LM3478 Compensation

Xavier Hanson

Hello everyone,

I'm having problems designing a compensation circuit.

I've been working on designing a power supply that delivers 45V at 0.2222A from a 14V source with this basic layout:

I initially tried out a webench design with a 100uH inductor, but am forced to use a 6.8uH inductor due to size constraints.

the other relevant component values:

Fs= 981 kHz

Cout=100uF (ESR at 100kHz = 28mOhm)

Rsn=0.07 Ohm

I've been using this Application Report to design the compensation circuit and when I try to compensate for the poles and zeroes I get rather strange values for Rc (14.47Mohm!) and Cc (13.8nF).

Am I missing something?

over 10 years ago

0 John Bowie over 10 years ago

TI__Intellectual 890 points

Hi, We'll have a look at this and get back to you tomorrow.

regards

John

0 Doyel Ghosh over 10 years ago

TI__Intellectual 770 points

Hi Xavier,

Please can you clarify if you are using the Apps note only, or are you using Webench? If you are using Webench, can you send me your design please?

Thank you

Doyel

0 Xavier Hanson over 10 years ago in reply to Doyel Ghosh

TI__Intellectual 925 points

I initially did a webench design, but the values for Rc and Cc were calculated entirely with the app note.

I wanted to calculate the values for the compensation circuit properly before tweaking in webench.

0 Xavier Hanson over 10 years ago in reply to Xavier Hanson

TI__Intellectual 925 points

Here is what I whipped together in Webench:

Again, this is the default design with the 100 uH inductor. I want to use the App note to get a better grip on the compensation circuit since the Cout and L values are different for my end application .

0 Doyel Ghosh over 10 years ago in reply to Xavier Hanson

TI__Intellectual 770 points

Hi Xavier,

Adding in here what I emailed earlier:

I’ve looked into the compensation values with your given conditions. Based on the equations in the application note, I get ωp1 to be 49.5rad/sec or 7.882Hz, RHP Zeros at 57KHz etc.
Because the pole is of a much lower frequency than the example, the gain rolls off sooner, thus a smaller compensation RC should be okay. For ωp2 , roughly 300 Hz (approx.), why don’t you try 100nF/5k or 10nF/50k ?

Hope this helps.

Thanks
Doyel

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LM3478 Compensation