All sense pins in this schematic are used with diodes before the ADC input (on the MCU). Anyone can explain? And what diodes will work here? No info in datasheet.

example implementation of DRV8305 with diodes on ADC inputs


1 Answer 1


They are protection diodes (aka clamping diodes) which are there to ensure that if the voltage starts to go above a certain amount (in this case the Vcc supply rail) that they will start conducting and try to prevent it rising any further.

Motors are noisy, and during switching you can get spikes at the centre of the h-bridges that are much higher than the motor supply rail (they can be on the order of tens or even hundreds of volts if not careful). This means that the spikes would cause the voltage at the sense pins to rise up much further than they are rated for.

By adding the diode, if the voltage rises up above Vcc, the diode conducts and increases the voltage drop across the upper resistor in the potential divider to clamp the voltage to no more than Vcc plus the diodes forward voltage. As such a Schottky type diode is used, as indicated by the box-like ends of the cathode bar in the symbol, as they have a much lower forward voltage than a standard PN diode which in turn reduces the amount above Vcc which the sense pin can reach.

  • \$\begingroup\$ Great explanation, thanks. What do you think, what are ratings of these diodes? Like Battery voltage times 3, or more? And current? \$\endgroup\$ Aug 17, 2016 at 21:41
  • \$\begingroup\$ Or it can be 5v and some milliamps? \$\endgroup\$ Aug 17, 2016 at 21:43
  • \$\begingroup\$ So, if I am right, even in case of 200V spike only several milliamps will flow. And If VCC is 3.3V, diode rating may be 5V? \$\endgroup\$ Aug 17, 2016 at 21:56
  • \$\begingroup\$ @RomanSimonyan Diode reverse rating need only be the supply voltage plus some leeway. 5V rating would probably be fine - though most will have a reverse rating much higher than that which is not an issue. Current rating as you say need only be a few milliamps with those resistor values - most available will be rated for something like 100mA or more, so you should have no trouble finding one. Probably go for something like a 0.5V forward voltage rating or lower. \$\endgroup\$ Aug 17, 2016 at 22:48

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