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LM5146-Q1: Unstable output voltage

Erik Langr
Prodigy 10 points
Part Number: LM5146-Q1
Other Parts Discussed in Thread: CSD19531Q5A, LM5146, LM5145

Tool/software:

Hello support team,

I'm trying to build a DCDC converter and used a controller LM5146QRGYRQ1. I'm completely new to power management and this is my first attempt. I started with WEBENCH Designer and built my first prototype, which was pretty unstable, so I did some research and in the datasheet, I found Quickstart Calculator. So I tried to calculate several versions and built them and the result is that some of them are gone and some unstable. Can someone look and tell me what am I doing wrong? I attached some pics and the Quick Start calculator which I used.

High side mosfet I've used CSD19531Q5A
Low side mosfet I've used BSC037N08NS5ATMA1
Output capacitors I phisically put 13x 22µF CL21A226MAYNNNE

Input voltage should be from 25V to 51V and output voltage12.2V & max output current 20A.

Thank you for your help.

3107.Pictures.zip
LM5146-q1 quickstart_25-51Vin_12.2Vout@23A -_130kHz_V1 .xlsm

  • 0
    TI__Guru 65997 points

    Hi Erik,

    If Vin-max = 51V, I recommend using 60V or 80V FETs, especially given the high current. Take a look at the LM5145 EVM, as the specs are close to your requirements.

    Just looking at the quickstart file, it's important to derate ceramic Cout for applied voltage. A 22uF cap is likely only 7-9uF at 12.2VDC. Try changing Cout to 100uF and recompensate. If Fsw = 130kHz, the loop crossover can only be ~15kHz, 20kHz max.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hi Tim,

    Thank you for you suggestion. I'll try it. 

    I'd like to ask you about the Fsw.. 130kHz is it OK or would be better to go with Fsw up for example 300kHz?
    As I mentioned it's my first attempt so please be patient with my question.

  • 0 in reply to Erik Langr
    TI__Guru 65997 points

    Erik,

    200-250kHz is a good tradeoff of efficiency vs. size at these high current levels. Note I chose 225kHz in the LM5145 EVM for this reason. I see you have a huge inductor, so increasing Fsw will help from that standpoint. Take a look at the Cyntec inductors in particular.

    Btw, here is Murata's tool to check cap derating vs. voltage: https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us&jis=false

    --

    Tim