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'[EE] FET question'
I have a circuit with a MOSFET driving a RCM4400W (A Rabbit module
with wifi, that need 80mA on startup and 450mA when the wifi module is on),
and it is not working.
The power supply is fine, I think, because if I connect the Rabbit
directly to the PSU it works.
The mosfet seems to be working, because I have 3.3V on the Rabbit
terminals when it is on.
The FET is driven with a PIC pin with a 5K6 resistor to gate.
The source of the FET is to GROUND and the drain is connected to the
NEGATIVE terminal of the Rabbit. The POSITIVE terminal of the rabbit is
directly connected to 3.3V.
There is nothing else in the circuit, shouldn't it be working?
The FET is an FDV305N, 0.9A and the Vgsth is 1.5V.
Microchip Design Partner
Although switching the ground side of the Rabbit might not be how I
would do it, it is probably OK. But only if you make sure that you are
not driving any of the Rabbit pins with 0 volts when the Rabbit is
unpowered. This can cause latchup and also other weird stuff.
I have found that chips are more sensitive to latchup when driven
negative than positive. But that is just my personal observation.
An example of weird stuff that isn't latchup is an LM324 opamp when
inputs are driven below the negative supply, it creates huge output
On Thu, 24 Jul 2008 18:50:41 -0300, "Mauricio Jancic"
<janso.com.ar> said: listas
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A few questions:
1) Besides 3.3V and the mosfet drain, what else is connected to the
rabbit module? (I suspect there may be some kind of ground leakage
2) What voltage is your PIC running at? (to determine how it is
driving the FET gate) I assume that the PIC shares the same ground as
3) Have you tried measuring the voltage across the rabbit module's
power input (pos to neg) when the FET is turned on? (trying to
determine whether there is a static on-resistance problem with the
4) Does the rabbit module have large capacitors on its input or a
switching power supply which may have a high inrush current? (This may
cause the voltage to drop sharply at startup, which may in turn
trigger the module to shut itself down immediately)
Also, I'd suggest replacing the rabbit module with a 7 ohm resistor
(approx) to simulate a 450mA load, then turn on your FET and measure
how much voltage there is across the FET and across the resistor.
You might also try removing the 5.6K resistor and just driving the FET
gate directly from the PIC.
On Thu, Jul 24, 2008 at 5:50 PM, Mauricio Jancic <janso.com.ar> wrote: listas
> 1) Besides 3.3V and the mosfet drain, what else is connected to the
> rabbit module? (I suspect there may be some kind of ground leakage
There is no other connection in or out the rabbit. I removed everything to
make tests. The only connections to the Rabbit are the 3.3V line straight
from the MCP1826 voltage regulator and the ground line going from the rabbit
to the GND path through the FDV305N FET.
> 2) What voltage is your PIC running at? (to determine how it is
> driving the FET gate) I assume that the PIC shares the same ground as
> the FET?
Yes, it is the same ground. The everything is running at 3.3V (PIC16F67J10)
> 3) Have you tried measuring the voltage across the rabbit module's
> power input (pos to neg) when the FET is turned on? (trying to
> determine whether there is a static on-resistance problem with the
There are 3.3V as expected. The ON time of the FET is 200 uS.
> 4) Does the rabbit module have large capacitors on its input or a
> switching power supply which may have a high inrush current? (This may
> cause the voltage to drop sharply at startup, which may in turn
> trigger the module to shut itself down immediately)
It doesn't seems to. However, it takes 450mA when the wifi module is on. I
know the rabbit works while the wifi is off, but it resets when the power
consumption increases (I should have mentioned this earlier, I know...)
> Also, I'd suggest replacing the rabbit module with a 7 ohm resistor
> (approx) to simulate a 450mA load, then turn on your FET and measure
> how much voltage there is across the FET and across the resistor.
I've done it with a 5.6 ohm. The weird thing is that it works just fine
(550+ mA). With the actual load (the rabbit) the PIC resets sometimes when
it is turned on, the 5.6ohm resistor has no negative effect however.
> You might also try removing the 5.6K resistor and just driving the FET
> gate directly from the PIC.
Donne that. This causes the PIC to reset as soon as the GATE line is taken
high. I put the resistor to solve this. I also placed a 1000 uF capacitor in
the output of the MCP1826 voltage regulator, to improve this reset problem,
that I trace down to the regulator not being able to regulate on the sudden
change of load.
