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LM66200: LM66200DRLR switchover output dip

Bhavesh Bhadricha
Prodigy 90 points
Part Number: LM66200
Other Parts Discussed in Thread: TPS2116

Hi,

we are using LM66200DRLR to switch between 5v USB input Vin1 and battery input 3V-4.2V VIn2.
when there is switching(system is running on battery and USB is connected OR when system is running on USB and USB is disconnected)  between the two there is a dip which results in our system reset.

  1. what can be the solution to stop the dip?
  2. do we have a part which resolves this issue with continuous power output management in same package
  • 0
    TI__Mastermind 19020 points

    Hi Bhavesh,

    A waveform with VIN1, VIN2, VOUT, and ST will help to definitively diagnose this. A schematic will also help.

    This is most likely due to one of two things: the output dropping during the switchover period or the inputs dropping due to the load transient. What is the load current, how much output capacitance do you have, and how much capacitance do you have on each input?

    This can likely be resolved by adding additional capacitance on one or all of the inputs and outputs.

    Thanks,

    Patrick

  • 0
    Prodigy 90 points

    Hi Patrick,
    there is a dip at C29 of 3V for 2ms from 5V to 2V which is causing the system reset during changeover from USB to Vbat

    we added 50uF at oring output  C29 but there was marginal changes in dip
    the datasheet mentions Tsw switchover delay but that is load dependent.

    1.>do you have any other suggestion than adding cap
    2.>any other IC which would resolve the issue (does not have Tsw switchover delay)


  • 0 in reply to Bhavesh Bhadricha
    TI__Mastermind 19020 points

    Hi Bhavesh,

    I would recommend adding more input capacitance on both inputs rather than output capacitance. The outputs may be dipping due to the sudden load transient, or the device may be switching rapidly when the input voltages reach similar voltages. I believe that this will solve your issue - but again, a waveform of this occurring will really help to diagnose this.

    If that doesn't work, or if you want to jump to another part right away, then I would recommend TPS2116/7, as you can program hysteresis into these parts. But I anticipate that you will still need more input capacitance with these devices, as this issue shows that the input sources' load transient performance is relatively weak.

    The switchover time is on the order of us, and is not what is causing the issue. When a substantial load is suddenly connected to a power source, the source's voltage will likely dip, sometimes below the LM66200 minimum voltage, causing a reset - this dip can be mitigated using more capacitance on that source.

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Prodigy 90 points

    I have added 100uf at VIN1-USB input and 470uf at Vout
    attached is the image requested
    we cannot increase cap across VUSB as per USB standard.


    system reset -->



    system not reset-->

  • 0 in reply to Bhavesh Bhadricha
    TI__Mastermind 19020 points

    Hi Bhavesh,

    Can you try adding 100uF on VBAT in addition to the 100uF you placed on VUSB, and remove the 470uF on VOUT?

    Then take waveforms with IN1, IN2, and VOUT. I believe that this will show how the input capacitance reduces dip seen on VBAT when the device switches from IN1 to IN2.

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Prodigy 90 points

    Hi Patrick

    this did resolve our issue. will do further testing to confirm.
    just one query , VBAT supply is form LiPo battery which is being charged by charger IC and sensed by fuel guage IC, would there be any concerns on connecting 100uF accros this VBAT

  • +1 in reply to Bhavesh Bhadricha
    TI__Mastermind 19020 points

    Hi Bhavesh,

    Great, please let me know if I can assist more, or if you have any further questions.

    I don't see how 100uF across the battery can pose any problems.

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Prodigy 90 points

    Hi Patrick
    with further testing it was found that as the Battery voltage decreases or load current increases the system resets when VUSB is removed.

  • 0 in reply to Bhavesh Bhadricha
    TI__Mastermind 19020 points

    Hi Bhavesh,

    This can likely be solved by adding more input capacitance (if the system is resetting due to the input voltage dipping right after switchover) or output capacitance (if the system is resetting due to the voltage dip during the switchover). Can you see where the voltage is dropping too low on a scope waveform then add more respective capacitance and see if that solves the issue?

    If this doesn't solve the issue, then I would suggest using TPS2116, which can be programmed to use one supply until it reaches a certain voltage, then switch over to the other supply.

    Thanks,

    Patrick