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Author Topic: SMD MOSFET to power down my SMD H-Bridge  (Read 4548 times)
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Alternatively, pick a mosfet with a higher Vgs rating but designed for 5V gate drive, such as TSM2313CX.
...still looking for something at Digikey. I can't get the one you mentioned  at Digikey, and at Mouser it's a non stocked item which you've gotta order 15000 of…
Digikey's filtering system doesn't  have the criteria of Vgs, so I'm having trouble selecting one. Once I find something I'll post my updated schematic.
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Perhaps PMV48XP then?
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Perhaps PMV48XP then?

Thanks!  Here's what I came up with...
Is R1 needed? I usually put something like that to make sure the motor is off while the Arduino boots up.
So nothing is needed between the Arduino pin and the gate of the BS1338?
Sure appritiate all the help!

I'll be glad to go this route rather than just a relay.


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R1 is worth including. Its purpose is to ensure the mosfet is kept off when the system is powered up but the Arduino pin has not yet been configured as an output. It isn't needed if the pin feeds a BJT, but then you need a series resistor instead.
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Looks OK to me. The Vgs rating of that P-channel mosfet is only 8V, uncomfortablly close to the 6V drive it is getting, so it is probably better to spit the 2K2 resistor into 2 x 1K and drive the gate from the junction of the two. Alternatively, pick a mosfet with a higher Vgs rating but designed for 5V gate drive, such as TSM2313CX.

I have changed from a 6 volt motor supply to a 9 volt supply, (and tapping off 4.5v for the 328 chip), but I don't understand this Vgs thing, so will this schematic still work to turn on/off Vcc to my motor driver chip?
Will I still need to have the 2k2 resister split into 2 1k resistors?  I don't understand thus part.
Thanks.


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Running from 9V, you definitely need R1, R2 to be 2 x 1K. However, your schematic is wrong. They need to be in series, not in parallel, and the gate drive to the P-channel mosfet needs to be taken from the junction of the two.

I would connect the 100nF capacitor to the source terminal of the P-channel mosfet instead of the drain terminal, and put a much larger electrolytic capacitor (e.g. 1000uF) in parallel with it. Position the P-channel mosfet and capacitors close to the 9986, and keep the traces connecting these 4 components short.
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OK!  I think I've got it.  I made the schematic on eagle, trying to make it all clear so I could get a second opinion on it.  If you could note that it looks okay, and that the mosfet are connected to the correct gates.  I've included datasheets for the components...
thanks again,


BSS138W N Channel Mosfet:
http://www.diodes.com/datasheets/ds30206.pdf

PMV48XP P Channel Moseft:
http://www.nxp.com/documents/data_sheet/PMV48XP.pdf

Si9986 H-Bridge
http://www.vishay.com/docs/70007/si9986.pdf


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As I tried to make clear in my previous reply, I think you should omit C3. Otherwise, every time you turn on the P-channel mosfet, there will be a large surge of current through the mosfet as C1 charges. If you do decide to keep C3, then you can omit C1 but I think you should also deliberately slow down the turn-on speed of the P-channel mosfet by connecting a capacitor of about 10nF in parallel with R2.

You will need to make sure that the 2 input signals to the 9986 are low before you turn off the P-channel mosfet, and remain low until after you have turned it on.
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I will just omit C3. I didn't realize it was a problem. I added C1 thinking that was what you meant to do. C3 is just what I put next to all chips I use as a decoupling cap. I didn't know it was interfering with the P channel MOSFET. Thanks for explaining that. I kinda understand it now.
I'll be sure to turn the h bridge signals off before turning off the P channel.
Thanks!
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On the same project, I need to turn off the Vcc to the DS3234 RTC chip. If I should start a new thread for that let me know.
The 3 chips in my project are:
Atmega328P-AU (first time to boot load the SMD variant)
si9986 H-Bridge
DS3234 RTC

Vcc for the Atmega328 & DS3234 is 4.5v (3 C cells)
...trying to last a year on batteries, I will power down the Atmega328P, and have the alarm from the DS3234 interrupt wake it up. But if the DS3234 has Vcc it uses a lot of current. If I turn Vcc off, it uses only 1-2 uA from Vbatt (a coin cell battery).
So I need to "disconnect" Vcc from the DS3234. Would this same technique I used on the si9986 work?
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I will just omit C3. I didn't realize it was a problem. I added C1 thinking that was what you meant to do. C3 is just what I put next to all chips I use as a decoupling cap. I didn't know it was interfering with the P channel MOSFET. Thanks for explaining that. I kinda understand it now.
I'll be sure to turn the h bridge signals off before turning off the P channel.
Thanks!

Just bear in mind that C1 is effectively the decoupling cap for the H-bridge, so it should be close to the chip and the traces between it, the chip and the P-channel mosfet should be kept short. If you are making a PCB, then I suggest you make a space for C1 anyway (and a capacitor in parallel with R2), just in case there are any problems with noise pickup when the mosfet is off.
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On the same project, I need to turn off the Vcc to the DS3234 RTC chip. If I should start a new thread for that let me know.
The 3 chips in my project are:
Atmega328P-AU (first time to boot load the SMD variant)
si9986 H-Bridge
DS3234 RTC

Vcc for the Atmega328 & DS3234 is 4.5v (3 C cells)
...trying to last a year on batteries, I will power down the Atmega328P, and have the alarm from the DS3234 interrupt wake it up. But if the DS3234 has Vcc it uses a lot of current. If I turn Vcc off, it uses only 1-2 uA from Vbatt (a coin cell battery).
So I need to "disconnect" Vcc from the DS3234. Would this same technique I used on the si9986 work?

The DS3234 takes less than 1mA. Power it from an Arduino output pin (possibly the same one that is controlling the power to the H-bridge), through a 150 ohm resistor, and put a decoupling capacitor between its Vcc and ground. Make sure you drive the 3 SPI control signals to it LOW before you power it off.
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Thanks!  I should've known that. In all the details of powering down the H bridge. I forgot.
I'll post my complete schematic here in a few days.
Trying to check anything that might draw current I thought about the 10K pull-up resistor on pin 1 of the Arduino. 5v through a 10K resistor = 500 uA. But maybe there's a lot more resistance in the Arduino chip when I put it to sleep?
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The pullup on the reset pin won't draw any current, unless something else pulls the pin low.
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Ok, thanks.
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