MP9989 - External VDD?

I’m thinking of using MP9989 to generate 4.35V.
Can I / Do I need to : feed the VDD from an external 12V, that are available ?

Hi Thomas: Welcome to our community !! I hope you enjoy your stay here and find the solutions you need for Power Management. The MP9989 does not need an Auxiliary Winding from an external 12V. So you don’t need to feed any 12V from external source. I am assuming you are using this in a Flyback converter. Is the output current at 4A or below? Also what is the turns ratio of the Flyback transformer? When the Main primary mosfet of the Flyback converter is OFF, the dot end is positive with respect to the no dot end of the secondary winding. It is during this time the internal Linear regulator works to charge the VDD pin and regulate the voltage across the cap between VDD and VS to 6.7V. The turns ratio will confirm that there is enough voltage during this time to provide enough headroom for the internal Linear regulator to regulate the cap to 6.7V.

Hi, and thanks!
Yes it’s a flyback that I’m trying to redesign to have a lower minimum input voltage, to be able to comply with ISO 7637-2 pulse 4 and ISO 16750-2 automotive cranking pulse, Level III. And on to that long cables. We have live problems in cars, especially the “Stop-Start” ones.
The current “Ideal Diode” is from TI, and doesn’t seem to like both CCM and DCM.
I think the transformer will be somewhere 4:4 to 4:6.
Might be a bit low headroom when input voltage are 3V for about say 50ms, and then 5V for up to 10s.
The output are 4.35V 0.2A to 2A, and an Aux 12V 10mA. Aux are used for other things as well.

Best Regards

Hi Thomas:
Thank you for your detailed answers. In you case there is not enough headroom as the input voltage is at 3V for 50 msecs and if I assume the turns ratio of 4:4 then only 3V is available on the secondary side at the input of the Linear regulator . Even at 5V it is not enough at 4:4 turns ratio. It is just enough at 5V when turns ratio is 4:6. So an auxiliary winding as you have shown would guarantee enough voltage at the input of the Linear regulator when the Flyback mosfet on Primary is ON. So in your case the extra secondary winding becomes a necessity.

I thought so, but was not sure.

Hi Thomas:
Yes and you already have some load that needs the 12V (10mA) and that makes it even more appropriate to use that available 12V the way you have done to bias the MP9989 and make its operation even more reliable. The MP9989 is now a happy component !!

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