1

LT1246/LT1247

1MHz Off-Line

Current Mode PWM

and DC/DC Converter

+

–

+

–

1

2

FEEDBACK

COMPENSATION

2.5V

2R

R

1V

1.5V

–

+

3

BLANKING

1mA

5.6V

4

OSCILLATOR

R

S

18V

UV

LOCKOUT

5

GND

6

OUTPUT

8

5V REF

MAIN BIAS

REFERENCE PULLDOWN

OUTPUT

PULLDOWN

REFERENCE ENABLE

LT1246 • BD01

DESCRIPTIO

S

FEATURE

S

APPLICATI

s

Current Mode Operation to 1MHz

s

30ns Current Sense Delay

s

< 250

µA Low Start-Up Current

s

Current Sense Leading Edge Blanking

s

Pin Compatible with UC1842

s

Undervoltage Lockout with Hysteresis

s

No Cross-Conduction Current

s

Trimmed Bandgap Reference

s

1A Totem Pole Output

s

Trimmed Oscillator Frequency and Sink Current

s

Active Pull-Down on Reference and Output During

Undervoltage Lockout

s

18V High Level Output Clamp

s

Off-Line Converters

s

DC/DC Converters

The LT

mode, pulse width modulators. These devices are de-

signed to be improved plug compatible versions of the

industry standard UC1842 PWM circuit. The LT1246/

LT1247 are optimized for off-line and DC/DC converter

applications. They contain a temperature compensated

reference, high gain error amplifier, current sensing com-

parator, and a high current totem pole output stage ideally

suited to driving power MOSFETs. Start-up current has

been reduced to less than 250

µA. Cross-conduction cur-

rent spikes in the totem pole output stage have been

eliminated, making 1MHz operation practical. Several new

features have been incorporated. Leading edge blanking

has been added to the current sense comparator. This

minimizes or eliminates the filter that is normally required.

Eliminating this filter allows the current sense loop to

operate with minimum delays. Trims have been added to

the oscillator circuit for both frequency and sink current,

and both of these parameters are tightly specified. The

on state. The output and the reference output are actively

pulled low during under-voltage lockout.

, LTC and LT are registered trademarks of Linear Technology Corporation.

I

DAGRA

BLOCK

Minimum

Start-Up

Operating

Maximum

Device

Threshold

Voltage

Duty Cycle

Replaces

LT1246

16V

10V

100%

UC1842

LT1247

8.4V

7.6V

100%

UC1843