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LM43603-Q1: PGOOD issue on large capacitor LOAD

Part Number: LM43603-Q1
Other Parts Discussed in Thread: LM43603

Hello,

We use LM43603 as buck +24V => +13V with a very large capacitor loads (4700µF//4700µF).
PGOOD is left floating .

I would like to know if PGOOD pin enable Hiccup protection (it seems to be the case), and what happened is PGOOD is left floating?
Thanks for your help.

  • Hello

    Leaving PGOOD floating should be OK.

    However, this device will not support 2x4700uF in parallel on the output.

    The device may be unstable and will probably not start up.  This device 

    has a hiccup type current limit that is activated during start-up that may

    prevent start-up with such a large output capacitance.

    As mentioned in the data sheet the maximum Cout must be less than 1000uF.

    Thanks

  • Hello 

    I hope your question has been answered.

    I will close this post due to inactivity.

    Thanks

  • Hello Frank,

    Thanks for your reply, it help me.
    I would be more precise about the Cout load : in fact we have 300µF "as Cout", then behind them we have an inductor of 2.2µH+2x4700µF. 

    Do to this very large capacitor load, we've put a large Css in order to have a 500ms softstart, in order to avoid Hiccup triggering.

    I understand it could generate some unstabilities and I agree : it's not the best way to design a buck power supply :/

    I would like to know if from your point of vue, could it have some damages when we switch off totally the DCDC (Vin=0V): due to full large capacitors which could empty through SW pin to Vin? (Is there any 'back' powering protection?)

    Thanks in advance.

  • Hello

    OK, I understand your question.

    Usually it is not a good idea to allow voltage on the output of a regulator when the input is zero volts (or even floating in some cases)

    This is because there is a diode between the drain(VIN) and source(SW) of the power MOSFET inside the converter.

    With a voltage on Vout, and the input held at zero volts, there will be a path from Vout to Vin through the inductor and this diode.

    Even if the input is floating, a current will flow and try to charge the input caps, possibly to a high voltage due to resonance

    with the caps and the inductor.  This phenomena has been seen in the past under some conditions.

    Allowing current to flow through this diode can cause a parasitic to turn on or even damage the diode.  Since it is not a "true" diode

    but a parasitic formed when the MOSFET is constructed on the die, it is not designed to take current for more than a few 10s-100s nsec.

    One way to avoid this issue is to use a diode from input to output to provide a path for the output capacitors to discharge around the device. 

    Thanks

  • Hello 

    I hope your question has been answered.

    I will close this post.

    Thanks