Power management forum

Fabio Sponticcia1

Part Number: LM5140-Q1

Hi,

I'm using LM5140-Q1 QuickStart tools Rev A2, I have a discrepancy between the Inductor value suggested using the tool and the datasheet formula (12): recommended value in the tool is 1.4uH and a comment in red states "Do not chose inductance greater than twice...", I used a 1.8uH 10mohm inductor, the ripple current (row 30) is 56%. Using datasheet formula (12) with Vout = 3.7V , FSW = 440KHz and Iout = 7.5A the inductor value should be more than 3.7uH (ripple current 30%) or 2uH (using 56% as ripple current). If I'm using the suggested value of 1.4uH I'm not respecting formula (12) , but if I use a 4uH I'm picking a value than is more than twice the suggested one... What I'm missing?

Regards

Fabio

over 3 years ago

0 Marshall_Beck over 3 years ago

TI__Mastermind 29250 points

Hello Fabio,

Discrepeancy arises as quite a few variables come in to play here and these established rule of thumbs will not always be applicable to every given application condition.

For your given application, what is the VIN?

Will this be operate as dual phase or single phase?

0 Marshall_Beck over 3 years ago in reply to Marshall_Beck

TI__Mastermind 29250 points

In addition, I was informed more appreciable inductor calculation was formed for this device that factors in sense resistor. Please find calculation. Do not choose inductor value that is more than 2X calculated if duty cycle is >50%

0 Fabio Sponticcia1 over 3 years ago in reply to Marshall_Beck

Prodigy 75 points

Hello Marshall,

The input voltage for this application can be either 12V or 24V.

Therefore are you going to update the datasheet in future? And we can use the Spreadsheet to design our dcdc?

Many thanks

Fabio

0 Marshall_Beck over 3 years ago in reply to Fabio Sponticcia1

TI__Mastermind 29250 points

Hello Fabio,

I confirmed with responsible engineer that we are working to update datasheet with more accurate equation.

Please use equation in determing inductance. Calculator and datasheet can be used, nevertheless, but revised equation is recommended for inductor selection.