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# LMZ14203: CFF selection and the limit of Ron

Part Number: LMZ14203

Hi There,

I would like to use a LMZ14203. The input is 26 V, and the output should be 5 V with up to 3 A.

According to Datasheet of LMZ14203 chap.8 Application and Implementation, there is no formula information to select CFF.

Regarding

As for the limit of  Ron, Ron >= Vin(Max) × 150ns /(1.3 × 10^-10) is given as equation (17).

But what is the limit of maximum Ron?

Although we simulated WEBENCH with our set-up and got specific values of Ron and Cff,

Thoery or Algorithim to decide Ron and Cff is needed.

Any mothods to calculate Ron and Cff for  input 26 V, output 5 V with up to 3 A?

My WEBENCH result summary is attached.

1145.WBDesign1.pdf

• Hi,

Per datasheet pin function description, the Ron values is  25 kΩ to 124 kΩ. Therefore i would use 124 kΩ as my maximum value.

For formula on Ron, you can refer to equation 13 on the datasheet.

For CFF, it is normally based on where you want to put an extra zero in your control loop. There is no set equation but the location of the zero can be calculated by a formula of fzero = 1 / (2*3.14*RFBT*CFF).

In this case webench should be used as a starting point for the design values for your specifications.

Thank you

-Arief

• Hi Arief,

Regarding Cff and Fzero, we have still questions.

>>For CFF, it is normally based on where you want to put an extra zero in your control loop.

>>There is no set equation but the location of the zero can be calculated by a formula of fzero = 1 / (2*3.14*RFBT*CFF).

We have to simulate LMZ14203 frequency response with SPICE model and measure LVM14203EVM in the lab to understand where to put an extra zero in cotrol roop?

or we should decided where to put an extra zero in our control roop by theoretical equations?

Do you have some materials or methods to understand fzero for the Vin=26V,Vout=5V,Iout=3A with LVM14203?

-Maki

• Hi Shunnosuke,

Yes that is correct. Normally the zero is added if you would like to increase the phase margin.
We normally measured the frequency response in the lab without CFF.
Then if you need more phase margin, you can add the CFF at a specific frequency in order to boost the phase.

Unfortunately on this product i could not find equations to decide on the CFF value.

Looking at the information on the datasheet, you may be able to use figure 24 and table 1 as starting value or using Webench for component selection
We also have evaluation module to evaluate the performance of the module
https://www.ti.com/tool/LMZ14203DEMO

One thing that you can do to assess the stability is to do a load transient and measure the amount of ringing in the output voltage based on this article