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I need to drive a small (3.6V, <1A) brushed motor bidirectionally with a PIC MCU. My available space is extremely limited, so I'd like to use a single 3.6V power supply for both the motor and PIC, and a minimum of drive circuitry.

I haven't found a dedicated motor driver IC that operates at this low voltage, so a discrete H-bridge seems the most suitable drive arrangement. I've identified the NXP PMV30UN and PMV32UP as suitable (N-type and P-type) drive MOSFETS (line card here).

Since both PIC and motor use the same power supply, is it possible to dispense with all the usual driving circuitry for an H-bridge, and connect the transistors near-directly to the MCU pins as follows? Are there any pitfalls with doing this that I should be aware of?

Schematic

(The inductor symbol is the motor and RB0 and RB1 are the PIC pins (CMOS outputs driven by the same 3.6v supply)).

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    \$\begingroup\$ You may want to consider external fly-back diodes on the MOSFET's to protect them from the back-emf. \$\endgroup\$ – helloworld922 Aug 22 '12 at 1:53
  • \$\begingroup\$ @helloworld922 - The thought has merit but in most cases in this arrangement "protection happens". If say top left and bottom right are on and this is swapped to top right and bottom left then the motor inductor will swap polarity and be connected to the new on switches. If you change from say TL-BR to BL-BR (motor is now not powered) you have a closed path through motor-FET1-ground-FET2-motor and you get motor kick as regenerative braking. The only time the motor is "unfettered", if then, is during switching crossover. Series opposed zeners across the motor catch that. \$\endgroup\$ – Russell McMahon Aug 22 '12 at 5:08
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It should work Ok as long as you aren't going to want to drive the FETs at ahigh frequency (and, as it is a motor, you shouldn't need to.)

You will get a degree of "shoot through" as you transit from high to low or back when both top and bottom FETs are on together. This will probably be cceptable as long as your supplies are not super capable and your switching super slow.

You probably do not need gate drive resistors at all as your drive current is limited and you can do with all you can get to speed uop transition times.

PMV30UN datasheet
Cin ~= 600 pF

PMV32UP datasheet
Cin ~= 1000 pF

P Channel has somewhat more gate capacitance. At say 10^-9 pF and 20 mA gate drive you get ~= 1 V gate V change per 10^-9/.02 = << 1 uS. = OK.

As FETs have very low Vgsth both will tend to be on as Vin transit. So motor is never disconnected. If you had higher Vgsth FETs so that at Vin = 1/2 supply both were off then motor may ring inductively - so series opposed zeners across motor would then be useful. As is a small cap across motor may be useful for comm noise suppression. Small.

Probably unneeded but a reverse biased small Schottky on gates right by FETs will help deal with gate ringing if it proves a problem. You'd need 4 - one per FET connected SG in each case.

To stop motor you remove power by switching both inputs up or both down. This will turn on both up FETs or boty down FEts and you'll get hard dynamic braking. This is fine as long as you expect it. You can't stop this happening with gates as shown. If that was not acceptable you can drive one FET pair on one side as shown and 2 separately on the other (3 drive leads) so you can have atrue off state. You will then also have turn off reactive kick from motor which will go looking for somewhere to be snubbed.

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