Question with TPS5420

Hi David,

I think some issues may be caused by the selected BOOT capacitor. This is recommended to be 0.01uF and C128 is 2.2uF in your circuit. Can you change this and see if there is any improvement?

Regards,
Anthony

Thanks.

I will change to 0.01uF and let you know the result.

after changing C128 to 0.01uF, there is still 1.5V input voltage.

I have used the same circuit for TPS5450 successfully with different Mosfet setting since TPS5450 generate higher current than TPS5420.

Maybe the problem is from P Mosfet.

Do you have any idea regarding to this problem?

Many thanks and Have a nice weekend.

David,

Is there any load on the output when shutting down the TPS5420 pulling it up to 1.9V? The 1.5V input voltage is likely from the output voltage through the body diode of the high side FET.

It could also be the remaining voltage on the output capacitor after shutting down the supply. Which is also why it appeared to not turn off when pulling ENA low. For testing purposes, can you add a resistive load to the output to allow the capacitor to discharge after shutting down?

Regards,
Anthony

1.5 V at Vin is not sufficient to overcome the internal UVLO.  So the device will not switch.  I suggest that you look at the load.  What is the output driving?  Are there any other active voltage rails on your board?  Most likely there is a leakage path that is sourcing 1.5 V to your output and is leaking thru the body diode of the internal high side FET to Vin

The load are some power supplies for MIC4451( Mosfet driver),SN74LVC1G17DBVR(digital buffer),74LVC4245APW,118( level shifter) and ADG708( analog switch).

Maybe I can put FDC6330L (load switch) between the output of TPS5420 and the load as I did in another design.

I will try again and let you know the result.

Have a nice weekend.

Hi John,

Would you give me an example Where I can find "active voltage rails" ?

Do you mean another switcher?

Many thanks.

Yes exactly.  Say you have your TPS5420 output voltage 5 V supplying some circuit that also has another supply.  When you turn off the 5 V output, sometimes that other supply can "leak thru" some piece of the circuit and onto the 5 V line.  There may be some losses thru the leakage path so the leaking voltage shows up as some seemingly random voltage like the 1.9 V you observe.

I know where the problem is:

It is from 74LVC4245APW,118( level shifter) , since 74LVC4245APW,118( level shifter) needs 3.3V be on first and 5V on secondly.

The 3.3V leaked to 5V section and generated 1.9V.

Many thanks