Need a part

It sounds like a difficult part to realize -- perhaps that's why?

Speeds, currents, type of load? Does it just need to source current, or does it need to sink it, too?

If your current and speed needs were modest, and an on-off source would work, then a depletion-mode N-channel FET would do, but you'd need to find one.

I know, from re-working Moto/ON-Semi logic to fit a new foundry process, that kind of tri-state is possible.

Source only... ~100mA.

Alternatively I could use a discrete P-channel capable of 10A+ with VGS=4V ...Jim Thompson

How about classic TTL output schematics?

VLV

How about:

back-to-back BSS84 PFETs (S-S), G-G shorted), resistor from S to G, NFET from G to ground.

Cheers

Phil Hobbs

yeh, if it is source only then a diode and any driver that can handle the current/voltage

-Lasse

For RS485:

LTC2862

and a number of other parts similar to this

Cheers

Klaus

Possible, yes, but easy?

I thought of that. But that only works if your output rail isn't going to float much above the input, or if you somehow pull the gate up to the highest of either the output or the input rail. And then it'll want to be slow.

I don't pay much attention to Vgs on FETs, except to know that if it's a

That's what made me think of depletion-mode N-fet: the higher the source gets, the more "off" the thing turns. But your P-channel is probably easier to find.

Thanks! I found a P-channel PowerMOS: CSD25401Q3 (TI) which will do the trick. The customer was playing a charge pump game to drive an NMOS, and I thought I was stuck with their approach, but the PMOS fixes the issue and loses not only a couple of capacitors, and a couple of diodes, but a power supply rail as well ;-) ...Jim Thompson

I'm rail value limited, so the Vbe drop would be a killer. But I found a PMOS solution: CSD25401Q3 (TI) ...Jim Thompson

I found: CSD25401Q3 (TI), solves the problem straight away. But thanks, anyway! ...Jim Thompson

Thanks, Klaus! I'll keep that in my bag of tricks for the next time. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

In the chip world? Trivial. Discrete world, too many components. Though an open drain high-side driver that could do a full tri-state disconnect from rail would have worked. I suspect there's one out there, I'm just not calling it by its right name.

Stuck with it. You know the drill, someone f...ed it up and I get called it to patch it, without changing anything ;-)

Yep. CSD25401Q3 ...Jim Thompson

I'm late to the party, but Micrel has some good switches

Cheers

Thanks, Martin! ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at