LT1512 Linear Technology, LT1512 Datasheet - Page 7

• Manufacturer Part Number: LT1512
• Description: SEPIC Constant-Current/ Constant-Voltage Battery Charger
• Manufacturer: Linear Technology

APPLICATIONS
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 – V
is less. Maximum V
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 A. 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 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 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
BAT
U
= 40V – V
SHUTDOWN
INFORMATION
THAN 3.5V. Q1 WILL NOT BE TURNED OFF
U
CONNECT D2 ANODE HERE FOR FULLY
CHARGED BATTERY VOLTAGE LESS
IN SHUTDOWN IF V
IN
.
W
S/S
LT1512
GND
V
BAT
IN
IN
Figure 3. Eliminating Divider Current
IS PRESENT
•
, whichever
U
V
L1 A
SW
FB
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
C2
STILL ACTIVE
loss toroid core such as KoolM
is recommended. Series resistance should be less than
0.1 for each winding. “Open core” 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 H and less than 0.2A with
L = 15 H. A low ESR 22 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 “hot switched” 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 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
the ground plane close to the LT1512.
KoolM is a registered trademark of Magnetics, Inc.
Metglas is a registered trademark of AlliedSignal Inc.
R3
L1 B
1N4148
D2
D1
R1
R2
Q1
2N7002
C6
470pF
+
1512 F03
R5
470k
®
, Molypermalloy or Metglas
LT1512
IN
pin and
7
®