CS3972 Datasheet, PDF (4/6 Pages) Cherry Semiconductor Corporation – 1.25A High Efficiency Switching Regulator

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.
V_IN
L1
Application Diagrams
D1
V_OUT
V_IN
Q1
L1
V_OUT
R₂
R₁
R₃
V_SW
V_IN
CS3972 ^VFB
V_C
Gnd
+
C₁
R₁
C₂
(1.24V)
+
C₃
R₃
V_SW
V_IN
CS3972 ^VFB
+
C₁
V_C
R₂
C₂
Gnd
D₁
(1.24V)
+
C₃
R₄
Figure 1: Boost Regulator
Figure 2: Buck Regulator with the External Switch
V_IN
TR1
C₄
R₁
C₁
V_SW
V_IN
V_FB
CS3972
V_C
Gnd
+
C_{2 R2}
C₃
V_OUT1
+
C₅
V_OUT2
+
C₆
R₄
(1.24V)
R₅
+V_IN
D₁
TR1
L₁
D₂
V_OUT
+
C₃
D₃
+
C₂
–V_IN
V_SW
V_IN
CS3972 ^VFB
R₁
V_C Gnd
R₂
C₃
1/2
OC1
D₁
R₃
C₅
1/2 ^R4
OC1
R₅
R₆
C₄
U₁
R₇
Figure 3: Flyback Multiple Output Regulator
Figure 4: Isolated Forward Regulator
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