LT1512
5
1512fc
For more information www.linear.com/LT1512
pin FuncTions
V
C
: The compensation pin is primarily used for frequency
compensation, but it can also be used for soft starting and
current limiting. It is the output of the error amplifier and
the input of the current comparator. Peak switch current
increases from 0A to 1.8A as the V
C
voltage varies from
1V to 1.9V. Current out of the V
C
pin is about 200µA when
the pin is externally clamped below the internal 1.9V clamp
level. Loop frequency compensation is performed with a
capacitor or series RC network from the V
C
pin directly to
the ground pin (avoid ground loops).
FB: The feedback pin is used for positive output voltage
sensing. This pin is the inverting input to the voltage
error amplifier. The R1/R2 voltage divider connected to
FB defines Li-Ion float voltage at full charge, or acts as a
voltage limiter for NiCd or NiMH applications. Input bias
current is typically 300nA, so divider current is normally
set to 100µA to swamp out any output voltage errors due
to bias current. The noninverting input of this amplifier is
tied internally to a 1.245V reference. The grounded end of
the output voltage divider should be connected directly to
the LT1512 ground pin (avoid ground loops).
I
FB
: The current feedback pin is used to sense charging
current. It is the input to a current sense amplifier that
controls charging current when the battery voltage is below
the programmed voltage. During constant-current opera
-
tion, the I
FB
pin regulates at –100mV. Input resistance of
this pin is 5k, so filter resistance (R4, Figure 1) should be
less than 50. The 24, 0.22µF filter shown in Figure 1 is
used to convert the pulsating current in the sense resistor
to a smooth DC current feedback signal.
S/S: This pin can be used for shutdown and/or synchro
-
nization. It is logic level compatible, but can be tied to V
IN
if desired. It defaults to a high ON state when floated. A
logic low state will shut down the charger to a micropower
state. Driving the S/S pin with a continuous logic signal of
600kHz to 800kHz will synchronize switching frequency
to the external signal. Shutdown is avoided in this mode
with an internal timer.
V
IN
: The input supply pin should be bypassed with a
low ESR capacitor located right next to the IC chip. The
grounded end of the capacitor must be connected directly
to the ground plane to which the GND pin is connected.
GND S, GND: The LT1512 uses separate ground pins for
switch current (GND) and the control circuitry (GND S).
This isolates the control ground from any induced volt
-
age created by fast switch currents. Both pins should be
tied directly to the ground plane, but the external control
circuit components such as the voltage divider
, frequency
compensation network and I
FB
bypass capacitor should 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
SW
: The switch pin is the collector of the power switch,
carrying up to 1.5A of current with fast rise and fall times.
Keep the traces on this pin as short as possible to mini
-
mize 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.
