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LMD18200: Bypass capacitance

Paolo Garavaglia
Prodigy 115 points
Part Number: LMD18200

We are developing a control board with LMD18200 motor driver for a welding equipment application [wire feeder].
The equipment as two functional operative work. In the first one the motor current in costant around 0.6 A max, while in the others the equipment work in “pulsed” mode where the motor is activated first in one direction for 300 ms, then in other direction for 200ms. That generates current pulse with peak of 10A and exponential decrease where current value get 0.6 A in around 150ms. RMS current value is around 4A.
The question concern which bypass value capacitors is/are suggested for LMD18200 Vs power supply. LMD18200 is supplied by a 42 V rail.

Thank you for your support.

  • 0
    TI__Guru** 114905 points
    Hi Paulo,

    Please note the SUPPLY BYPASSING section of the datasheet:

    It is normally necessary to bypass the supply rail with a high quality capacitor(s) connected as close as possible
    to the VS Power Supply (Pin 6) and GROUND (Pin 7). A 1 μF high-frequency ceramic capacitor is recommended.
    Care should be taken to limit the transients on the supply pin below the Absolute Maximum Rating of the device.
    When operating the chip at supply voltages above 40V a voltage suppressor (transorb) such as P6KE62A is
    recommended from supply to ground. Typically the ceramic capacitor can be eliminated in the presence of the
    voltage suppressor. Note that when driving high load currents a greater amount of supply bypass capacitance (in
    general at least 100 μF per Amp of load current) is required to absorb the recirculating currents of the inductive
    loads.
  • 0 in reply to Rick Duncan
    Prodigy 115 points

    Hi Ricky,

    Thank you for your prompt answer.

    I took care about datasheet recommendation.

    My question is about the last point, since we have an inductive load which is driven in "current pulse" mode. So, I have to foresee the bypass capaictance take into account 10A peak [100uF x 10], 4A RMS value [100uF x 4 ]  or 0.6A  nominal value [100 uF x 0.6] ?

    Thank you.

    Kind Regards

    Paolo

  • 0 in reply to Paolo Garavaglia
    TI__Guru** 114905 points

    Hi Paolo,

    At 42V and 10A for possibly tens of ms, I would recommend having the footprints for 10A x 100uF. As you evaluate your system, you may find that you can reduce the capacitance. 

  • 0 in reply to Rick Duncan
    Prodigy 115 points

    Thank you for the answer.

    We will take it into consideration.

    Kind regards