LM20146: LM20146

Shafeeq Muhammed

Part Number: LM20146

I want to use LM20146 for an output of 3.3V,6A. My input is 5V. When I calculate the value of Rc1 (Cc1 as 4.7nf) i got the value as 4.9k. But when i use WEBENCH it shows Rc1 as 15.4k and Cc1 as 4.7nf. Which value should i take?

L=4uH

Cout=100uH

fsw=750khz

over 7 years ago

0 JohnTucker over 7 years ago

TI__Guru**** 191445 points

Muhammed,

I think you made an error. I calculate Rc1 as 5.61 k for your conditions. However I think your inductor value is too high. In your case, I would enter your conditions in webench. If you want 750 kHz switching frequency, you have to enter that change in the "advanced options" section on the left after the initial design is created. Then use the exact schematic and BOM as recommended.

0 Shafeeq Muhammed over 7 years ago in reply to JohnTucker

Expert 1190 points

I got the inductor value from WEBENCH

0 Shafeeq Muhammed over 7 years ago in reply to JohnTucker

Expert 1190 points

When i give the Frequency as 750 KHz in WEBENCH It shows L=950nH, Cc1= 2.7nf , Rc1=4.32k . is this sufficient? For this what will be the ripple current?

0 JohnTucker over 7 years ago in reply to Shafeeq Muhammed

TI__Guru**** 191445 points

Muhammed,

My simulation returned 1 uH instead of 950 nH. I think there is negligible difference. You will probably find 1 uH is much more widely available. I got the same compensation values.

ILp-p = (Vout / Vin) * (Vin - Vout) / (Lout * Fsw)

0 Shafeeq Muhammed over 7 years ago in reply to JohnTucker

Expert 1190 points

If i give the inductor value as 1uh then the ripple current will be 1.496A. So is it possible this much ripple current? Is there any problem by giving 4uH as L?

0 JohnTucker over 7 years ago in reply to Shafeeq Muhammed

TI__Guru**** 191445 points

Muhammed,

for an output current of 6 A, 1.496 A ripple current is 25%. 0.25 is a good value for ripple current factor ( between 0.2 and 0.3). Use 1 uH as advised.

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LM20146: LM20146