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PMP7282: Minimum Sense Resistor Value

Nesh Johny
Expert 1310 points
Part Number: PMP7282
Other Parts Discussed in Thread: LM5116,

Dear Sir,

Is there any maximum limitation for LM5116 when going for high output current rating like 40A each phase (total 12V/80A)  with very less current sense resistor value(1mE) and with the help of additional MOSFET drivers?

what is the minimum sense resistor which I can use which can be used for high current rating like 40A, that can be used for 2phase buck converter also to ensure current sharing?

thanks and Regards,

Nesh

  • 0
    TI__Intellectual 1235 points

    Hi Johny,

    The maximum limitations are due to the power stage design and the ability of the LM5116 to drive the selected MOSFETs. PMP7282 does not use a current sense resistor, but instead uses the MOSFET Rds(on). For 3 FETs in parallel the approximate value is 1-2mOhm depending on temperature and operating conditions. For PMP7282 at 25A/phase, you can estimate the current sense voltage to be 25A*2mOhm=50mV. This leaves 50mV of the full scale 100mV for slope compensation.

    The LM5116 has an adaptive dead-time circuit to ensure that high and low side FETs do not have any overlap conduction. If you choose to use an external driver, you will need to carefully evaluate the dead time since the adaptive circuit will time out.

    I have not done any study for a minimum current sense resistor value. The full scale current sense voltage of 100mV should be used at the maximum load current. There is an LM5116 design calculator that can help determine appropriate values for the sense resistor and slope compensation.

    Best Regards,

    Bob Sheehan

    Power Design Services

  • 0 in reply to Bob Sheehan
    Expert 1310 points

    Dear Sir,

    Thanks for the reply.

    Is there any chances of the adaptive timer getting partial damage due to high voltage spikes at SW node (due to snubber tunning and track inductance due to layout). which all pins of LM5116 are worst affected due to noise in current sense, or Compensation. 

    There was an observation during testing while probing the compensation components the IC got partial damage. the output started going up and down.

    Any particular reasons, or precautions to be taken to avoid this.

    thanks and regards,

    Nesh

  • 0 in reply to Nesh Johny
    TI__Intellectual 1235 points

    Hi Nesh,

    The SW and VIN pins both have maximum rating of 100V. Without some failure analysis I can't say which internal circuits got damaged by such an event.

    The current sense inputs are the most noise sensitive, and need to be routed differentially to the sense element, either resistor or MOSFET. Noise will not hurt the IC directly, but may cause abnormal operating.

    Probing the COMP pin with a normal high impedance scope probe should not cause any issue. Probing the FB pin directly can cause abnormal behavior due to noise injection. I usually use a 1000 ohm resistor in series with the probe when measuring any high impedance amplifier inputs.

    Best Regards,

    Bob Sheehan

  • 0 in reply to Bob Sheehan
    Expert 1310 points

    thank you sir for the reply