Package Pin Description
PACKAGE PIN #
PIN SYMBOL
FUNCTION
1
BOOST
The BOOST pin provides additional drive voltage to the on-chip NPN
power transistor. The resulting decrease in switch on voltage increases
efficiency.
2
3
VIN
This pin is the main power input to the IC.
VSW
This is the connection to the emitter of the on-chip NPN power transistor
and serves as the switch output to the inductor. This pin may be subject-
ed to negative voltages during switch off-time. A catch diode is required
to clamp the pin voltage in normal operation.This node can stand −1.0V
for less than 50ns during switch node flyback.
4 (CS51412/ 14)
5 (CS51411/ 13)
BIAS
The BIAS pin connects to the on-chip power rail and allows the IC to
run most of its internal circuitry from the regulated output or another
low voltage supply to improve efficiency. The BIAS pin is left floating if
this feature is not used.
SYNC
This pin provides the synchronization input.
5 (CS51412/ 14)
4 (CS51411/ 13)
SHDNB
The shutdown pin is active low and TTL compatible. The IC goes into
sleep mode, drawing less than 85µA when the pin voltage is pulled
below 1V. This pin should be left floating in normal operation.
6
7
Gnd
VFB
Power return connection for the IC.
The FB pin provides input to the inverting input of the error amplifier. If
VFB is lower than 0.29V, the oscillator frequency is divided by four, and
current limit folds back to about 1 ampere. These features protect the IC
under severe overcurrent or short circuit conditions.
8
VC
The VC pin provides a connection point to the output of the error amplifi-
er and input to the PWM comparator. Driving of this pin should be
avoided because on-chip test circuitry becomes active whenever current
exceeding 0.5mA is forced into the IC.
Block Diagram
SHDNB
SYNC
VIN
5µA
Shutdown
Comparator
2.9V
LDO Voltage
Regulator
+
Thermal
Shutdown
Artificial
Ramp
Oscillator
BIAS
BOOST
-
+
-
1.3V
Output
Driver
S
R
Q
VSW
∑
+
-
Current
Limit
Comparator
PWM
Comparator
IREF
+
1.46V
-
-
-
V
+
FB
IFOLDBACK
+
Frequency
and Current
Limit Foldback
+
Gnd
0.32V
-
+
-
Error
1.270V
Amplifier
VC
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
