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CS3972
1.25A High Efficiency Switching Regulator
Description
The CS3972 is a 1.25A, 60V, current
mode, high efficiency, switching
regulator circuit. It can be config-
ured in buck, boost, forward, iso-
lated and non-isolated topologies,
using a single-ended switch.
The IC contains a 1.25A high effi-
ciency output switch, precision
bandgap reference, voltage regula-
tor, error amplifier, oscillator, con-
trol, and protection circuitry.
Output transistor efficiency is
achieved by keeping the switch in
quasi-saturation when it is on. This
minimizes the switch turn-off
delay time and power dissipation,
an important consideration for
applications with wide ranging
loads.
The precision reference is internally
trimmed to 1.24V, while the oscilla-
tor frequency is internally trimmed
to 40kHz. The on board 2.3V regu-
lator powers the internal circuitry.
By keeping the voltage regulator at
2.3V, the IC is able to operate down
to 3V and maintain a constant input
supply current of 6mA(typ).
Synchronization mode is achieved
by applying a pulsating VC voltage
of less than 0.2V with a duration of
about 0.5µs to VC pin. The switch-
ing frequency can be adjusted from
50kHz to 70kHz. The IC's synchro-
nization mode is set externally.
Protection circuitry includes a vari-
able current limit with external
voltage clamp, under-voltage and
over-temperature protection.
Block Diagram
VIN
2.3V
Reg
40kHz
OSC
Logic
Output
Protection
Driver
VSW
1.25A,
75V
Switch
Features
s S75wViMtcAhX, 2APEAK Internal
s 3V to 60V Input Voltage
Range
s 6mA Input Supply
Current
s 1.24V Precision Reference
s 40kHz Switching
Frequency
s Variable Current Limit
with External Voltage
Clamp
s Under-Voltage Protection
s Over-Temperature
Protection
s External Synchronization
Package Options
16L SO Wide
NC 1
VC
VFB
NC
NC
NC
NC
NC
Gnd
NC
E2
VSW
NC
E1
NC
VIN
COMP
+-
VC
VFB -
Error
Amp
+
1.24V
Ref
+
Current
Amp
-
0.22
0.22
*Always connect E1 to Ground Pin on DIP package. Both emitters are tied to ground on TO-220 Package.
Rev. 6/23/99
1
8L PDIP
5L TO-220
Tab (Gnd)
E2*
E1*
Gnd
Gnd 1
VC
VFB
NC
E2
VSW
E1
VIN
1
1 VC
2 VFB
3 Gnd
4 VSW
5 VIN
Cherry Semiconductor Corporation
2000 South County Trail, East Greenwich, RI 02818
Tel: (401)885-3600 Fax: (401)885-5786
Email: info@cherry-semi.com
Web Site: www.cherry-semi.com
A Company
®

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Absolute Maximum Ratings
Input Voltage Range ........................................................................................................................................................3V to 60V
Output Switching Voltage.........................................................................................................................................................75V
Voltage Transient (tr=10µs, tf=400ms) ......................................................................................................................................60V
ESD (Human Body Model; R=1.5k, C=100pF) ...................................................................................................................2kV
Junction Temperature Range...............................................................................................................................-40°C to +150°C
Storage Temperature Range ................................................................................................................................-65°C to +150°C
Lead Temperature Soldering
Wave Solder (through hole styles only)......................................................................................10 sec. max, 260°C peak
Reflow (SMD styles only) ......................................................................................60 sec. max above 183°C, 230°C peak
Feedback Pin Voltage (1ms transient)...................................................................................................................................±15V
Electrical Characteristics: For 3VVCC60V, -40°CTA125°C unless otherwise specified.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
s
Reference Voltage
Line Regulation
Feedback Input Current
Minimum Input Voltage
Supply Current
Supply Current Increase
during switch on time
Switching Frequency
Maximum Duty Cycle
VC Threshold
VC=0.8V; VFB=VREF
VC=0.8V; VFB=VREF, 3VVIN60V
VC=0.8V; VFB=VREF
VC=VFB=VREF
VC=0.6V; 3VVIN60V
VC=open, 1.0VVFB1.5V
VC=VFB=VREF; VSW=15V
VC=2V; VFB=open, VSW=15V
VFB=open; VSW =15V,
duty cycle=0%
1.214
33
0.60
1.244
350
2.6
6.0
25
40
92
0.90
1.274
0.03
1100
3.0
9.0
47
1.25
s Error Amplifier
Transconductance
Source/Sink Current
High Voltage Clamp
Low Voltage Clamp
Gain
VC=0.8V; VFB=VREF, IC=±25µA
VC=1.5V
VFB=1.0V
VFB=1.5V
0.9VVC1.4V, VFB=VREF
s Output Switch
Breakdown Voltage
On Resistance
Control Voltage to Switch
Current Transconductance
Current Limit
VC=0.5V; 3VVIN60V, IOUT=5mA
VC=open, VFB=0.8V, IOUT=1.25A
1.2VVC1.8V
Duty Cycle50%, TJ25°C
Duty Cycle50%, TJ<25°C
Duty Cycle=80%
Duty Cycle=80% (Note 1)
2400
120
4400
200
2.0
0.38
800
7000
400
75 90
0.6 1.0
2
1.25 3.00
1.25 3.50
1.00 2.50
0.50 2.50
Note 1: 16 lead SO wide package only.
UNIT
V
%/V
nA
V
mA
mA/A
kHz
%
V
µmho
µA
V
V
V/V
V
A/V
A
A
A
A
2

