LM25085: Vout dropping out at higher Vin

Looks like my images didn't work in the original post so here they are again:

All components are on the front of the board.

Front fill:

Back fill with front silk/pads

Hello

Please use the component calculator that is on our website to double check your component values.

Even though you used Webench, it is good to check.

Can you send a scope shot of the switch wave-form for good and bad behavior.

It looks like your input capacitor is quite far away from the device.  Can you take the input capacitor

and place it near the VIN pin and ground near the device, and test.

Thanks

Do you have a link to the component calculator you mentioned?

By the 'switch wave-form' do you mean the junction of FET, Diode and inductor (T1, D6 and L1)?

Thanks!

-Sam

In both scope captures below

CH1 (yellow): Vout

CH2 (blue): switch output FET, Diode and inductor (T1, D6 and L1)

Edit: There is no load on Vout.

When VS = 19VDC      Vout = 5V

When VS = 20VDC   Vout = 1.38VDC

Hello

I have attached the calculator.

Do you have a load applied when you took these plots?

Thanks

8117.LM(2)5085_Quick_Start.xls

No, no load in the above images.  

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:

I appreciate your help with this, thanks!

-Sam

I plugged everything into the calculator spreadsheet you linked.  In general everything seems to check out except the values in step 7. 

The calculator doc recommends:

C1 = 3300pF

C2 = 0.1uF

R3 = <42.7K

where as Power Designer suggested:

C1 = 10nF

C2 = 560pF

R3 = 75K

Do you think that could be part of the issue or just optimized differently?

Thanks!

-Sam

I took some more measurements this morning.

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

Please let me know what other measurements might be helpful, or what other troubleshooting steps I can try.

Thanks!

-Sam

Hello

Since the values are very different in your previous post, then it is possible that could be the issue.

I would substitute the values and test.

Thanks

I changed to

• C1 = 3300pF
• C2 = 0.1uF
• R3 = 33K

with no real change.  Vout is more noisy near the 'good to bad' VS threshold of 13V, but no improvement.

Any other thoughts or suggestions?

Thanks!

-Sam

Hello

At this point the best thing I can suggest is to get an EVM for this device and test in your system.

Then you can slowly change the components to your configuration and test.

Thanks