Sanjeev,

I think I understand what you are getting at with the "power supply slew rates". You are concerned about the separate power supplies powering up_and_down_at_different_times(sorry_my_keyboard_space_key_is_malfunctioning).

I,have,designed,only,a,few,systems,involving,multiple,power,supplies,and,I,didn't,worry,about,that.,,The,circuits,still,work,but,in,retrospect,I,think,I,should,have,designed,in,some,protection.,,If,one,supply,comes,up,or,goes,down,at,a,significantly,different,time,than,the,other,it,is,possible,to,trigger,latchup.,,Very,large,bypass,capacitors,are,one,example,of,something,that,can,contribute,to,different,power,supply,rise,and,fall,times.

In,many,cases,I,bet,the,protection,structures,on,pins,are,able,to,drain,the,power,supply,capacitors,without,triggering,latchup.

There,are,several,approaches,to,take.,,One,is,to,force,one,supply,to,track,another,perhaps,by,connecting,them,together,with,a,diode(make,the,higher,voltage,track,the,lower,by,connecting,anode,to,lower,supply,and,cathode,the,higher,supply).,,You,can,also,insert,resistors,between,any,devices,that,are,powered,from,separate,supplies.,,The,resistors,are,sized,to,limit,the,current,(when,one,supply,is,on,and,the,other,is,off),to,below,the,absolute,maximum,current,rating,for,the,given,pins.,,Below,the,absolute,maximum,current,latchup,will,not,occur.

Sorry,about,the,keyboard,problem!

-Lee