Schottky diode connected to gate drive output of PWM controller

[eem2am]

Hello,

I am doing an offline flyback isolated SMPS of 22W with LT1241 PWM controller

LT1241 DATASHEET:
http://www.datasheetcatalog.org/datasheet/lineartechnology/1241fa.pdf

On page 12 of the LT1241 datasheet, there is a diode D4 connected to pin 6 (gate drive output) of the LT1241.

Diode D4:
http://i56.tinypic.com/egu0j7.jpg

do you know what this diode D4, is for?

 

[FvM]

It's funny, that you are using the chip with the diode since a year (according to your edaborad posts) and are finally asking what it's good for. The question is however answered completely in the datasheet. It's to prevent parasitic substrate transistor action of an - apparently - badly designed chip.

 

[eem2am]

thankyou,

yes, this is my favourite chip as it doesn't have loads of obscure protections that i then have to disable.

I wondered of this problem is only apparent in CMOS based PWM controllers?

e.g. the BJT based UC3843 would not have this problem?

 

[FvM]

Yes, parasitic transistor action and even latchup are mostly known from CMOS devices. But the problems can be handled in chip design.