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LM25085: Vout dropping out at higher Vin

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Kristoff:
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







jkostb:
Have you checked that your design meets minimum on time and off time time conform data sheet? If you increase the input supply voltage then the buck regulator needs to decrease duty cycle (=VOUT/VIN). But if you decrease duty cycle then the on time will decrease. All buck converters can only regulate if on time is higher than parameter specified in datasheet.

Further have you verified that buck converter is still working in continuous mode when the voltage drops under loaded condition?

Kristoff:
Hey jkostb,

Thanks for the suggestions.

From the data sheet: The minimum on-time, which occurs at maximum VIN, should not be set less than 150ns
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At Vin = 40V I calculate Ton as 192.9nS which is within the specified range.  I'm not sure about Toff.  I calculated it to be 1.350 nS as Vin=4V, but I'm not sure if that is correct or what it should be.

How can I verify that the converter is in continuous mode?

Thanks!






SiliconWizard:
Also, you said that the outputs vary depending on the load.
Is there a minimum load above which it starts regulating properly?

Kristoff:
A higher load appears to lower in Vin value at which the output becomes unstable.

With no load a Vout is a stable 5V up to Vin of 19V.
With a 12Ohm load Vout is stable at 5V up to Vin of 13V.

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