2

On the L298 datasheet (http://www.sparkfun.com/datasheets/Robotics/L298_H_Bridge.pdf), the Bidirectional motor control application example (Fig 6) shows that the need to be two diodes connecting the ground. I understand that you connect a flyback diode to the V_supply for EMF protection by allowing the current to flow back into the motor itself. However, why is there a need for a diode connecting to ground? I don't see how the diode can be made forward biased.

tuzzer
  • 269
  • 1
  • 2
  • 6
  • 1
    Interesting link, but it's not the L298 datasheet! Without reading the datasheet, I'd guess that it's because the polarity pf supply reverses, so the diode to ground is used in that case. – AndrejaKo Feb 19 '13 at 20:24
  • Oops, corrected the link – tuzzer Feb 19 '13 at 22:23

1 Answers1

2

If you pull an inductance to ground (with a low side driver, the inductor's other terminal being connected to VCC) and then turn off the driver, the inductor will generate a positive spike - this forward biases the diodes (D1 or D3 in figure 6 on this datasheet) and conducts the spike into the positive supply.

But if you pull the inductor high (with a high side driver) and then turn off the driver, the inductor will generate a negative spike - this will forward bias diodes D2 or D4 and conduct the spike into the negative supply (ground).

In a bridge driver, both of these are happening (one at each terminal of the motor) so both pairs of diodes are necessary.