Switching a low voltage to a load

solid state switch, no relays

Is the 1.2V supply _all_ that you have? Nothing higher? "Digitally" controlled... what signal level(s)? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

What sort of control voltage (and polarity) do you have? Is the entire circuit 1.2 volts?

How about a solid stat relay? digikey lists some that turn on at 1.1 to 1.2 volts... (just enough to turn on a led.)

George H.

If you don't have a voltage greater than 1.2V for the gate, bite the bullet and make one. You shouldn't need any appreciable current, so a capacitive step-up chip should do. I'd go with a logic-level NMOS running common drain.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

I have +3.3V available as my control voltage.

I need to pass/switch +1.2V to my load.

I'm looking at pmos transistors, but I don't think they'll work out for me because I don't have anything lower than ground. Largest Vgs I can supply them is -1.2V

I found this guy:

Looks like he'll do the job. 150mOhm at -1.2V Vgs, not bad.

Much thanks for the replies.

I've seen logic-level FETs that go down to a Vgs of 2.4V -- maybe do some looking and see if there are some 2V Vgs N-channel FETs out there?

3.3 would easily double to over 6V, which would give you more Vgs than you need. For that matter, you could make -2.5V fairly easily at low current.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

I'm just using the nmos to switch the pmos, I can switch the nmos no problem with 3.3V. But it's the pmos that worried me cause I could only hold it at -1.2V. since there is 1.2V at it's source.

I know zero about them, just remember reading an article some time ago. Don't know if they have anything that can do the current you need.

why not use an nmos? if you have a 3.3V control switching a 1.2V source, you have >2V Vgs

120mOhm at vgs 1.8V

-Lasse

low side / high side switch reasons. I don't want to switch ground.

Optical coupled MosFet.

YOu can keep the outside from the inside and it only takes no more than 1.2 volts to turn it on at ~8 mA.

As long as the long isn't that high of course :)

Jamie

if you have 3.3V control for a 1.2V source, you can put the nmos on the high side, you get 3.3V-1.2V = 2.1 Vgs

-Lasse

Absolutely! Much thanks Lasse, I was ignoring nmos. This sounds good to me.

Yep. Just make sure that the source is connected to the load side so that the body diode doesn't provide a sneak path.

I don't know why some manufacturer doesn't turn out some low voltage power MOS with 0.6um gate spacing (limits VDS operating voltage to

7-8V max) _but_ threshold is ~0.6V. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

You're in the right voltage range for the most basic type of blocking oscillator Joule thief - the flyback pulses from that should be sufficient for a bootstrap rail to bias a N-MOSFET.

A logic level MOSFET would probably help - the ones on old PC motherboard fit the bill, Vds can be as low as 20V - 30V is fairly typical, Id ratings of 80A are fairly usual. Datasheets for typical devices may or may not say logic level in so many words - but they're usually in the right ball park.

Which is what I was trying to say a long time ago when I said "Common drain".

It should work a charm.

--
www.wescottdesign.com

Why is a PNP transistor now anathema?

RL