CS3845B
CS3845B
Current Mode PWM Control Circuit
with 50% Max Duty Cycle
Description
The CS3845B provides all the neces-
sary features to implement off-line
fixed frequency current-mode control
with a minimum number of external
components.
The CS3845B incorporates a precision
temperature-controlled oscillator to
minimize variations in frequency. An
internal toggle flip-flop, which blanks
the output off every other clock cycle,
ensures that the duty-cycle never
exceeds 50%. The undervoltage lock-
out feature ensures that VREF is stabi-
lized within specification before the
output stage is enabled.
The CS3845B
has been optimized for lower start up
current (500µA max).
Other features include 1% trimmed
band gap reference, pulse-by-pulse
current limiting, and a high-current
totem pole output for driving capaci-
tive loads.
Features
s
Optimized for Off-line Use
s
Temperature
Compensated Oscillator
s
50% Maximum Duty-cycle
Clamp
s
Low Start-up Current
(500µA max)
s
Pulse-by-pulse Current
Limiting
s
Undervoltage Active Pull
Down
s
Double Pulse Suppression
s
1% Trimmed Bandgap
Reference
s
High Current Totem Pole
Output
Absolute Maximum Ratings
Supply Voltage (I
CC
<30mA) ..........................................................Self Limiting
Supply Voltage (Low Impedance Source)...................................................30V
Output Current ...............................................................................................±1A
Output Energy (Capacitive Load) .................................................................5µJ
Analog Inputs (V
FB
, V
SENSE
)...........................................................-0.3V to 5.5V
Error Amp Output Sink Current...............................................................10mA
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
Block Diagram
Package Options
8 Lead PDIP & SO
V
CC
V
CC
Undervoltage Lock-out
V
CC
Pwr
COMP
V
FB
1
2
3
4
8
7
6
5
V
REF
V
CC
V
OUT
Gnd
34V
Gnd
8.4V/7.6V
V
FB
Error
Amplifier
-
+
OSC
2.50V
Set/
5.0 Volt
Reset Reference
Internal
Bias
R
V
REF
Undervoltage
Lockout
Toggle
Flip-Flop
V
REF
Sense
OSC
14 Lead SO Narrow
COMP
1
NC
2
NOR
V
OUT
14
13
12
11
10
9
8
COMP
V
REF
NC
V
CC
V
CC
Pwr
V
OUT
Gnd
Pwr Gnd
R
Oscillator
V
FB
3
2 R
R
Sense
1V
Current PWM
Sensing Latch
Comparator
S
Pwr Gnd
R
NC
4
Sense
5
NC
6
OSC
7
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
Rev. 11/19/98
1
A
®
Company
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
