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In TPL7407L's datasheet, there is a stepper motor driver schematic as below

enter image description here

There is a zener diode denoted as "optional". However the 'COM' pin is connected to 'VSUP' directly. The 'optional' means I can remove (not short!) it, right? Then what's the purpose of the zener?

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  • \$\begingroup\$ flyback ? to control inductive kick ? \$\endgroup\$
    – efox29
    Commented Mar 30, 2015 at 5:27
  • \$\begingroup\$ If we remove it, it still can supply a path for the inductive kick back, right? (It is similar to ULN2803, with a free-wheel diode integrated.) \$\endgroup\$
    – diverger
    Commented Mar 30, 2015 at 5:31
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    \$\begingroup\$ The zener could be used to allow the magnetic feild to collapse faster by clamping it. A huge chuck of that energy gets dropped across the zener. \$\endgroup\$
    – efox29
    Commented Mar 30, 2015 at 5:37

1 Answer 1

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9.2.2.1 Design Requirements - [...] Whenever a Zener diode is used between Vcom and the motor supply, the Vcom pin will slew from the coil supply to a voltage that is the sum of the Zener voltage and the coil supply when there is a flyback event. Depending on the coil inductance & resistance, this can be very rapid. Whenever the COM pin may experience a slew rate greater than 0.5 V/ μ s a capacitor must be added to limit the slew to < 0.5 V/ μ s. See Power Supply Recommendations for further explanation.

So in this particular case you should either:

  1. Add a zener diode from motor supply to V(COM) and forget the V(SUP) at the V(COM) node.
  2. Short both nodes V(COM) to motor supply.

The advantage for 1. is that the energy stored in the motor coils is being broken down more quickly and the stepper response will be 'snappier'. Notice that the motor supply increased with the zener voltage must not exceed 42V (absolute maximum, so you want to be well below that).

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  • \$\begingroup\$ Yes, IMO, the schematic is not right, at least not clear enough. There shouldn't two nodes labeled 'VSUP', because they short the zener actually, right? \$\endgroup\$
    – diverger
    Commented Mar 30, 2015 at 6:08
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    \$\begingroup\$ Agree the circuit is not clear, took me a moment to figure that out too, but the text in the datasheet was helpful. \$\endgroup\$
    – jippie
    Commented Mar 30, 2015 at 6:24

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