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PACKAGE PIN #
16L 5L
SO Wide TO-220
8L
PDIP
16 3
1
21
2
3
1,4,5,6,7,8,
10,12,15
9
11
2
5
3
4
5
6
13 4
14
7
8
Package Pin Description
PIN SYMBOL
FUNCTION
Gnd
VC
VFB
NC
VIN
E1
VSW
E2
Logic and power ground on TO-220. Logic ground only on 8L
PDIP.
Control voltage can be used for frequency compensation, cur-
rent limit threshold, frequency synchronization, and soft start.
Feedback voltage is the negative input to the error amplifier.
No connection.
Supply input voltage with a range of 3V to 60V.
Emitter One is a power ground which should always be con-
nected to logic ground since it is used as the negative input to
the current sense amp.
Collector of internal 75V NPN power switch.
Emitter Two is a power ground which can be left disconnected
to reduce the switch rating to 625mA.
Reference Voltage vs. Temperature
1.274
1.264
1.254
1.244
1.234
1.224
1.214
-50 -25
-0 25 50 75 100 125 150
Temperature (°C)
Typical Performance Characteristics
Switching Frequency vs. Temperature
47
45
43
41
39
37
35
33
-50 -25
-0 25 50 75 100 125 150
Temperature (°C)
Driver Current vs. Switch Current
100
90
80
70
60
50
40
30
20
10
0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Switch Current (A)
Maximum Duty Cycle
97
96
95
93
92
91
90
-50 -25
-0 25 50 75 100 125 150
Junction Temperature (°C)
Switch Saturation Voltage
1.6
Tj=25°C
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.0
Switch Current (A)
3

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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 (0.11) 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.
VIN L1
Application Diagrams
D1
VOUT
VIN
Q1
L1 VOUT
R2 R1
R3
VSW
VIN
CS3972
VFB
+
C1
VC
R1
Gnd
C2
(1.24V)
+
C3
R3
+
C1
VSW
VIN
CS3972
VFB
VC Gnd
R2
C2
D1
(1.24V)
+
C3
R4
Figure 1: Boost Regulator
Figure 2: Buck Regulator with the External Switch
VIN TR1
C4
R1
C1
VSW
VIN
VFB
CS3972
+
C2
VC
R2
Gnd
C3
VOUT1
+
C5
VOUT2
+
C6
R4
(1.24V)
R5
+VIN
D1
TR1
L1
D2
VOUT
+
C3
D3
+
C2
–VIN
VSW
VIN
CS3972
R1
VC Gnd
VFB
R2
C3
1/2
OC1
D1 R3
1/2 R4
OC1
R5
C5
R6
C4
U1
R7
Figure 3: Flyback Multiple Output Regulator
Figure 4: Isolated Forward Regulator
4

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Package Specification
PACKAGE DIMENSIONS IN mm (INCHES)
PACKAGE THERMAL DATA
Lead Count
8 Lead PDIP
16 Lead SO Wide
Metric
Max Min
10.16 9.02
10.50 10.10
D
English
Max Min
.400 .355
.413 .398
Thermal Data
RΘJC
RΘJA
typ
typ
5L
TO-220
1.7
50
8L
PDIP
52
100
16L
SO Wide
23
105
˚C/W
˚C/W
Plastic DIP (N); 300 mil wide
7.11 (.280)
6.10 (.240)
3.68 (.145)
2.92 (.115)
8.26 (.325)
7.62 (.300)
1.77 (.070)
1.14 (.045)
2.54 (.100) BSC
.356 (.014)
.203 (.008)
REF: JEDEC MS-001
0.39 (.015)
MIN.
.558 (.022)
.356 (.014)
D
Some 8 and 16 lead
packages may have
1/2 lead at the end
of the package.
All specs are the same.
Surface Mount Wide Body (DW); 300 mil wide
7.60 (.299)
7.40 (.291)
10.65 (.419)
10.00 (.394)
0.51 (.020)
0.33 (.013)
2.49 (.098)
2.24 (.088)
1.27 (.050) 0.32 (.013)
0.40 (.016) 0.23 (.009)
REF: JEDEC MS-013
5
D
1.27 (.050) BSC
2.65 (.104)
2.35 (.093)
0.30 (.012)
0.10 (.004)