CS2844/CS3844
CS2845/CS3845
Current Mode PWM Control Circuit
with 50% Max Duty Cycle
Description
Features
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 VREF 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.
Absolute Maximum Ratings
Supply Voltage (ICC<30mA).........................................................................Self Limiting
Supply Voltage (Low Impedance Source) .................................................................30V
Output Current..............................................................................................................±1A
Output Energy (Capacitive Load) ................................................................................5µJ
Analog Inputs (VFB, VSENSE) .........................................................................-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
VCC
Gnd
VFB
COMP
OSC
Sense
VCC Undervoltage Lock-out
34V
16V/10V
(8.4V/7.6V)
Error
Amplifier
-
+
2.50V
Oscillator
Set/ 5.0 Volt
Reset Reference
R
VREF
Undervoltage
Lockout
R
Toggle
Flip-Flop
NOR
Internal
Bias
2R
R
S
R
1V
Current PWM
Sensing Latch
Comparator
( ) indicates CS-2845/3845
VCC Pwr
VREF
V O UT
Pwr Gnd
s Optimized for Offline
Control
s Temp. Compensated
Oscillator
s 50% Max Duty-cycle Clamp
s VREF 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
Package Options
8 Lead PDIP & SO Narrow
COMP 1
VFB 2
Sense 3
OSC 4
8 VREF
7 VCC
6 VOUT
5 Gnd
14 Lead SO Narrow
COMP 1
NC 2
14 VREF
13 NC
VFB 3
NC 4
Sense 5
NC 6
12 VCC
11 VCC Pwr
10 VOUT
9 Gnd
OSC 7
8 Pwr Gnd
16 Lead SO Wide
NC 1
16 NC
NC 2
COMP 3
VFB 4
SENSE 5
OSC 6
NC 7
15 VREF
14 VCC
13 VCCPwr
12 VO
11 Gnd
10 Pwr Gnd
NC 8
9 NC
Rev. 3/17/99
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
1 A ® Company
