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Topic: Mosfet and switching regulator (Read 952 times) previous topic - next topic

iggykoopa

Jun 05, 2012, 07:18 pm Last Edit: Jun 05, 2012, 07:49 pm by iggykoopa Reason: 1
I have a bare 328p hooked up to a 3 volt switching regulator off of a 7 volt lipo. I am also running a 5 volt switching regulator to supply power to some servo's that the 328p is controlling that won't run off of the 3 volt. I'm trying to cut power to the 5 volt regulator when I'm not using the servo's to conserve power. This works correctly if I hook the servo's ground up to the drain on the mosfet that I'm using, but it doesn't work if I hook up the regulator's ground to the mosfet, is this not going to work? I'd like to conserve as much power as possible when it's in sleep mode. I'm using a http://www.digikey.com/product-detail/en/NTD3055L104-1G/NTD3055L104-1GOS-ND/1484764 for the mosfet and these http://www.digikey.com/product-search/en/power-supplies-board-mount/dc-dc-non-isolated-linear-regulator-drop-in-replacement/4326593?k=oki for the regulators. Thanks

dc42

#1
Jun 05, 2012, 09:37 pm Last Edit: Jun 05, 2012, 09:42 pm by dc42 Reason: 1
When you say "it doesn't work", what is it that doesn't work? A schematic would be helpful. For that scheme to work, you would need a delay between powering up the regulator and powering up the servo, to allow the regulator output to reach 5V.

Generally, regulators are not designed to have their ground lines switched. It's better to use a P-channel mosfet (driven by an NPN transistor) to switch the regulator input instead.

To conserve power in sleep mode, you should replace the 3v switching regulator (which has 5mA no-load current) by a micropower linear regulator - but that may be less efficient when the mcu is not in sleep mode, depending on the mcu current consumption.
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iggykoopa

#2
Jun 06, 2012, 06:13 am Last Edit: Jun 06, 2012, 06:21 am by iggykoopa Reason: 1


Sorry for the crappy schematic. U2 is the 3.3v regulator, it is attached to the 328p with appropriate filtering caps (I just didn't add them in because I have to head to work in a minute). U1 is the 5v regulator. R1 is a pulldown resistor and R2 is a current limiting resistor.

When I set pin 14 Low I'm expecting to cut power to the 5v rail, this is not happening. If I connect the servo ground to the fet drain instead of the regulator ground to the fet drain, then set pin 14 low, the servo will not move even when it is pulsed. That is functioning as I would expect. If I manually disconnect ground from the regulator then the 5v rail loses power as I want. It's just not working when wired as the schematic I've attached. If it won't work this way I'll look into getting some P-channel mosfet's, I don't currently have any.

would you recommend something like this http://www.digikey.com/product-detail/en/LP2985IM5-3.3%2FNOPB/LP2985IM5-3.3CT-ND/334984 to replace the 3.3v regulator?

dc42

The schematic looks OK to me. However, most regulators will leak some current between input and output when you disconnect the ground wire.

What is the current draw of the servo when you are not pulsing it? If it is very low, then there is no point in removing its supply. You could even use 2 or 3 silicon rectifier diodes in series to drop the voltage from 7V to 5V, instead of the regulator (saving the regulator ground pin current).

For micropower regulators, I like the MCP1702 series, which has just 2uA quiescent current and is available in a TO92 package.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

iggykoopa

Thanks for the help. I'm actually running two servo's off the regulator, just trying to keep the schematic simple. It's for a pair of cat ears mounted on a headband. I'll check on the current draw when I get back from work.I'm trying to keep the circuit size small so that all the components can fit inside the ears, otherwise I would just use one mosfet per servo. Besides saving power I was wanting to cut power to the servo's because even when I don't write to them they tend to jitter a little, but that may be my code. I'll take a look at the mcp1702's.

MarkT


The schematic looks OK to me. However, most regulators will leak some current between input and output when you disconnect the ground wire.

What is the current draw of the servo when you are not pulsing it? If it is very low, then there is no point in removing its supply. You could even use 2 or 3 silicon rectifier diodes in series to drop the voltage from 7V to 5V, instead of the regulator (saving the regulator ground pin current).

For micropower regulators, I like the MCP1702 series, which has just 2uA quiescent current and is available in a TO92 package.


No, you can't control a series-regulator by switching the sense pin like this - it doesn't work because you are not controlling the flow of current.  You are simply fooling the regulator about the output voltage and making it turn into a short between input and output rails.  The way a regulator works is by sensing the voltage between the output rail and the sense ("ground") pin and controlling a transistor that allows current to flow from input rail to output rail.

I suspect you need to use high-side switching on the input of the regulator for the servos, which will be awkward at 7V, or low-side switching _before_ the regulator for the servos.  In the latter case you'll need to add protection resistors (10k?) in series with the servo control pins to prevent damage to the Arduino input protection diodes).

Another approach is to find a series regulator with an enable-input.
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iggykoopa

So something like this http://www.digikey.com/product-detail/en/LP38692MP-5.0/LP38692MP-5.0CT-ND/723513 would probably work better? Thanks again for the help, I didn't even think to look for regulators with an enable pin. Should I use a current limiting resistor on the enable pin if I end up going with one of those?

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