Post subject: Switching Negative Supply line using Positive
control Posted: Mon Aug 27, 2007 4:52 pm
Joined: Fri Jan 26, 2007 2:40 am
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.
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.
Post subject: Posted: Tue Sep 11,
2007 5:14 pm
Joined: Fri Feb 02, 2007
Location: Overland Park, KS
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.