4/90 Rev 1.1 2/94

Copyright

© 1994

11861 Western Avenue

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(714) 898-8121

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SG1540/SG2540/SG3540

OFF-LINE START-UP CONTROLLER

FEATURES

••••• Useable with primary and secondary side PWM

controllers

••••• Micropower comparator / switch

- Internal 2.5V bandgap reference

- 50mA power switch

••••• Squarewave oscillator

- 500Hz to 200KHz operation

- 200mA totem pole outputs

••••• Eliminates bulky, expensive 50/60 Hz transformer

••••• Minimizes high voltage bleeder current

••••• Programmable start-up voltage and hysteresis

••••• Internal and programmable overvoltage crowbar

latch

••••• Available in 8 pin DIP, 10 pin flat pack, and 16 pin

widebody SOIC

HIGH RELIABILITY FEATURES - SG1540

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♦ Available to MIL-STD - 883

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♦ LMI level "S" processing available

DESCRIPTION

The SG1540 is an integrated circuit designed to efficiently provide

start-up power from a high-voltage DC bus to a PWM control circuit

in a switching power supply. When used on the primary side, it

reduces start-up current to less than 1mA and allows any standard

PWM control circuit to be used as a primary-side controller. When

used to power a controller on the secondary side, it efficiently

eliminates the need for a heavy 50/60Hz line transformer with its

associated low frequency magnetic fields.

The circuit consists of three sections: a micropower bandgap

comparator/power switch referenced to 2.5 volts which isolates the

start-up capacitor from its load; a high frequency square-wave

oscillator with 200mA totem-pole output for driving an isolation

transformer; and a second bandgap comparator with latching

crowbar to protect against overvoltage faults while starting or

running.

The SG1540 is specified for operation over the full military ambient

temperature range of -55°C to 125°C. The SG2540 is character-

ized for the industrial range of -25°C to 85°C, and the SG3540 is

designed for the commercial range of 0°C to 70°C.

BLOCK DIAGRAM