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I am designing the buck with LMR14050 .My question is :
I can not obtain the stable output. Please see below waveform:
Figure 1 Input 24V-Ouput 12V/2A-RIpple Voltage 2.6V(Peak -Peak)
Figure 2 Input 24V-Ouput 12V/2A-SW node
The circuit is below:
Please give your advice,
Do you have a snapshot of your layout?
As Arief's request, it's easier to analysis the problem if you can provide the layout. Also the the part number of Cout.
Right now, you can try to remove Cff and add a 100pF~220pF cap parallel with the Rfbb to see if it can solve the problem.
In reply to Arief Hernadi:
In fact, I changed some parameters on LMR14050QDPREVM. Please see layout as below:
In reply to Luke Pan:
Hi, Luke The list of Cout is below: 1 x 270uF AluminumPolymer（ESR 8 m Ohm.) + 2 x 47uF MLCC Thanks Devin
In reply to user1876340:
Your inductor seems quite large for your frequency selection. Could you try a 10uH inductor and see the difference in behavior?
Also do you have the picture of the board with your modifications?
I spent a few days on purchasing inductor samples . I modified the EVM with 10uH inductor ,but it did not work.
I think I may omit something. So I read the EVM user manual again and find a caution:
----R9 must be removed if the output voltage is changed higher than 7 V (see page 2 of the attached file)
R9 is connected to Vout and PGOOD, the maximum input voltage of PGOOD is 5V. When output is 12V, the voltage is higher than 5V.Obviously , I made a mistake, not removing R9. The LMR14050 works normally after R9 is removed.
I'm curious ,why over voltage on PGOOD has effect on the stable of output?
The PGOOD pin is an open drain FET, therefore maybe the FET got damaged when you have output higher than 7V as stated by the abs max.
I guess it could be the FET got damaged and it pull the output low.
Thank you ,Arief
There is another question. I've changed the inductor and capacitors on the LMR14050 EVM. Please see schematic as below:
Fig-1 LMR14050 Input :24V Output:12V/3A
I tested SW node and the waveform is as below:
Fig-2 Diode -(PDS760-13)
There is high over shoot and under shoot while I change diode from B540C-13F to SS54,Please see waveform :
Will the CT of diode cause over shoot ? Is there any methods to solve the problem?
The spec of diode has been attached here:SS54 (2).pdfDiode-(PDS760-13)-PowerDI-Diode.pdf
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