I need to be able to switch power to a section of my circuit off and on
under PIC control. I've been looking at analog switches, but they all
have fairly high "on" resistance (tens of ohms or more). The problem is
that I need to provide a nicely regulated 5V at a widely varying current
(fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
resistance will drop the voltage significantly at those high currents.
In a message dated 98-10-23 10:58:16 EDT, you write:
<< I need to be able to switch power to a section of my circuit off and on
under PIC control. I've been looking at analog switches, but they all
have fairly high "on" resistance (tens of ohms or more). The problem is
that I need to provide a nicely regulated 5V at a widely varying current
(fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
resistance will drop the voltage significantly at those high currents.
Any ideas? >>
You may want to consider powering that part of your circuit from a separate
regulator, such as a 7805, and switching the input to this regulator.
> I need to be able to switch power to a section of my circuit off and on
> under PIC control. I've been looking at analog switches, but they all
> have fairly high "on" resistance (tens of ohms or more). The problem is
A P-channel MOSFET should do the job. If size is no object then a TO-220
would be cheap and effective. Otherwise an SO-8 package.
> I need to be able to switch power to a section of my circuit off and on
> under PIC control. I've been looking at analog switches, but they all
> have fairly high "on" resistance (tens of ohms or more). The problem is
> that I need to provide a nicely regulated 5V at a widely varying current
> (fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
> resistance will drop the voltage significantly at those high currents.
I think that you want a VFET. Try a BSS (BSS 170 etc).
There are power supply regulators available with enable/inhibit pins.
Don't have the part numbers to hand at the minute, will look them up
after the weekend (if you don't get sorted before then). Also good
for supplies that need clean, well defined startup characteristics.
You could try switching the INPUT side of a small regulator (78L05?).
A simple PNP transistor and two resistors should do the job nicely.
> I need to be able to switch power to a section of my circuit off and on
> under PIC control. I've been looking at analog switches, but they all
> have fairly high "on" resistance (tens of ohms or more). The problem is
> that I need to provide a nicely regulated 5V at a widely varying current
> (fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
> resistance will drop the voltage significantly at those high currents.
>
> Any ideas?
>
> Dave Johnson
At 07:57 AM 10/23/98 -0700, Dave Johnson wrote...
>I need to be able to switch power to a section of my circuit off and on
>under PIC control. I've been looking at analog switches, but they all
>have fairly high "on" resistance (tens of ohms or more). The problem is
>that I need to provide a nicely regulated 5V at a widely varying current
>(fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
>resistance will drop the voltage significantly at those high currents.
>
>Any ideas?
If you can use an SMT device, the Toko TK11450MTL will do this, it's a 180ma +5
regulator with an on/off control input. Available from Digikey. I'm sure there a
re others.
> > I need to be able to switch power to a section of my circuit off and on
> > under PIC control. I've been looking at analog switches, but they all
> > have fairly high "on" resistance (tens of ohms or more). The problem is
> > that I need to provide a nicely regulated 5V at a widely varying current
> > (fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
> > resistance will drop the voltage significantly at those high currents.
>
> I think that you want a VFET. Try a BSS (BSS 170 etc).
>
> Peter
A highly-saturated medium power PNP transistor in series with the supply
(emitter to unswitched) in a common base configuration works well. Use
a VFET to switch the base to ground (through a resistor). This
configuration only drops a few tenths of a volt.
Or, National Semi make a few switchable low dropout regulators - check
their web site. I've used the LP2957 but it may not satisfy your output
requirements.
> > > I need to be able to switch power to a section of my circuit off and on
> > > under PIC control. I've been looking at analog switches, but they all
> > > have fairly high "on" resistance (tens of ohms or more). The problem is
> > > that I need to provide a nicely regulated 5V at a widely varying current
> > > (fraction of a mA up to 100 mA) when the power is on. Even a 10 ohm
> > > resistance will drop the voltage significantly at those high currents.
> >
> > I think that you want a VFET. Try a BSS (BSS 170 etc).
> >
> > Peter
The neat thing with this BSS VFET is, you need NOTHING to switch 5V with
it. Check the specs, guys, It turns ON in the proper Vgs range for direct
drive from a CMOS output. I don't remember the exact type number. It may
have been 175 ?
> The neat thing with this BSS VFET is, you need NOTHING to switch 5V with
> it. Check the specs, guys, It turns ON in the proper Vgs range for direct
> drive from a CMOS output. I don't remember the exact type number. It may
> have been 175 ?
