1

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

Package Options

Current Mode PWM Control Circuit

with 50% Max Duty Cycle

CS3845B

Description

Block Diagram

Absolute Maximum Ratings

1

COMP

2

3

4

Sense

OSC

Gnd

8

7

6

5

8 Lead PDIP & SO

10

7

14

13

12

8

1

2

3

4

5

6

11

9

COMP

NC

NC

Sense

NC

OSC

NC

Pwr Gnd

Gnd

14 Lead SO Narrow

December, 2001 - Rev. 7

ON Semiconductor

2000 South County Trail, East Greenwich, RI 02818

Tel: (401)885–3600 Fax: (401)885–5786

N. American Technical Support: 800-282-9855

Web Site: www.cherry–semi.com

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.

Supply Voltage (Low Impedance Source)...................................................30V

Output Current ...............................................................................................±1A

Output Energy (Capacitive Load) .................................................................5µJ

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

Gnd

COMP

OSC

Sense

Undervoltage

Lockout

Internal

Bias

S

R

PWM

Latch

Current

Sensing

Comparator

R

2 R

1V

Error

Amplifier

-

+

2.50V

Set/

Reset

34V

R

R

Pwr Gnd

Oscillator

Toggle

Flip-Flop

5.0 Volt

Reference

8.4V/7.6V

NOR