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LM74720-Q1: Inrush Current due to CAP and VS capacitors

Alex Fields1
Prodigy 125 points
Part Number: LM74720-Q1
Other Parts Discussed in Thread: LM7480-Q1

Hello,

In trying to use LM74720-Q1 as an ideal diode OR controller with inrush protection, there is still 2uF of capacitance that will see an instantaneous dV/dt (purple boxes below, from the EVM design) if a battery or power supply is plugged in live. In bench testing, this causes >40A inrush current spikes. Is the only mitigation for this to implement a series hotswap controller, or can we reduce the cap values circled? If a hotswap controller is the only solution, is there a TI part that is recommended to use in series? 

 

  • +1
    TI__Guru 57737 points

    Hi Alex,

    Yes, C1 and C8 can cause some inrush current during input power supply hotplug. The C9 capacitor will not cause inrush current during hotplug because this is the output capacitor of the internal boost converter. 

    Are you okay to use LM7480-Q1 instead of LM74720-Q1 ? LM7480-Q1 just needs 2x 0.1uF capacitors (CVS and CIN in the image below) unlike 1uF required for LM74720-Q1.

  • 0 in reply to Praveen GD
    Prodigy 125 points

    Since I'm using an ideal diode with load sharing with another power source, I'd like to use the linear regulation scheme of 74720 or 74800. However, only 74720 and 74801 are in stock anywhere. 74801 uses comparator only. Is there any problems with comparator only I should be concerned about? 

  • 0 in reply to Alex Fields1
    TI__Guru 57737 points

    For comparator only device there needs to be a minimum reverse current flow (= VSD(REV)/ FET Rds(on)) for the controller to detect it and turn OFF the FET. The advantage of comparator only device is that when the load current is > VSD(REV)/ FET Rds(on), the FET gate is fully enhanced allowing the FET to operate at its lowest resistance (Rds(on))  irrespective of the load current. 

  • 0 in reply to Praveen GD
    Prodigy 125 points

    And in your experience there is no problem with fully enhancing the mosfet in ORing applications with another ideal diode connected to the center node of the back to back NMOS's? Not only for ORing but for load sharing when the voltages are roughly equal

  • 0 in reply to Alex Fields1
    TI__Guru 57737 points

    Hi Alex,

    If the power supplies are very close to each other and the amount of reverse current flow in one path is less than VAC(REV)/ FET Rds(on) then the controller will not turn OFF the FET. This is the only problem that can happen. But for this to happen the difference between power supplies has to be less than  VAC(REV) = 4.5mV which is very narrow voltage range.