>
Siliconix makes VFETs also - we use the VN0610LL. You can get them with
or without diode protected inputs (gates).
>> I think that you want a VFET. Try a BSS (BSS 170 etc).
> A highly-saturated medium power PNP transistor in series with the
> supply (emitter to unswitched) in a common base configuration works
> well.
I think you really mean common-emitter.
> Use a VFET to switch the base to ground (through a resistor). This
> configuration only drops a few tenths of a volt.
There's the rub. You can suggest logic VFETs, but are these (readily)
available in P-type?
> Or, National Semi make a few switchable low dropout regulators - check
> their web site. I've used the LP2957 but it may not satisfy your
> output requirements.
Now there's an idea. Lateral thinking; you could even use a
switchable regulator as a relay driver or whatever!
--
Cheers,
Paul B.
Paul B. Webster VK2BZC wrote:
>
> Matt Bonner wrote:
> > A highly-saturated medium power PNP transistor in series with the
> > supply (emitter to unswitched) in a common base configuration works
> > well.
>
> I think you really mean common-emitter.
>
No. Common base. You've forced me into ASCII art
Q1
5v |-----\ /------|5Vswitched
| EBC
1K2 |
|----| Q1 is a medium power PNP (eg: 2N4033)
| Q2 is an N Channel enhancement mode VFET
1K2 The 1.2K resistors ensure high saturation
(only
| drawing current when power is switched on)
|-|
On/Off ---| Q2
|-|
|
|
---
Gnd
That's actually a common emitter circuit IMHO (circuit analysis
shields up) but is a very workable solution.
Zetex make some very nice transistors with very low Vsat (which is
what you need here), Imax of about 2 amps and Beta of several 100 at
currents over 1 amp - these are in their eline packages which are
similar in size to the small TO92 transistors. I use their ZTX749 in
this application.
Some of the regulators in Natsemis LM292x - LM293x series have this
capability
Russell McMahon
From: Jason Bent <.....MBCJABTKILLspam@spam@MAIL1.MCC.AC.UK>
>There are power supply regulators available with enable/inhibit
pins.
>Don't have the part numbers to hand at the minute, will look them up
>after the weekend (if you don't get sorted before then). Also good
> Q1
> 5v |-----\ /------|5Vswitched
> | EBC
> 1K2 |
> |----| Q1 is a medium power PNP (eg: 2N4033)
> | Q2 is an N Channel enhancement mode VFET
> 1K2 The 1.2K resistors ensure high saturation (only
> | drawing current when power is switched on)
> |-|
> On/Off ---| Q2
> |-|
> |
> |
> ---
> Gnd
Ok. I don't get it. Doesn't this more or less give you the disadvantages
of both mosfets AND bipolar transistors? It would appear to be a way of
getting a "high side" driver from a mosfet without needing to worry much
about Vgs issues, but why not use just the mosfet in a low-side
configuration, or just a bipolar (NPN) in a more traditional base-driven
configuration (insufficient gain for high current loads? Use two.)
This has plenty of advantage over a bipolar for Q2(is that what you meant?)
This circuit draws tiny leakage current when off, allows a big range of
voltages to be switched, (the FET can be chosen to take 400 volts if you
want), and has a small power dissipated by Q1 when on.
If you choose Q1 for power dissipation, and heat sink ability, you can get a
soft start/stop by an analog signal to the gate of Q2.
> Ok. I don't get it. Doesn't this more or less give you the disadvantages
> of both mosfets AND bipolar transistors? It would appear to be a way of
> getting a "high side" driver from a mosfet without needing to worry much
> about Vgs issues, but why not use just the mosfet in a low-side
> configuration, or just a bipolar (NPN) in a more traditional base-driven
> configuration (insufficient gain for high current loads? Use two.)
>
Fewer parts and better thermal characteristics using the VFET as a
simple digital switch. I believe the reason I chose a PNP (this is
going back 10 or so years when I first put this together) is the lower
Vce(sat) over NPNs. Moot point anyway - this was before low quiescent
switched output low-dropout linear regulators (which I recommend for
similar applications).
Aside to Russell and Paul. OKay, you caught me, it's common emitter.
Time to get away from embedded code, op-amps, sigma-delta etc and get
back to transistor amplifier design. What's hfe? :-)
man.ac.uk
