Buck-Boost Inductor design calculation

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newbie_hs

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Hi,

I am designing with Buck boost converter with the following parameters:

Vin: 2.5 - 4.2 V

Vout: 3.3V

Iout Max: 1A

Fsw: 1250Khz (From Dtasheet Page 3)

Mine is a battery powered application.

The inductor design calculation in the datasheet is given below.



I believe the 0.3A is the allowed ripple current.Please correct me if I am wrong.

The datasheet also says the below thing.



When I plug my values in the above equation I am getting L1 = 1.89uH and L2 = 1.62uH.

I have 2 Questions:

  1. May I know where I went wrong.
  2. How they obtained the recommended value of 1.5uH to 4.7uH
 

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newbie_hs said:
I believe the 0.3A is the allowed ripple current.Please correct me if I am wrong.
Click to expand...
Correct. It's unclear why they chose to set ripple current to 0.3A for these equations...
newbie_hs said:
When I plug my values in the above equation I am getting L1 = 1.89uH and L2 = 1.62uH.

I have 2 Questions:

  1. May I know where I went wrong.
Click to expand...
Your calculations correct.
newbie_hs said:
  1. How they obtained the recommended value of 1.5uH to 4.7uH
Click to expand...
It's likely that this covers all L1 and L2 values for all possible combinations of Vin (1.8-5.5V) and Vout (1.2-5.5V), with 0.3A of ripple and fsw=1.25MHz.
 

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KlausST said:
...Why do you call it wrong? Both your values are within the recommended range.

Klaus
Click to expand...
How they decided the range.
Why they took 2.2uH
--- Updated ---

KlausST said:
...Why do you call it wrong? Both your values are within the recommended range.

Klaus
Click to expand...
How they decided the range.
Why they took 2.2uH
 

Upvote 0 Downvote
No need to be accurate. The regulation loop does it's job. Higher inductivity is more safe but higher cost and size
 

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