What is puzzling is that with the 5.6 ohm resistor everything works as
expected, there are 3.3V as needed, but the rabbit won't work.
Microchip Design Partner
can you check with a 'scope if the voltage drops momentarily below the
rabbit's lower VDD threshold when the Wireless module turns on? If you have
a DSO, you should be able to set a trigger w/ falling edge at the threshold
I measured that and there are not voltages drop below 3.2 V. the rabbit
should work down to around 3.15V
Microchip Design Partner
As an experiment, what happens if you use a relay instead of a FET ?
On Fri, Jul 25, 2008 at 5:50 AM, Mauricio Jancic <janso.com.ar> wrote: listas
> The FET is driven with a PIC pin with a 5K6 resistor to gate.
> The source of the FET is to GROUND and the drain is connected to the
> NEGATIVE terminal of the Rabbit. The POSITIVE terminal of the rabbit is
> directly connected to 3.3V.
Like Bob Blick, I do not like to switch the ground side of the Rabbit
either. A better method may be to have the switch in series with
the supply path. An LDO with a shut-down pin will be very good
for this application.
Anyway, I also think 5k6 is too high. The PIC pin may not have
enough driving capability to the FET. Try to reduce 5k6 to lower
value. Or better add a buffer.
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something I can think of:
Is your power supply current limited?the gate resistor is way too high for me, but it's not a problem since I assume the load is always on but not switching, right?
another thing is how much current your PIC can give at the output, maybe it's working in linea region or so.
Why not connect your rabbit POSITIVE to the FET source and NEGATIVE to ground? I don't know how it should be working, though.
> From: janso.com.ar> To: listasmit.edu> Subject: [EE] FET question> Date: Thu, 24 Jul 2008 18:50:41 -0300> > Hi,> I have a circuit with a MOSFET driving a RCM4400W (A Rabbit module> with wifi, that need 80mA on startup and 450mA when the wifi module is on),> and it is not working.> > The power supply is fine, I think, because if I connect the Rabbit> directly to the PSU it works.> > The mosfet seems to be working, because I have 3.3V on the Rabbit> terminals when it is on.> > The FET is driven with a PIC pin with a 5K6 resistor to gate.> > The source of the FET is to GROUND and the drain is connected to the> NEGATIVE terminal of the Rabbit. The POSITIVE terminal of the rabbit is> directly connected to 3.3V.> > There is nothing else in the circuit, shouldn't it be working?> > The FET is an FDV305N, 0.9A and the Vgsth is 1.5V.> > Regards,> > Mauricio Jancic> Microchip Design Partner> Janso Desarrollos> > -- > piclisthttp://www.piclist.com PIC/SX FAQ & list archive> View/chan
ge your membership options at> mailman.mit.edu/mailman/listinfo/piclist
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2008/7/25 Jinx <clear.net.nz>:joecolquitt
> As an experiment, what happens if you use a relay instead of a FET ?
Or even a toggle switch?
At 06:38 PM 7/24/2008, you wrote:
That's not weird, it's expected. The module probably has a bunch of
bypass capacitors and you're momentarily killing the supply charging them.
I suggest a separate regulator with shutdown input is probably the best
way to handle this (effectively a high-side switch with current limiting)
The problem with switching the low side is that any logic pins that
connect to the PIC or whatever could get pulled way below
"ground" from the perspective of the the Rabbit module (eg. the PIC
is "low", a great deal of current will flow through the protection
diodes on the Rabbit module, possibly damaging it or causing latchup
which would result very quickly in damage due to high currents.
Listas de Correo
Unfortunally, we already have 100 boards made. We can make small changes,
like putting a FET on the high side of the rabbit, but making big changes
will be difficult.
I will see tomorrow, while I'm at the office, if it is possible to change
the layout easily from low side switch to High side switch.
by the way, if I connect the rabbit directly to ground (or with a switch or
a relay) it works just fine. So, the problem is with the MOSFET...
On Thu, Jul 24, 2008 at 10:43 PM, Spehro Pefhany <interlog.com> wrote: speff
> I have a circuit with a MOSFET driving a RCM4400W (A
> Rabbit module
> with wifi, that need 80mA on startup and 450mA when the
> wifi module is on),
> and it is not working.
FET should tolerate a zero ohm connection from PIC.
- Set up system as described.
- Add switch across FET drain-source.
- Open switch
- Enable system via FET.
