voltage sag when switching on circuit

If there's no load on the MOSFETs, then you have to ask, where's the current going that makes the power supply droop? Remove all the MOSFET circuitry and see what happens. The problem may have nothing to do with them.

And while you're at it, please post the *complete* circuit so we can see it.

-- Joe

Electrolytics of that capacitance most likely won't be able to do much about current spikes in the usec range. Sprinkle some 1uF ceramics and a few 0.1uF. Short leads, best is SMT. I hope you have a full ground plane. If not I guess it may be time for a re-layout.

WRT the LDO I'd be careful. Don't know this type but I've seen lots of grief with LDOs. A switcher may be much better and many processors can start on a slower clock as low as 2V. At least the MSP430 can.

obviously you have a heavy load coming on and maybe from capacitance ? you could try using a series R to the gate of the Fet for one thing.. and, maybe a diode to the Vcc side of the Pic from the supple with a cap there to ground. this way when a load takes place, the PIC will still have power from the cap and the rest of it will have to suffer for that moment.

Thanks for the response, Joe.

I was thinking of posting the circuit (it's fairly small) as a .jpg but I didn't want to offend anyone.

Is it acceptable to attach a small .jpg?

Dave

Thanks for the response, Jeorg.

I chose the TC1108 (Microchip part) for the very low drop-out voltage and pin-out with the pads on the circuit board. Since I'm working with a 3.6VDC source and need a regulated 3.3VDC for the PIC and associated circuitry, I needed a fairly small drop-out device.

The circuit has 1uF ceramic 0805 SMD caps on the input and output of the TC1108 regulator. I also have a 0.1uF ceramic cap across all components in the 3.3VDC circuit. The total current draw from the 3.3VDC circuit varies from 80mA to about 110mA. It goes to 110mA at power-up/reset.

Dave

Thanks for the response.

Could you elaborate a little on what you mean by series resistor to the gate of the MOSFET? Do you mean a small (100-ohm or so) resistor from the PIC pin to the gate of the N-Channel control MOSFET? And the diode/cap... do you mean diode anode to the 3.3VDC supply, diode cathode to the PIC Vcc input, cap from PIC Vcc input to ground? I have a 0.1uF cap from Vcc to ground now.

Dave

Do you have a web site to post you .jpg ?

There are many available for just this sort of thing.

More information about the sag would be a good thing.

Like a scope trace if you have it.

donald

First of all. I'd skip the regulator. The PIC should handle the full battery voltage. Add an RC low pass filter between the supply and the PIC. Something like 100 ohms and 100 uF should do it.

It sounds like the regulator in the supply has a fairly low frequency response, so keep the 1000 uF across the supply to help it with the load transients. You may need even more for the battery. Keep in mind that not all electrolytic capacitors are made equal. Some have lots lower or higher equivalent series resistance and inductance. This series is pretty good for the money.

Post it on alt.binaries.schematics.electronic and then post the subject line in this thread.

I mean both.. You shouldn't drive the gate directly.. it has capacitance, how much I don't know how ever, It would help the PIC out in current inrush on the output. something like a 1k R to the gate should be fine. And for the VCC on the pic, I would use a diode to isolate from the main source to prevent back feed when supply voltage is getting dragged down and add a semi large by pass cap there to maintain the charge for that brief moment, that should take care of the PIC rebooting. 100 Uf should do it .. P.S. You'll get a little drop in voltage to the PIC using a Diode... You could go with a resistor and cap. like maybe a 150 ohm and a 470 uf cap. it depends on the current demands of the PIC.

if the system runs on 3.6 and the voltage regulator is 3.3 may be there is the problem you need more voltage

...snip...

Per a suggestion by Paul Hovnanian (I just realized I mispelled your last name in the binaries post, Paul... sorry:) ), I posted the applicable circuit section and scope trace on alt.binaries.schematics.electronics with the heading of "3.6VDC voltage sag on switching".

A suggestion was made to just run the circuit on 3.6VDC, as the PIC will tolerate this voltage. While this is true, the circuit has some additional circuitry (a GPS, and RS232 interface chip, a voltage monitoring circuit, etc.) that I did not include in the schematic. The GPS is pretty picky about the operational voltage. Please understand I can't post the complete schematic as this is an on-going project, etc. The GPS is active at the time of the switching attempt, though, and is drawing about 90mA, max, at that time. The PIC itself and all remaining circuitry draw less than 20mA at 3.3VDC.

Another suggestion was to include a small series resistor to the N-Channel MOSFET gate from the PIC output. Good suggestion. I will try that. Another response stated the votlage regulator might not have enough margin. The TC1108 is designed to work with a very small margin (typical around

The cap and resistor (or cap and diode) to the power supply input line is also a good suggestion. I will see if I can cut the trace to the Vcc pin going to the PIC to insert this. I'm a little concerned about this lowering the available Vcc to the PIC as I use the Vcc for a reference on an ADC input. I can always derate the value for the full-scale Vcc, though. The analog value is not that critical anyway.

If anyone sees anything blatantly standing out after examining the trace and circuit, please post a reply.

Thanks for all the help so far.

Dave

Dave, you may have a layout issue then. Enough of those ceramic caps should definitely buffer such weak 2usec spikes. Might be the time to haul in an expert, preferably someone close by.

Had a similar PIC "self reset" problem the other week. Was switching

Thanks for the response. I do have a 0.1uF cap right at the Power/Ground pins but what you've stated made me re-examine my layout. This is a two-sided board design (I know that's at least part of the problem... no internal power and ground planes) and the power and ground traces are all over the board. I may try to route the power for the PIC a little more directly from the +3.3VDC regulator or (as was suggested in another response) through a Schottky diode and to the PIC from the +3.6VDC side directly with a fairly large (10uF to 22uF) cap to ground at the power pin of the PIC.

Dave

Use jamie's idea. Dump the LDO and isolate the power like this, usng a Schottky diode.

• 3.6----+-----to other stuff | +--->|---+---To PIC | [1000uF] | Gnd

That is step 1. Of itself, it won't keep spikes out of the PIC, but it will ensure that the PIC's Vcc doesn't sag. Right now, that is your major problem.

Ed

... snip...

I've posted some resultant traces on the alt.binaries.schematics.electronics newsgroup under the heading "3.6VDC voltage sag on switching" after implementing some fixes.

Thanks much to all who responded.

Dave