plus output voltage.
The LT1513 has a maximum input voltage of 30V and a
maximum switch voltage of 40V, so this limits maximum
input voltage to 30V, or 40V – VBAT, whichever is less.
R2(V
1.245 +R2(0.3µA)
– 1.245)
BAT
R1=
VBAT = battery float voltage
0.3µA = typical FB pin bias current
Shutdown and Synchronization
A value of 12.4k for R2 sets divider current at 100µA. This is
a constant drain on the battery when power to the charger is
off. If this drain is too high, R2 can be increased to 41.2k,
reducing divider current to 30µA. This introduces an addi-
tionaluncorrectableerrortotheconstantvoltagefloatmode
of about ±0.5% as calculated by:
The dual function S/S pin provides easy shutdown and
synchronization. It is logic level compatible and can be
pulled high or left floating for normal operation. A logic low
on the S/S pin activates shutdown, reducing input supply
currentto12µA. Tosynchronizeswitching, drivetheS/Spin
between 600kHz and 800kHz.
±0.15µA(R1)(R2)
V
Error =
BAT
1.245(R1+R2)
Inductor Selection
L1AandL1Barenormallyjusttwoidenticalwindingsonone
core, althoughtwoseparateinductorscanbeused. Atypical
value is 10µH, which gives about 0.5A peak-to-peak induc-
tor current. Lower values will give higher ripple current,
whichreducesmaximumchargingcurrent.5µHcanbeused
if charging currents are at least 20% lower than the values
±0.15µA = expected variation in FB bias current around the
nominal 0.3µA typical value.
With R2 = 41.2k and R1 = 228k, (VBAT = 8.2V), the error due
to variations in bias current would be±0.42%.
A second option is to disconnect the divider when charger
powerisoff. ThiscanbedonewithasmallNFETasshownin
7
Linear [ Linear ]
