Chip options for relay driver

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

First off, I am sorry for this being off topic. That being said...
I am looking for a simular chip to the STPIC6C595. It is like the 74HC595 serial to parallel chip.((But it can drive larger currents)) I want to find one that has the sink pin seperate from the chip ground. This is because I want
to drive sink with a NMOS and PWM. I want to drive it hard for the relays' pull-in current. Then drop off to a hold current. Perhap I can use some constant current driver circuits(but that isn't the problem).

http://www.st.com/stonline/books/pdf/docs/7726.pdf

Does anyone know of a simular chip to STPIC6C595 that has a seperate pin for the drive?

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

How about applying the PWM to the /G line, pin 8? From Fig 3 of the datasheet this pin looks like it should do just what you want.
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treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

I'll give it a try, and let you know how it works. I assumed that doing it through the logic would be too slow. ie I need it fastenough to not have the relays "chatter".

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

By the spec sheet the propogation time from the /G pin to the output is typically 130 nanoseconds, which much faster than any mechanical relay could ever dream of!
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The search for better is endless. Instead simply find very good and get the job done.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

I'm usually driving Hamlin HE3621A0510 relays (DigiKey HE207-ND) directly from PCF9554 (or PCF9555 if I need 16 bits) I2C i/o expander. I can drive only two at a time, though.