Why a separate power supply is needed for L293D while interfacing with microcontroller.

I have searched it but didn't got any proper/relevant answer.

  • \$\begingroup\$ This datasheet says "A Vcc1 terminal, separate from Vcc2, is provided for the logic inputs to minimize device power dissipation". Is that what you're asking about? \$\endgroup\$ May 5 '15 at 11:18
  • \$\begingroup\$ @RogerRowland why we don't give same 5v to L293D which is given to microcontroller \$\endgroup\$
    – Saad Rafey
    May 5 '15 at 11:21
  • \$\begingroup\$ Where did you read that you can't do that? I can't see any reason why you can't supply the microcontroller and Vcc1 from the same 5V supply. You just have to make sure that Vcc2 is not less than 5V. Can you post a link to where you saw that restriction? \$\endgroup\$ May 5 '15 at 11:25
  • \$\begingroup\$ I have done it and it drops voltage by 0.01V which in turn distorting ADC output in my case \$\endgroup\$
    – Saad Rafey
    May 5 '15 at 11:29
  • \$\begingroup\$ I'm not sure I understand what this has to do with ADC. You may need to explain more and show a schematic, I think your suspicion about the cause of the problem may be wrong. \$\endgroup\$ May 5 '15 at 11:31

Driving motors is a dirty business, especially when you're driving them with PWM - you're switching relatively high currents on & off at kHz, which generates a lot of noise, and is fairly demanding of the power-supply; and that's when everything is going well! If a motor stalls due to a heavy load, some motors can draw even more than their usual current. Motors also put inductive spikes back into the power supply, which is why you try to deal with as much of that crap with the fly-back diodes, but diodes aren't perfect.

This is about the worst kind of power-supply you want to use to also power the MCU that's controlling your motors! If your MCU locks up because Vcc sags too low the MCU stops working completely, or if the AVcc or Vref (reference voltage needed by the ADC) inputs to the ADC are noisy then your analog readings are less accurate, all of which can be annoying at best, and dangerous to human life & damaging to property at worst. This is why a separate power supply is used for the MCU & related digital peripherals, or at the very least a separate voltage regulator.

  • \$\begingroup\$ yes absolutely - if Vcc is noisy, then so too will be the digital values you get from the ADC! \$\endgroup\$
    – Techydude
    May 5 '15 at 12:05

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