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LM9036Q-Q1: ESR requirement with 200 Ohms in-line

Fabio Posser
Prodigy 245 points
Part Number: LM9036Q-Q1


Hi,

To share the losses on my application I had to add a 200ohms resistor in series with the LM9036QM-5.0_NOPB as you can see below.

The datasheet bring the information for output capacitance of 0.3Ω ≤ ESR ≤ 8Ω, for DC and very low frequency, this requirement is met by the capacitors used in my design.

Unfortunately, the control loop characteristics like step response, phase margin over frequency, etc. are not specified over frequency.

What is the advice about ESR of output-Cs vs. stability and about effective input capacitance with 200 Ohms in-line?

Thank you.

  • 0
    TI__Expert 6640 points

    Hi Fabio,

    Thanks for reaching out.

    I recommend adding a 300mΩ resistor in series with the output capacitors to fulfill the ESR requirement.

    There are no stability concerns associated with adding a 200Ω resistor to the LDO input; the input capacitor value does not need to be altered.

    I want to double-check any power dissipation concerns as well, are you able to share the expected nominal input voltage to the LDO and expected load current?

    You can also connect pins 4 and 5 to GND for thermal improvements, as neither of these pins are internally connected.

    Regards,
    Kelsey

  • 0 in reply to Kelsey Farr
    Prodigy 245 points

    Ok, I will connect the pin 4 and 5.

    The input voltage is up to 22V on Vcc, and output load is 20mA nominal.

    I'm evaluating to remove the 200R in the input, otherwise short circuit protection of the IC will not work I believe, right?

  • 0 in reply to Kelsey Farr
    Prodigy 245 points

    Regarding your recommendation for adding a resistor to increase the ESR, the main focus of this +5V output is to feed amp op and digital/analog insulator.

    You think is still needed to increase the ESR in the output?

    This IC's in the output are decoupled with local capacitors, frequency of these signals on the digital insulator are below 200khz.

  • 0 in reply to Kelsey Farr
    Prodigy 245 points

    Are you 100% sure I can connect pin 4 and 5 to GND?
    I found nothing about it on datasheet and also on the application note / Examples boards on the product page.

  • 0 in reply to Fabio Posser
    TI__Expert 6640 points

    Hi Fabio,

    With 22V on the LDO input, 5V at the output, at 20mA nominal load, this is 0.34W of power dissipation, which, assuming a JEDEC-standard Rja value of 140C/W, would result in a 47.6C temperature rise of the LDO on top of ambient conditions. Thermal shutdown typically occurs at the abs max temperature value of 150C, so you'd have to ensure the LDO stays under 150C. That would impose a limit on the system ambient temperature to 102.4C for the LDO to perform as expected.

    The effective Rja value can be reduced further by optimizing the PCB for thermal relief, see guidelines for that in this app note we have available. This may also aid in your desire to remove the series resistor at the LDO input.

    It would be ideal to remove the series resistor at the LDO input, but not necessarily because of the current limit feature of the LDO. It's more because in periods of high load currents (ex. during cold crank or a short circuit event), the battery voltage at the input of the LDO dips, and the extra resistor there may cause the IN voltage to the LDO to dip low enough to cause the LDO to not regulate. You can design around and test expected circuit behavior with the series input resistor, but in general it is preferred for there to not be an extra resistor at the LDO input for design ease.

    Thanks for sending the information about the ESR characterization of the capacitors you're using and a bit more about the surrounding components. I think regardless of this, it's still recommended to add a small resistor (ex. 300mΩ) in series with the capacitors. This is because the relevant frequency range when discussing ESR has to do with the LDO's bandwidth, rather than that of the digital isolator. I unfortunately don't have data on the LDO's bandwidth available since this is a legacy device, and as you mentioned previously, relevant graphs are not available in the datasheet (or that I was able to find in our internal system). Given that, it's best to ensure the minimum ESR condition is met by including the resistor.

    And yes, you can connect pin 4 and 5 to GND. I checked (and double-checked!) the design file for this device that includes the internal bond wire connections, and pins 4 and 5 are not internally connected.

    Regards,

    Kelsey