Pushing the Vin to Vout range of the LTM4600HV.

I have a circuit that uses the LTM4600HV to create a 1.2V supply from 24V.  Somehow this design made it into production, despite the fact that this Vin-Vout ratio is outside the range of the part.  Not surprisingly, the circuit often oscillates.  I've found that if I add a 499K resistor from Fadj to Vin (24V) I can stabilize the circuit.  This results in a Ton time of 93ns, which is less than the 100ns minimum.  I realize I'm off the map for this part, but how much further can I go without falling over the edge, and what dragons might I encounter on the way?

Update:  I've gone with a 402K resistor, and therefore an 89ns pulse width without issue on a half-dozen units.  I I also had to remove the recommended 100pF cap between Vout and Voset, and instead connect a 270pF cap from Voset to GND.  I'm sure this slightly compromises transient response but it keeps the output stable.



Added the "Update" comments
[edited by: Charlie_99 at 4:24 PM (GMT 0) on 14 Dec 2018]
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  • Problem solved.  The circuit I'm dealing with had the feed forward cap connected outside of an output filter.  This put a 0.22uH inductor in series with the feed forward cap.  With the feed forward path connected directly to the output I can now get the circuit to be stable with a 400kHz frequency, as suggested.  Thanks.

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  • Problem solved.  The circuit I'm dealing with had the feed forward cap connected outside of an output filter.  This put a 0.22uH inductor in series with the feed forward cap.  With the feed forward path connected directly to the output I can now get the circuit to be stable with a 400kHz frequency, as suggested.  Thanks.

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