Can LMR14050 output 12V/3A?

user1876340

Prodigy 170 points

Other Parts Discussed in Thread: LMR14050, LMR14050QDPREVM

Hello,

I am designing the buck with LMR14050 .My question is :

Input: 24V
Output :12V/3A

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,

Thanks

Devin

cid:image012.png@01D2361A.08FD3FF0

over 7 years ago

0 Arief Hernadi over 7 years ago

TI__Mastermind 23815 points

Hi,

Do you have a snapshot of your layout?

Thanks

-Arief

0 Luke Pan over 7 years ago

TI__Intellectual 2790 points

Hi,

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.

Luke

0 user1876340 over 7 years ago in reply to Arief Hernadi

Prodigy 170 points

Hi, Arief

In fact, I changed some parameters on LMR14050QDPREVM. Please see layout as below:

Thanks

Devin

0 user1876340 over 7 years ago in reply to Luke Pan

Prodigy 170 points

Hi, Luke
The list of Cout is below:
1 x 270uF AluminumPolymer（ESR 8 m Ohm.) + 2 x 47uF MLCC
Thanks
Devin

0 Arief Hernadi over 7 years ago in reply to user1876340

TI__Mastermind 23815 points

Hi,

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?

Thanks

-Arief

0 user1876340 over 7 years ago in reply to Arief Hernadi

Prodigy 170 points

Hi， Arief

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?

Thanks

-Devin

LMR14050 evm.pdf

0 user1876340 over 7 years ago in reply to Luke Pan

Prodigy 170 points

Hi,
As your suggestion ,the LMR14050 does not work normally.

Thanks for your reply!

-Devin

0 Arief Hernadi over 7 years ago in reply to user1876340

TI__Mastermind 23815 points

Hi,

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.

THanks

-Arief

0 user1876340 over 7 years ago in reply to Arief Hernadi

Prodigy 170 points

Thank you ,Arief

0 user1876340 over 6 years ago in reply to Arief Hernadi

Prodigy 170 points

Hi 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 :

Fig-3 Diode-SS54

Will the CT of diode cause over shoot ? Is there any methods to solve the problem?

The spec of diode has been attached here:5707.SS54 (2).pdf Diode-(PDS760-13)-PowerDI-Diode.pdf

Devin

0 Andy Chen over 6 years ago in reply to user1876340

TI__Mastermind 27045 points

Hi Devin,

I think you working in China as the Osope language is Chinese.

Did you measure the SW waveform by shortest probe grounding?

You may add 5.1 ohm in series with C4 to limit switching speed and resuce the leading edge ringing.

B R

Andy Chen

Power management

Power management forum

Can LMR14050 output 12V/3A?