WEBENCH® Tools/LM5163: About selection of Cout

Koji Hayashi92

Mastermind 7030 points

Part Number: LM5163

Tool/software: WEBENCH® Design Tools

Hi TI-team

I calculated the following condition using WEBENCH.

Vinmin=30V, Vinmax=90, Vout=15V Iout=0.5A.

In additon, I optimized with Small Footprint Design.

Then Cout is 3.3μF.

The data sheet recommends 22μF in consideration of the voltage coefficient, but isn't it considered in WEBENCH ?

Should I choose a capacitance that becomes 3.3μF when Vout = 15V ?

Best Regards,

Koji Hayashi

over 5 years ago

0 Iven Su over 5 years ago

TI__Expert 8245 points

Hi,

You can just follow suggestions on datasheet (8.2.2.4), use a larger Cout to ensure actual capacitance at a certain voltage is great than the result of formula 21.

0 Koji Hayashi92 over 5 years ago in reply to Iven Su

Mastermind 7030 points

Hi Iven-san

Thank you for quick response.

8.2.2.4 of the data sheet recommends 22μF when 3.1μF is obtained from Equation 21.

Since the calculation result of WEBENCH is 3.3μF, it is necessary to set Cout to 22 μF.

If it is correct, I think the calculation result on WEBENCH should be 22μF.

What do you think ?

Best Regards,

Koji Hayashi

0 Iven Su over 5 years ago in reply to Koji Hayashi92

TI__Expert 8245 points

Hi,

22uF in datasheet is designed for more margins. Sometimes they are based on difference calculations/considerations, normally the results are reasonable then both of them are correct.

0 Koji Hayashi92 over 5 years ago in reply to Iven Su

Mastermind 7030 points

Hi Iven-san

Thank you for response.

Is it necessary to change the constants of other external elements calculated by WEBENCH when the capacitance is 22μF ?

Is it safe to use more than 22μF ?

For example, 27μF

Best Regards,

Koji Hayashi

0 Iven Su over 5 years ago in reply to Koji Hayashi92

TI__Expert 8245 points

Hi,

If you change the output capacitance, according to table 1 on its datasheet, you don't need to change RA, CA and CB. But you still cannot use a too large one because it will make loop response slow. 27uF might be OK and you need to verify it on your bench to make sure it meets your requirements on transient, ripple, etc. Thanks.

0 Koji Hayashi92 over 5 years ago in reply to Iven Su

Mastermind 7030 points

Hi Iven-san

Thank you for quick response.

Is "your bench" WEBENCH ?

Since WEBEN displays "Simulation not enabled for this design", transient simulation could not be performed.

Best Regards,

Koji Hayashi

0 Iven Su over 5 years ago in reply to Koji Hayashi92

TI__Expert 8245 points

Hi,

"Your bench" means your PCB board, it can help guarantee the actual working performance.

0 Koji Hayashi92 over 5 years ago in reply to Iven Su

Mastermind 7030 points

Hi Iven-san

Thank you for response.

I understand.

Best Regards,

Koji Hayashi

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WEBENCH® Tools/LM5163: About selection of Cout