Hey all,
I posted this on the TI forums
here, but haven't made any progress, hoping someone here may be able to help.
I used WEBENCH® Power Designer to design the DC part of a 24 VAC to 5V 3A and 3.3V 3A supply using the LM25085SD/NOPB. The 5V and 3.3V circuits are identical except for the R17 and R23 which are used to set the output voltage.
When the circuit is powered via 24VAC transformer VS is typically around 34VDC and both the 5V and 3.3V hover around 2V.
Lowering the input voltage such that VS is ~12VDC results in the expected 5V and 3V output. If I slowly increase the input voltage, eventually the rails will drop to again. I've primarily been experimenting with the 5V rail and the drop out point seems to be around VS=19VDC, but also seems to vary with the output load.
I'd appreciate any suggestions / troubleshooting steps. Please let me know what other info I can provide to help.





In both scope captures below
CH1 (yellow): Vout
CH2 (blue): switch output FET, Diode and inductor (T1, D6 and L1)
There is no load on Vout.
When VS = 19VDC Vout = 5V

When VS = 20VDC Vout = 1.38VDC

I attached a 12 Ohm load the the 5V output and repeated the measurements. With the 12 Ohm load Vout drops when VS is greater than ~12V:
CH1 (yellow): Vout
CH2 (blue): switch output FET, Diode and inductor (T1, D6 and L1)
Here is VS=12V with 12 Ohm load on Vout:

Here is VS = 13V with 12 ohm load on Vout:

Same setup with time scale adjusted to see multiple switch events:

The positive side of the input cap and the voltage at Vin on the LM25085SD are steady at VS in both the good and bad scenarios. No dips, very little ripple.
The voltage on the source side of the FET is also a very steady VS.
Here are some captures of the PGATE output with a 12 Ohm load (~400mA draw on Vout)
CH1 (Yellow) is Vout
CH2 (Blue) is the PGATE output of the LM25085SD
VS=11V Vout = ~4.96V

VS = 12V Vout = ~4.96V

Vs = 13V Vout = ~2.05 V

VS = 15V Vout = ~0.07V
