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CS-52845
(Rev.:1997)

Current Mode PWM Control Circuit with 50% Max Duty Cycle

Cherry semiconductor 

CS-52845

Current Mode PWM Control Circuit

with 50% Max Duty Cycle

Description

The CS-52845 provides all the neces-

sary features to implement off-line

fixed frequency current-mode control

with a minimum number of external

components.

The CS-52845 incorporates a new pre-

cision temperature-controlled oscilla-

tor to minimize variations in frequen-

cy. An internal toggle flip-flop, which

blanks the output every other clock

cycle, limits the duty-cycle range to

less than 50%. An undervoltage lock-

out ensures that VREF is stabilized

before the output stage is enabled. In

the CS-52845 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

Reflow (SMD styles only) ...........60 sec. max above 183¡C, 230¡C peak

Block Diagram

VCC

VCC Undervoltage Lock-out

34V

Gnd

VFB

COMP

OSC

Sense

8.4V/7.6V

Set/ 5.0 Volt

Reset Reference

Error

Amplifier

2.50V

Oscillator

VREF

R Undervoltage

Lockout

R

Toggle

Flip-Flop

2R

R

S

R

1V

Current PWM

Sensing Latch

Comparator

NOR

Internal

Bias

VCC Pwr

VREF

VOUT

Pwr Gnd

Features

s Optimized for Off-line

Control

s Temperature

Compensated Oscillator

s 50% Maximum 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

Reference

s High Current Totem Pole

Output

Package Options

8L SO Narrow

COMP 1

VFB 2

Sense 3

OSC 4

8 VREF

7 VCC

6 VOUT

5 Gnd

14L SO Narrow

COMP 1

NC 2

VFB 3

NC 4

Sense 5

NC 6

OSC 7

14 VREF

13 NC

12 VCC

11 VCC Pwr

10 VOUT

9 Gnd

8 Pwr Gnd

Cherry Semiconductor Corporation

2000 South County Trail

East Greenwich, Rhode Island 02818-1530

Tel: (401)885-3600 Fax (401)885-5786

email: info@cherry-semi.com

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