LM25085 EVAL Board incorrect

Hi Steve,

are you referring to AN1905?

Steve,

There are 2 boards that are look almost identical.  They are the  LM25085AMYEVAL and the LM25085MYEVAL.  The layouts are similar, but not identical. The components are mostly different, as well as the target specs.

Contact me if there are other issues.

Regards,
Richard Levin
Manager SVA Eval Board Group

Hi Steve,

Can you please tell us what problems you faced with the Webench design?

Thanks,
Akshay

it just didnt work very well, it had problems with a load over 2 amps or so. i have given up on the webbench design after seeing the performance of the eval board, much bigger inductor, fet and diode.

too bad the documentation here is also incorrect in the app note.

steve

Hi Richard,

The PCB I recieved is 551600255-001, so i guess this is the older version

Can I use the design of the EVAL board with the newer A part or do other values change too - 12V in, 5V 5A out.

Both app notes have different value parts than the EVAL

is it possible to call me at the phone number in my profile?

Thanks,

Steve

Was the design current limitting at loads above 2A?

The sizes of the components can be changed on Webench using the 'Optimization Knob'. The default design returns with the knob positioned at position 3. If you turn it towards 1, you will get the smallest components at a slightly lower efficiency. Turning the knob towards 5 will give you the highest efficiency at a higher BOM size.

Regards,
Akshay

Project data (Buck):
Vin: 24V
Vout: 12V (+-2%)
Iout: 5A

Guys, I had a very similar problem.
For over a week we concentrate our forces to put the project generated by Webench to work.
No charge, all wonder Vout = 12V, but with dozens of miliamper load, Vout drops drastically. I think all the possible options were tested, including bypassing Rsense (short circuiting it), changing various parameters, inspecting the PCB, the devices ...
Now, the purpose of comparison, we resorted to the mounting Evaluation Board LM25085A. The readjusted to Vout = 12V. With a load 1A works, but for 2A, Vout drops by half.

Hello Flavio,

Please send the design schematic that you created using Webench and also the modified schematic that you used on the eval board. Without pertinent information, it is hard to understand where the problem would be. 

Typically if the Vout is in regulation at light loads and is out of regulation at higher loads, it is due to premature current limiting. That can happen if the components are not sized appropriately or if there is too much noise on the sense lines. The LM25085(A) devices use a Rsense resistor in series with the FET or the MOSFET rdsON. Using the RdsON is not generally advised because the value of RdsON is not a fixed number and would change with temperature. When you mention that you have modified the eval board for a 12V operation, which components did you swap out? Further, are you monitoring your input voltage? The power supply at the input should also be capable of providing the necessary current. If your power supply is not able to provide that, then too you would see this behavior.

Those were some points to look at. I hope our Webench team would be able to look at the schematic that you send.

Regards,
Akshay 

Hello, Akshay,

I attached what you asked: The schematic suggested by webench (which rode faithfully, checking the correct assembly of the MOSFET, ok?), and schematic evaluation board (for conference), where the values ​​of Rfb1 and Rfb2 were readjusted for Vout 12V.
Note1: in both attempts  RDSon and Rsense were used to ISense, but, unsuccessfully.

In this last attempt (Eval. Board), also changed gradually Rt (potentiometer) thus increase Ton to support greater loads. We were progressing in this way, however, we realize that Iout only reaches less than 2A, otherwise Vout falls from 12V to 6V.

Note2: Vin was monitored all time at oscilloscope.

Best regards.

Hi Flavio,

I am looking into this and will get back to you.

Hi Flavio,

Do you have the experimental waveforms showing the inductor current, SW node, and the output voltage in this conditions? I want to check if the converter is in current limit.

Please also note the frequency of operation when regulating and when not regulating.

Thanks and regards,

Vijay

Hi Vijay,

I agree in this point, always the behavior was as a current limit situation, but, what would you say if we put a short-circuit there in Rsense?

Now, the project was interrupted, if we need go back to it, I´ll attached more information.

Thanks for try help us, and best regards.

Flávio