Driving 5V DC motor from MCU output

[alexwryt]

I am trying to drive a 5V DC motor using a MCU output. The motor is run directly off the +5V line which is down-converted from 18V using a TPS5430 Step-down converter.

The MCU coding is basically that when the inputs are in the correct state, the output to the FET turns on for a number of milliseconds before shutting off again. My problem is that as soon as the motor turns on, the voltage from the supply surges down for a split second and kills my MCU with a POR reset. I think it is because of the initial great current demand for starting the motor from stop. Is there any way I can modify my circuit to protect against this effect?

[PLAIN]http://myimgs.net/images/ydhx.jpg

 

[Bob S]

I suggest splitting the +5 volts at the source, running 1 leg directly to the circuit, and putting a series resister R (first) and a shunt filter capacitor C (next) on the other leg in front of the motor. The values of R and C depend on the motor, but for a small motor drawing say 0.1 amps, use R=5 ohms and C= 10 mF for starters. If this doesn't work, put a separate smaller RC filter on the other leg.

Bob S

 

[vk6kro]

The Micro needs to have its own regulator. A LM7805 would be OK.

To do this, you would have to bring a supply voltage of 7 volts or more to the board with the Micro on it and mount a 5 volt regulator there.

If this wasn't possible, you could try this arrangement:

[PLAIN]http://dl.dropbox.com/u/4222062/supply%20filtering.PNG

The idea is that the large capacitor holds enough charge to supply the Micro while the motor starts.
The Schottky diode stops the voltage from the capacitor discharging into the motor.
Unfortunately, this diode also drops the supply voltage of the Micro by about 0.2 volts, even using a Schottky diode.
And the voltage will still fall when the motor starts, although not as much as without the diode and capacitor.

I have added a diode across the motor. This is a conventional way of protecting the FET against destructive pulses from an inductive load.
It is cheap insurance.