LT1512 Linear Technology, LT1512 Datasheet - Page 5

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

PIN
BLOCK
be connected directly to the GND S pin or to the ground
plane close to the point where the GND S pin is connected.
V
carrying up to 1.5A of current with fast rise and fall times.
Keep the traces on this pin as short as possible to minimize
OPERATION
The LT1512 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage or current. Referring to the
Block Diagram, the switch is turned “on” at the start of each
oscillator cycle. It is turned “off” when switch current
reaches a predetermined level. Control of output voltage
and current is obtained by using the output of a dual
feedback voltage sensing error amplifier to set switch
current trip level. This technique has the advantage of
simplified loop frequency compensation. A low dropout
internal regulator provides a 2.3V supply for all internal
circuitry on the LT1512. This low dropout design allows
input voltage to vary from 2.7V to 25V. A 500kHz oscillator
SW
U
: The switch pin is the collector of the power switch,
FUNCTIONS
U
DIAGRAM
I
FB
FB
U
1.245V
GND S
REF
5k
U
62k
S/S
W
–
–
–
+
+
DELAY AND RESET
SYNC
I
FBA
EA
SHUTDOWN
500kHz
OSC
LOW DROPOUT
Figure 2
V
2.3V REG
C
COMP
LOGIC
V
IN
radiation and voltage spikes. In particular, the path in
Figure 1 which includes SW to C2, D1, C1 and around to
the LT1512 ground pin should be as short as possible to
minimize voltage spikes at switch turn-off.
is the basic clock for all internal timing. It turns “on” the
output switch via the logic and driver circuitry. Special
adaptive antisat circuitry detects onset of saturation in the
power switch and adjusts driver current instantaneously to
limit switch saturation. This minimizes driver dissipation
and provides very rapid turn-off of the switch.
A unique error amplifier design has two inverting inputs
which allow for sensing both output voltage and current. A
1.245V bandgap reference biases the noninverting input.
The first inverting input of the error amplifier is brought out
for positive output voltage sensing. The second inverting
input is driven by a “current” amplifier which is sensing
output current via an external current sense resistor. The
A
V
6
IA
+
–
DRIVER
ANTI-SAT
GND
SW
SWITCH
0.08
1512 F02
LT1512
5