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Switching Negative Supply line using Positive control - RF Cafe Forums

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jom

Post subject: Switching Negative Supply line using Positive control Posted: Mon Aug 27, 2007 4:52 pm

Captain

Joined: Fri Jan 26, 2007 2:40 am

Posts: 14

OK, I've got this problem where I have to turn be able to control power to sections of my board during TX or RX mode. For example lets say I've got a +5V supply that runs to some Op Amps. Some are used in TX mode and some are used in RX mode but that they aren't on all at the same time. We've used a control line (coming from a PIC, 0 to 3 V) to drive a NMOS Fet which in turn drives current thru a gate resistor of a PMOS device to either open or short the PMOS to connect the +5V line to the Op Amps. We repeat this circuit for TX and RX.

That works fine for a +5V but what about the -5V? The first problem is the control line, which is too high a voltage to switch a Fet. I obviously have to level shift but I can't seem to get any fet to drive completely ON since I also have the problem in that I don't have access to a voltage lower than -5V.

I've tried to run a search on the net for this but I keep coming up with "Switching power supplies" or some such. Any idea what this kind of problem is called? That might help my search.

Thanks

jom

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madengr

Post subject: Posted: Tue Sep 11, 2007 5:14 pm

Captain

Joined: Fri Feb 02, 2007 5:22 pm

Posts: 5

Location: Overland Park, KS

How about 3V uC drives gate of open collector inverter or buffer.

Buffer drives gate of PMOS. Drain of PMOS is at 5V. Gate of PMOS pulled to drain via 1k.

Source of PMOS tied to gate of NMOS. Source of NMOS tied to -5V. Gate of NMOS pulled down to source via 1k. Drain of NMOS drives load.

I have used this configuration to gate-switch large GaAs FETs and it works well.

Posted  11/12/2012

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