I need to eliminate the MOSFET drive circuit from my PCB by replacing uln2004 or uln2803, should I still keep the back EMF diode across the dc motor?
motor supply 5V current 70mA
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If the load has inductance, you must use freewheeling diode. Otherwise, you will probably damage the supply and/or MOSFET when breaking the current.
You cannot change current of an inductor instantly. So you have to provide a path for inductor to “discharge itself” safely. If inductor does not have that path, it will reverse the voltage as high as needed to keep the current flowing. You can see this in light switches as a blue light (arc) when you try to turn off the light.
Note that relays and motors have coils, and they should always have a freewheeling diode.
The ULN200x and ULN2803 have built-in diodes, all connected to a COM pin. Tie this pin to +VSUP, and you do not need external diodes.
Marko Gulin
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You may have missed the point of the question. The ULN2003, for example, has snubber diodes built in. – Transistor Jun 22 '20 at 06:13
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I was just reviewing the datasheet of the chip when you posted a comment. – Marko Gulin Jun 22 '20 at 06:16
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@Marko Gulin you meant The "ULN200x and **ULN2803** have built-in diodes" ? – oppo Jun 22 '20 at 07:04
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Yes. Check the block diagram in the datasheet, more specifically, check the COM pin. You will see that cathodes of all built-in diodes are connected to this pin. – Marko Gulin Jun 22 '20 at 07:11