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DRV8802 Absolute Maximum Rating on ISENSEx pin voltage

Other Parts Discussed in Thread: DRV8813

On the datasheet, Absolute Maximum Rating on ISENSEx pin voltage is specified as "-0.3V to 0.8V".

While in OCP event, in approximately 2.5us, as the OCP deglitch time expires, the current is still high level. If a 0.5 ohm sense register is used, and the current is 2A, the voltage on ISENSEx will be 1V. It is over the Absolute Maximum Rating on ISENSEx.

1)Should I keep specified ISENSEx pin voltage(~0.8V) while in OCP event?

2) Is adding a capacitor in parallel with sense register effective to reject spike noise?  Is there any specifications for capacitor value?

Best Regards,

Hide

  • Hi Hide

    1) Yes. You can lower sense resistor and Vref together to make Vsense lower but the same current limit setting.
    Even there should be little chance to damage the sense pin with 2.5us peak over spec to ISEN pin, using a sense resister bigger may easily found the OCP nFAULT tripped on noise but a true OCP.

    2) It is good to do so as all our EVM as the 0.1uF cap on ISENs pin.
    0.1uF is a good value you can use. Also note the cap is better placed on ISEN pin other than just parallel with sense resister on the layout, although the same thing in schematic, but with different noise filter effect.

    Best regards,
  • Hi Wilson,

    Thanks for your explanation.
    Let me ask some questions.

    1) In case ISENx pin voltage over the Absolute Maximum Rating specified 0.8V, which part of internal circuit is possibility to damage? I guess it is the comparator which is input from DAC, and it will influence current limitation control. Is this correct? Is there any possibility to damage?

    2) Is there no protection circuit before the comparator?

    3) Do you have information about response time of OPAmp which is the most closed ISENx terminal?

    Best Regards,
    Hide
  • Hi Hide,

    Let's check with design team about your question and get back later.

    Best regards,
  • Hi Wilson,

    Thanks for your kindly support.

    On the Application Report SLVA505–February 2012 "Understanding Motor Driver Current Ratings" (www.tij.co.jp/.../slva505.pdf), Figure 2 shows an oscilloscope capture of a short circuit event using a DRV8813.

    ------------------------
    Initially, the current rises quickly. After a brief overshoot, which is not a problem for the output stage, the current is limited to approximately 9 A by the analog current limit. In approximately 2.5 μs, as the OCP deglitch time expires, the current is still at the analog current level of 9 A, exceeding the OCP level. In this case, the OCP level is approximately 3 A. At this point, the output is disabled and the current drops to zero. Shortly thereafter, the fault signal is driven low indicating that the OCP event has occurred to the rest of the system.
    ------------------------

    DRV8813 is specified ISENSEx pin voltage up to 0.8V on the ABSOLUTE MAXIMM RATINGS. To keep 0.8V on the ISENSEx pin, it's very difficult to decide sense register value. Could you please confirm again that we should keep ISENSEx pin voltage up to 0.8V while in OCP event?

    Best Regards,
    Hide

  • Hi Hide,

    One thing for sure is that we don't have to the keep the rule even in OCP event. Our device will not get damaged if just there is a small peak voltage >0.8V on this pin. I have asked designer about how much risk it will have.

    Anyway you can refer to our EVM design on choosing the R sense resister. Maybe you will still think it is possible to go higher than 0.8V. But it is a robust prooved design. The general rule is to keep the R sense as small as possible if the current regulation is working ( mostly we see customer using from 100 mohm to 500 mohm).

    Best regard,

  • Hi Hide,

    Checked with designer that the limit of the voltage on the ISENs pin was due to the ESD cell will should be kept within -0.6V to 2.1V. Other wise there will be break down leakage current which may damage the ESD cell or make the sense accuracy much worse. So we give the spec of -0.3 to 0.8v for enough headroom of the sensing.

    Best regards,