CS3972
Circuit Description
The CS3972 has an on board, high current NPN transistor
in its output stage. The output transistor is switched on
every 25µs by the on board 40 kHz oscillator. The output
is switched off as soon as the output current reaches a
dynamically controlled level. That level is determined by
three factors: the current drawn by the load, the input
voltage, and the output voltage. This control scheme has
several benefits. The IC reacts quickly to overvoltage and
short circuit conditions and it is less sensitive to input
voltage fluctuation.
Current drawn by the load influences when the output
stage turns off. The voltage across the internal high preci-
sion resistor (0.11Ω) in the emitter of the output transistor
generating the input to the current sensing amplifier. The
output of the current sensing amplifier provides one of the
inputs to the comparator.
The other input to the comparator connected to the out-
put of the error amplifier whose inverting input is con-
nected to the output voltage through the feedback circuit-
ry while its noninverting input is set to an internal refer-
ence voltage of 1.24V. When the current sensing amplifier
output exceeds the error amplifier output, the comparator
switches off, triggering the logic which turns off the out-
put stage.
All on board amplifier, logic and protection circuitry uses
the internal 2.3V reference to minimize quiescent current
drain (6mA) and permits IC operation down to 3V.
The anti-saturation circuitry minimizes output switch
turn off time and power dissipation, by keeping the
power NPN at the edge of saturation.
Application Diagrams
V
IN
L1
D1
V
OUT
V
IN
L1
V
OUT
Q1
R
2
R
1
R
3
V
SW
V
IN
V
SW
V
IN
CS3972
V
C
+
C
1
R
1
C
2
Gnd
V
FB
(1.24V)
+
C
3
+
C
1
V
C
R
2
CS3972
Gnd
V
FB
(1.24V)
+
C
3
R
4
R
3
D
1
C
2
Figure 1: Boost Regulator
Figure 2: Buck Regulator with the External Switch
V
OUT1
V
IN
C
4
R
1
C
1
+
C
6
V
OUT2
D
3
C
5
1/2
OC1
R
4
(1.24V)
C
2
+
R
1
V
SW
R
4
R
5
R
6
C
4
TR1
+
+V
IN
C
5
D
1
TR1
+
D
2
C
3
L
1
V
OUT
V
IN
V
SW
V
FB
V
IN
CS3972
V
C
Gnd
V
FB
CS3972
V
C
+
C
2
R
2
C
3
Gnd
R
5
–V
IN
R
2
C
3
D
1
1/2
OC1
R
3
U
1
R
7
Figure 3: Flyback Multiple Output Regulator
Figure 4: Isolated Forward Regulator
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
