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UC3845BDG

High Performance Current Mode Controllers

ON Semiconductor 

UC3844B, UC3845B,

UC2844B, UC2845B

High Performance

Current Mode Controllers

The UC3844B, UC3845B series are high performance fixed frequency

current mode controllers. They are specifically designed for Off−Line

and dc−dc converter applications offering the designer a cost−effective

solution with minimal external components. These integrated circuits

feature an oscillator, a temperature compensated reference, high gain

error amplifier, current sensing comparator, and a high current totem pole

output ideally suited for driving a power MOSFET.

Also included are protective features consisting of input and reference

undervoltage lockouts each with hysteresis, cycle−by−cycle current

limiting, a latch for single pulse metering, and a flip−flop which blanks

the output off every other oscillator cycle, allowing output deadtimes to

be programmed from 50% to 70%.

These devices are available in an 8−pin dual−in−line and surface

mount (SOIC−8) plastic package as well as the 14−pin plastic surface

mount (SOIC−14). The SOIC−14 package has separate power and

ground pins for the totem pole output stage.

The UCX844B has UVLO thresholds of 16 V (on) and 10 V (off),

ideally suited for off−line converters. The UCX845B is tailored for

lower voltage applications having UVLO thresholds of 8.5 V (on) and

7.6 V (off).

Features

• Trimmed Oscillator for Precise Frequency Control

• Oscillator Frequency Guaranteed at 250 kHz

• Current Mode Operation to 500 kHz Output Switching Frequency

• Output Deadtime Adjustable from 50% to 70%

• Automatic Feed Forward Compensation

• Latching PWM for Cycle−By−Cycle Current Limiting

• Internally Trimmed Reference with Undervoltage Lockout

• High Current Totem Pole Output

• Undervoltage Lockout with Hysteresis

• Low Startup and Operating Current

• Pb−Free Packages are Available

VCC 7(12)

Vref

8(14)

RT/CT

4(7)

Voltage

Feedback

Input

2(3)

Output/

Compensation

1(1)

5.0V

Reference

R

Vref

R

Undervoltage

Lockout

Oscillator

Latching

PWM

Error

Amplifier

VCC

Undervoltage

Lockout

GND 5(9)

Pin numbers in parenthesis are for the D suffix SOIC−14 package.

Figure 1. Simplified Block Diagram

VC

7(11)

Output

6(10)

Power

Ground

5(8)

Current

Sense Input

3(5)

© Semiconductor Components Industries, LLC, 2005

1

November, 2005 − Rev. 4

http://onsemi.com

8

1

14

1

8

1

PDIP−8

N SUFFIX

CASE 626

SOIC−14

D SUFFIX

CASE 751A

SOIC−8

D1 SUFFIX

CASE 751

PIN CONNECTIONS

Compensation 1

Voltage Feedback 2

8 Vref

7 VCC

Current Sense 3

6 Output

RT/CT 4

5 GN

D

(Top View)

Compensation 1

NC 2

Voltage Feedback 3

NC 4

Current Sense 5

NC 6

RT/CT 7

14 Vref

13 NC

12 VCC

11 VC

10 Output

9 GND

8 Power Ground

(Top View)

ORDERING INFORMATION

See detailed ordering and shipping information in the package

dimensions section on page 15 of this data sheet.

DEVICE MARKING INFORMATION

See general marking information in the device marking

section on page 16 of this data sheet.

Publication Order Number:

UC3844B/D

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