LM25010 quick start calculator different to work bench - I get no output current

Hello,

 I followed your link to WB and was unable to iopen the design?

I am not sure where the Ron value you mentioned above are correct?  I am not sure why the calculator is telling you its not possible?  Can you please post the calculator with your entries so I can take a look. 

Thanks.

I have attached the spread sheet, if you try to use the values in the design below, it will tell you its wrong.

Maybe try this link again, it worked for me on two computers, it just takes a while to load

https://webench.ti.com/appinfo/webench/scripts/SD2.cgi?ID=185910::power::carl@edsengineering.com.au

if now here is a screen capture:

LM25010QuickStartCalculator.xls

Hi,

Thanks for the spreadsheet. I was still unable to open the webench design. But that aside, I chose an Fsw of 250 kHz and an Ron of 891k. I think we should move on to your issue. Can I have a look at your schematic? Also, please confirm, your issue right now is the Vout takes a long time to come up to set Vout and when you load the output, the voltage collapses? Please confirm. Thanks.

Yes, I can confirm the issue is a REALLY slow power up time. Seems fine when i use the 5V design, instant on, current works, but at 27.6V output it works nothing like at 5V.

I will be changing the design to the other chip with current control as i have to limit the output to 750mA as to not overload my input.

You can create it quickly using WEBENCH and select 27.6V 750mA wiht 33 to 38V input and it will make the design in a few seconds as shown below.

Hi,
The Webench design looks ok to me. The only potential issue I see with the webench design is the Cout value. You can try increasing Cout from 2.2uF to 4.7uF. I don’t think this is the issue causing your problem. This brings me to my next question. I am assuming you have built up the board? What components did you use on your test board? Can you detail the Cin’s ,Couts , D1 and also the inductor you used and any other component values that deviates from your posted design?

Thanks.

Have you tried putting the values from the WEBENCH example I posted into the spread sheet? It tells you it exceeds the limits... To make it work the coil must be 150uH and the Ron must be around 820Kohm if you use the spread sheet.

What I have built up on a test board is the WEBENCH example without the snubber circuit and 2200uF input capactior and 47uF output capacitor.
Here is the coil:
BOURNS SRU1048-680Y Surface Mount Power Inductor, SRU1048 Series, 68 µH, 1.4 A, 1.25 A, Shielded, 0.127 ohm
Newark / Element14 link here:
au.element14.com/.../ProductDisplay

Here is the diode:
FAIRCHILD SEMICONDUCTOR SS26 Schottky Rectifier, 60 V, 2 A, Single, DO-214AA, 2 Pins, 700 mV
Neward / Element 14
au.element14.com/.../ProductDisplay

Everything else is pretty standard 50V 0603 parts.
Input is very stable, no ripple, sitting at 33V.

In this conflagration It should still "Work" and still deliver 1Amp of current to load and regulate close enough to 27.5 volts. (I am driving a dumb load with this that does not care about noise / stability / ripple as long as its within a few hindered mV my application will work fine).

With a 47uF output cap it comes up to 27.5V very quickly (I did have 150uF + 2 x 1000uF + small load), but the second I put any load (even 3mA) it will drop several volts. This circuit essentially cannot deliver any current even with a stable input voltage.

My theory is the WEBENCH is broken for calculating circuit requirements with high Vout values and works fine for lower voltages. I cant imagine to many people want to regulate high voltages like 27.5V.

According to the spread sheet calculator I should change Ron to 820Kohm and the inductor to 150uH.

Maybe I will buy this coil and try it? Or is there some other issue?

This would be the coil I would order to try:
au.element14.com/.../2329188