- Close switch.
IF it works under that condition the FET is suspect as an on
FET should have about 0.15 ohm on resistance as described.
Should be OKish although the module spec is 3V3 +/- 5% so
you are outside that range at 450 mA.
If it fails to work then the system is probably being
disabled in some manner by the load side open switch.
eg having the module connected to Vdd and stray paths from
I/O to ground may do funny things to it.
Also: RF may be getting into FET. All bets off if that
Listas de Correo
I have tried connecting and disconnecting the ground wire of the rabbit,
directly, to GND and it successfully starts every time.
The 450 mA at 0.15 ohm represent a voltage drop of 0.0225 V, that is 0.68%
of 3.3V, so the Rabbit should work with that voltage, shouldn't?
I'm trying not to consider RF yet...
On Thu, Jul 24, 2008 at 11:00 PM, Apptech <paradise.net.nz> wrote: apptech
The Fet itself might be ok with a 0 ohm drive but the PIC probably wont
be ;-> you could well be asking for a few amps of drive current (for a
very short time).
I'm wondering given the OP is using a 5.6K resistor if the miller charge
is turning the fet off again as the rabbit comes up to power (as
somebody else said its probably charging some caps and asking for a few
amps of current)
To limit the initial current you want about a 170 ohm resistor and that
should hopefully be stiff enough to handle any of that other weirdness
that goes on with fets.
Your sure the fet is into its saturated region at 3.3V gate drive?
that's quite low, not impossible just uncommon is all.
Don't know how it'll work with the rabit, but try sticking a 1 ohm
resistor in line with it, might help reduce the switch on thump.
> Unfortunally, we already have 100 boards made
Just curious - how did you come to be in this position ?
> The 450 mA at 0.15 ohm
This d/s says 0.2R @ 4.5V, 0.3R @ 2.5V
Take a look at Figures 2 and 4, you might find that Ron is higher than
you planned for
If I'm asssuming correctly, quick look, and the normalised Ron is *1.1
@ 3.5V and 0.5A, then that would mean actual Ron in circuit is 0.33R,
so you'd be losing 0.450A * 0.33R = 0.15V. If so, that puts the voltage
available for the Rabbit at 3.15V
Can you bump up the regulator output a little ?
> Your sure the fet is into its saturated region at 3.3V
> gate drive?
> that's quite low, not impossible just uncommon is all.
Looks OK enough from the spec sheet curves.
Looking at V across module with a scope at turn on may give
An easy enough test is to try a high side driver.
Disconnect FET drain from model gnd.
Ground module gnd.
Small pnp transistor with at least 500 mA rating (BC327 or
Beta of 100 at least at 500 mA.
Emitter to V+
Collector to module V+
Base via say 470 ohms to FET drain.
You now have a high side switch with good drive
If that works it gives you a clue.
If not then the clue is different but not so clear.
I assume that you have the MOSFET gds connected OK :-).
Listas de Correo wrote:
> I have tried connecting and disconnecting the ground wire of the rabbit,
> directly, to GND and it successfully starts every time.
I'm ready to suggest that when using the FET the voltage rises too
slowly for the Rabbit to start.
Can you test again using the FET but driving the gate manually, first
with a jumper wire, and if that works, then with a resistor? Because if
it starts with the jumper driving the FET but not the resistor, that is
a risetime problem.
Maybe he should try the FDN377N.
On 25 Jul 2008 at 15:15, Jinx wrote:
M. Adam Davis
That particular Rabbit core has several power supplies for the radio,
and while the radio may be off the power supplies do start up on power
This means that the core has a significant inrush current due to all
the bypass capacitores used for filtering - 0.9A may not be enough,
the Ron of the FET may not be low enough, etc, etc.
Measure the voltage across the FET with a high speed scope to see what
the inrush current looks like in combination with the turn on time of
Also, are you able to reset the Rabbit when it doesn't come up using
the FET to power it? If it resets ok, then you may need to control
the reset along with the power suplly. If it doesn't reset then
you'll need to beef up the power supply and switching mechanism (the
FET and FET driver). You may need to drive the FET very hard to get
it to turn on quickly enough especially since you don't have much
You might consider trying the same thing with another Rabbit 4000 core
to determine if it's the added radio circuitry, or if there's a
problem with the reset and cpu itself. It should just plug right in.
On 7/24/08, Mauricio Jancic <janso.com.ar> wrote: listas
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