TPS923654: Trouble calculating inductor value

Eric Steimle

Part Number: TPS923654

Page 22 equation 2 of the datasheet is for calculating the inductor value. When I do the math with your example values I get a much different L value. What am I doing wrong?

This comes out to 26.67uH for me. Is the peak current not 1A for that design?

If I change it to 3A the answer matches yours. Also if I take your values for IL(peak) and IL(ripple) and solve for IL(max) in equation 4 I also get 3A.

Just confused

Thank you.

13 days ago

0 Steven Li 13 days ago

TI__Genius 13120 points

Hi Eric,

You can refer to my answers in this E2E thread - TPS923654: Can TPS923654 be used for LED lighting with Vin=24V, Vout(Vf)=50V and Io=3.5A conditions? - Power management forum - Power management - TI E2E support forums

Hope this helps.

Best Regards,

Steven

0 Eric Steimle 13 days ago in reply to Steven Li

Expert 2220 points

Okay that helped using 100% efficiency I get the same answer now. Then by using 10uH I get the same values for ripple, peak, and rms current. But now I tried to match your calculations for Vin ripple (equation 8) and you get 400mV but I get around 11mV. I'm using:

ILripple = 1.6A (from datasheet previous section)

Cin = 33uF + 1uF + 0.1uF (from datasheet this section)

fsw = 0.5MHz

I get

0.012V = 1.6 / (8 * 34.1 * 0.5)

but the datasheet says 400mV

Thank you

0 Steven Li 12 days ago in reply to Eric Steimle

TI__Genius 13120 points

Hi Eric,

I am currently out of office. I will reply to you on May 6th when I am back to office. Thanks for your understanding.

Best Regards,

Steven

0 Steven Li 2 days ago in reply to Eric Steimle

TI__Genius 13120 points

Hi Eric,

I think you are referring to Equation (6) from the datasheet. Am I correct?

If so, then please be noticed that the 33-uF electrolytic capacitor should not be taken into calculation. The MLCCs are mainly responsible for filtering the input voltage and should be used in the Equation (6). The electrolytic capacitor is mainly responsible for energy storage during load change / line change and should not be used in the Equation (6).

Best Regards,

Steven