Datasheet

LT1512
8
1512fb
For more information www.linear.com/LT1512
adequate drive to the transistor (even when the V
IN
voltage
is at its lowest operating point of 2.4V), the FET gate is
driven wth a peak detected voltage via D2. Note that there
are two connections for D2. The L1 A connection must be
used if the voltage divider is set for less than 3.5V (fully
charged battery). Gate drive is equal to battery voltage
plus input voltage. The disadvantage of this connection is
that Q1 will still be “on” if the V
IN
voltage is active and the
charger is shut down via the S/S pin. The L1 B connection
allows Q1 to turn off when V
IN
is off or when shutdown is
initiated, but the reduced gate drive (= V
BAT
) is not adequate
to ensure a Q1 on-state for fully charged battery voltages
less than 3.5V. Do not substitute for Q1 unless the new
device has adequate V
GS
maximum rating, especially if
D2 is connected to L1A. C6 filters the gate drive and R5
pulls the gate low when switching stops.
Disconnecting the divider leaves only D1 diode leakage
as a battery drain. See Diode Selection for a discussion
of diode leakage.
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 out
-
put 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
BAT
, whichever
is less. Maximum V
BAT
= 40V – 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 12µA. 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 33µH, which gives about 0.25A peak-to-
peak inductor current. Lower values will give higher ripple
current, which reduces maximum charging current. 15µH
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 loss toroid core such as KoolMµ, Molypermalloy or
Metglas 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 electronics close to the charger.
Input Capacitor
The SEPIC topology has relatively low input ripple current
compared to other topologies and higher harmonics are
applicaTions inForMaTion
Figure 3. Eliminating Divider Current
LT1512
V
IN
L1 A
L1 B
GND
FB
1512 F03
V
SW
SHUTDOWN
D2
1N4148
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
CONNECT D2 ANODE HERE FOR FULLY
CHARGED BATTERY VOLTAGE LESS
THAN 3.5V. Q1 WILL NOT BE TURNED OFF
IN SHUTDOWN IF V
IN
IS PRESENT
S/S
C2
C6
470pF
•
R1
R5
470k
R2
R3
Q1
2N7002
+
D1