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DRV8251A: Maximum current from IPROPI pin?

David Thomas
Prodigy 40 points
Part Number: DRV8251A


Tool/software:

Hi All,

The maximum output current specified under Recommended Operating Conditions is 4.1A. This is also listed as one of the key features of the part.

The current mirror mechanism generates a current (IIPROPI) from the IPROPI pin proportional to output load current with a scaling factor of 1575uA/A.

Therefore the mirror current generated at the maximum output current would be: 4.1 x 1575uA/A = 6458 uA = 6.458mA.

However, the maximum IIPROPI Peak output current listed under Recommended Operating Conditions is only 3mA.

If IIPROPI = 3mA, the peak output current would be only 1.9A which is substantially less than the rated 4.1A.

Can anybody explain this disparity?

Thanks in advance.

  • 0
    TI__Mastermind 20150 points

    Hello,

    The IPROPI peak output current spec is a Recommended Operating Condition that is recommended to not exceed. However, the overcurrent protection (OCP) threshold is 4.1A when VM > 5.5V. The OCP is the max amount of current the device can handle. If the OCP is exceeded for the specified deglitch time (typically 1.5us) then the outputs will disable until the current reduces below threshold. The recommended IPROPI max spec of 3mA keeps the devices continuous current consumption in the recommended level for thermal performances and operating conditions. 

    How much peak and continuous current is needed for your application?

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 40 points

    Hello David,


    Many thanks for your response.

    What you have described is exactly my understanding of the operation of the device so thanks for that confirmation.

    I understand that the maximum current capability is underpinned by the thermal constraints and limitations and that steady-state operation at currents approaching the maximum rated 4.1A should be addressed with extreme caution and with due regard for maximum junction temperature. I can appreciate that a best practice type of application for this device in order to achieve a safe and robust implementation might well involve restricting the output current to a sensibly conservative value, perhaps in the region of 1.9A (which would keep IIPROPI within recommended limits).

    Our application involves only short current pulses at a low duty. Typically this will consist of 1.5msec pulses repeated every 1.5 seconds. The current during each pulse will have a peak value of somewhere in the region of 2.5A. This is above the 1.9A limit where the corresponding IIPROPI current exceeds the datasheet recommended maximum of 3mA. The load is purely resistive. Our extensive evaluation of this set of operating conditions suggests that the application is robust and we have experienced no failures or performance peculiarities which give us cause for concern.

    My concern however is that we are operating the device outside of recommended operating conditions simply due to the peak magnitude of IIPROPI. What I don't understand is how the recommended operating condition for peak output current is specified as 4.1A but that this output current would necessarily cause IIPROPI to exceed its recommended maximum peak output current. Surely the maximum recommended values of these two currents should obey the 1575uA/A relationship (within tolerance limits of course).

    For what it's worth we are indeed using the IIPROPI output with a resistor to 0V in order to get current feedback. It has occurred to us that it may be intended by TI that the IPROPI output must simply be disconnected if the device output current exceeds 1.9A. This would ensure that IIPROPI remains within the 3mA limit (by reduced it to zero), although this would obviously impact the usefulness of the device.

    Any further input you might have to this dilemma would be most welcome.

    Best regards,
    Dave.

  • 0 in reply to David Thomas
    TI__Mastermind 20150 points

    Hi Dave,

    I understand the concern. We appreciate the feedback and I'll mention to the systems team on how to better display this information in the datasheet for the next revision. However, for your application I don't see any issues with this implementation. The output current is still within operating range and under OCP threshold. The short pulses with low duty cycle should cause no issues with thermal performance either.

    Best,

    David