THAN 3.5V. Q1 WILL NOT BE TURNED OFF
鈥?/div>
L1 A
D2
1N4148
C2
V
IN
V
SW
R1
SHUTDOWN
S/S
LT1512
L1 B
Q1
2N7002
GND
FB
R3
R2
D1
C6
470pF
R5
470k
CONNECT D2 ANODE HERE IF FULLY CHARGED
BATTERY VOLTAGE IS GREATER THAN 3.5V AND
Q1 MUST BE TURNED OFF IN SHUTDOWN WITH
V
IN
STILL ACTIVE
Figure 3. Eliminating Divider Current
Maximum Input Voltage
Maximum input voltage for the circuit in Figure 1 is partly
determined by battery voltage. A SEPIC converter has a
maximum switch voltage equal to input voltage plus output
voltage. The LT1512 has a maximum input voltage of 30V
and a maximum switch voltage of 40V, so this limits
maximum input voltage to 30V, or 40V 鈥?V
BAT
, whichever
is less. Maximum V
BAT
= 40V 鈥?V
IN
.
Shutdown and Synchronization
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
current to 12碌A(chǔ). To synchronize switching, drive the S/S pin
between 600kHz and 800kHz.
Inductor Selection
L1A and L1B are normally just two identical windings on one
core, although two separate inductors can be used. A typical
value is 33碌H, which gives about 0.25A peak-to-peak induc-
tor current. Lower values will give higher ripple current,
which reduces maximum charging current. 15碌H can be
used if charging currents are at least 20% lower than the
values shown in the maximum charging current graph.
Higher inductance values give slightly higher maximum
charging current, but are larger and more expensive. A low
U
W
U
U
+
1512 F03
loss toroid core such as KoolM碌
廬
, Molypermalloy or Metglas
廬
is recommended. Series resistance should be less than
0.1鈩?for each winding. 鈥淥pen core鈥?inductors, such as rods
or barrels are not recommended because they generate
large magnetic fields which may interfere with other elec-
tronics close to the charger.
Input Capacitor
The SEPIC topology has relatively low input ripple current
compared to other topologies and higher harmonics are
especially low. RMS ripple current in the input capacitor is
less than 0.1A with L = 33碌H and less than 0.2A with
L = 15碌H. A low ESR 22碌F, 25V solid tantalum capacitor
(AVX type TPS or Sprague type 593D) is adequate for most
applications with the following caveat. Solid tantalum ca-
pacitors can be destroyed with a very high turn-on surge
current such as would be generated if a low impedance input
source were 鈥渉ot switched鈥?to the charger input. If this
condition can occur, the input capacitor should have the
highest possible voltage rating, at least twice the surge input
voltage if possible. Consult with the capacitor manufacturer
before a final choice is made. A 2.2碌F ceramic capacitor such
as the one used for the coupling capacitor can also be used.
These capacitors do not have a turn-on surge limitation. The
input capacitor must be connected directly to the V
IN
pin and
the ground plane close to the LT1512.
KoolM碌 is a registered trademark of Magnetics, Inc.
Metglas is a registered trademark of AlliedSignal Inc.
7
1
2
3
4
5
6
7
8
9
10
11
12
