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Power sequencing for LMH7324 comparator

Other Parts Discussed in Thread: LMH7324

In one of my designs i use several LMH7324 comparators with the following power supplies:  VCCI = +5V, VCCO=+3.3V and VEE = -5V. Due to the power sequencing the +3V3 starts some time after the +5V and -5V.

This power sequencing was a poor choice... Obviously the LMH7324 chips immediately starts drawing current from the VCCO supply, which goes down to approximatly -1V. I suspect the LMH7324 chips are now "powered" trough a protection diode on the output of the +3.3V regulator. This prevents the +3.3V regulator from starting.

What would be the recommended power sequencing for the LMH7324 is one can not guarantee, that all supply voltages ramp up at the same time?


In my current design i hope to fix this problem with a low drop schottky diode from ground to the +3.3V rail. This should prevent it from going too far into the negative, and allow the +3.3V regulator to start. However, i am worried what the digital outputs of the LMH7324 chips will do while VCCO is powered from ground. The outputs are connected to LVPECL chips with a typical 3x50Ohm Y-termination to ground. The inputs of the LVPECL chips may not receive a negative voltage. Can the outputs of the LMH7324 create a negative voltage in this configuration? Figure 14 in the datasheet shows open emitter outputs, which can not sink current, but i am not sure if that is only for illustration.

  • Hi Markus,

    The LMH7324 outputs have diodes to VCCO and VEE as shown in the same Figure 14.

    I have not made start-up / transient current measurements on the VCCO pin myself and I'd need to some time to setup to do that, if necessary. So, I'd like to ask some questions to see if we can resolve the problem more quickly by other means?

    1. From your explanation, I'm not sure why your 3.3V regulator is failing to power up? Is it because you exceed the regulator current rating at this instance? Do you know what that rating is (and how many LMH7324 are being powered off of it?). I'm hoping that your regulator rating is not set too close to what your devices may need.

    2. Are you in a position to remove the LMH7324's VCCO pins from the 3.3V regulator and confirm that the regulator does power up without any issues? That my not be an easy thing in your setup, but I thought I ask anyway.

    3. Can you alter the power sequencing on your setup as an experiment (e.g. VCCO up before VCCI) to see if the 3.3V power up problem is affected?

    4. If you were to replace the 3.3V regulator on your board with a lab power supply instead, can you start up the 3.3V supply this way? Can you measure the transient current at start-up?

    5. Is it feasible to try a series R, shunt cap on the VCCO bus that powers every LMH7324? I'm thinking this could be a low pass RC arrangement, that is sometimes used to isolate / filter the supply noise, but it could also act to reduce the start-up current visa vis a vis the series R.

    Please let me know and we'll try and figure out a way out of this.

    Regards,

    Hooman