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Original question:

OPA564: Question about Schottky protection diodes

OPA564: Question about back-EMF protection

Sylvain
Intellectual 465 points
Part Number: OPA564

Hi,

I want to use the OPA564 with a single supply (0->12V) in order to amplify a sinus signal from a DDS to feed a coil.

If I understood the principle of the back-EMF problem, I believe that, because my supply voltage is +12V, a back-EMF can only be negative. If this consideration is true, do I need the diodes S1 and Z1 and S2 (figure 55 of the datasheet of the OPA564)? I believe I only need the Z2 zener diode with a low zener voltage (as low as 0.4V in order to be sure that the voltage at the output of the OPA never goes below GND-0.4V)?

In this case, if the voltage generated by the back-EMF goes below the 0V, the zener diodes becomes conducting and the current can flow through it.

Am I right?

Thank you!

  • +1
    Guru 257270 points

    If the current through the coil flows only in one direction, then the back EMF indeed can only be negative.

    Z2 itself is not connected to the OPA output; how would Z2 alone be able to protect the output? You still need S2.

    But you do not need Z2, because in a single-supply circuit, S2 already shunts the current to ground.

  • +1
    TI__Guru* 93105 points

    Hello Sylvian,

    Clemens raises the point if the current through the coil only flows in one direction than the back EMF can be of only one polarity. If however, the current can reverse direction as in the case of an AC signal than the back EMF may be of either polarity. The voltage generated by the collapsing current in the coil follows VL = L di/dt. so it just depends on when the output waveform in the case of AC gets applied or turned off what the back EMF polarity could be. 

    If the output is DC or an AC signal that always collapses at the same point of the output waveform, then a single diode from the output to the appropriate supply rail should clamp the back EMF voltage and protect the OPA564 output transistor subject to it. In the case of a positive going EMF the upper output transistor would be vulnerable and that is where the diode clamp is needed. Likewise, for the lower output transistor of the back EMF is of a polarity that it tries to drive the OPA564 output below ground.

    Frankly, if you don't know for sure what the OPA564 output might see in term of a back EMF voltage it is best to have both diodes in place. There are dual diodes in a single 3-lead package that take up only slightly more board space and then the circuit is fully protected.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0
    Intellectual 465 points

    Thank you for your answers!

    I will feed the coil with an AC signal, but between 0 and 6~10V, never below the ground potential. So the back-EMF can only be negative.

    Thank you,

    Sylvain

  • 0 in reply to Sylvain
    TI__Guru* 93105 points

    Hi Sylvain,

    Sounds good. Hope all goes well with your OPA564 application.

    Regards, Thomas

    Precision Amplifiers Applications Engineering