CS2844/3845 SERIES
CS2844/CS3844
CS2845/CS3845
Current Mode PWM Control Circuit
with 50% Max Duty Cycle
Description
The CS3844/45 provides all the neces-
sary features to implement off-line
fixed frequency current-mode control
with a minimum number of external
components.
The CS3844 family incorporates a new
precision temperature-controlled oscil-
lator to minimize variations in fre-
quency. An internal toggle flip-flop,
which blanks the output every other
clock cycle, limits the duty-cycle range
to less than 50%. An undervoltage
lockout ensures that V
REF
is stabilized
before the output stage is enabled. In
the CS2844/CS3844 turn on occurs at
16V and turn off at 10V. In the
CS2845/CS3845 turn on is at 8.4V and
turn off at 7.6V.
Other features include low start-up
current, pulse-by-pulse current limit-
ing, and a high-current totem pole out-
put for driving capacitive loads, such
as gate of a power MOSFET. The out-
put is low in the off state, consistent
with N-channel devices.
Features
s
Optimized for Offline
Control
s
Temp. Compensated
Oscillator
s
50% Max Duty-cycle Clamp
s
V
REF
Stabilized before
Output Stage is Enabled
s
Low Start-up Current
s
Pulse-by-pulse Current
Limiting
s
Improved Undervoltage
Lockout
s
Double Pulse Suppression
s
1% Trimmed Bandgap Ref.
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
Package Options
8 Lead PDIP & SO Narrow
COMP
V
FB
Sense
OSC
1
2
3
4
8
7
6
5
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
V
REF
V
CC
V
OUT
Gnd
14 Lead SO Narrow
V
CC
V
CC
Undervoltage Lock-out
COMP
1
14
13
12
11
10
9
8
V
REF
NC
V
CC
V
CC
Pwr
V
OUT
Gnd
Pwr Gnd
V
CC
Pwr
NC
2
V
FB
3
34V
Gnd
16V/10V
(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
NC
4
Sense
5
NC
6
OSC
7
COMP
16 Lead SO Wide
NC
1
NC
2
16
15
14
13
12
11
10
9
NC
V
REF
V
CC
V
CCPwr
V
O
Gnd
Pwr Gnd
NC
R
NOR
V
OUT
COMP
3
V
FB
4
Oscillator
2 R
R
Sense
1V
S
Pwr Gnd
R
Current PWM
Sensing Latch
Comparator
SENSE
5
OSC
6
NC
7
NC
8
( ) indicates CS-2845/3845
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. 3/17/99
1
A
®
Company
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
