# LT4320 when using RDSON small

Hi,

I would like to use LT4320.

So, I have a question.

#1   When DC voltage 24 V is inserted and IAVG is set to 1 A, it is necessary to use RDSON with 30 mΩ or more.

However, is there a disadvantage when using it at 30 mΩ or less?

#2   Is there a problem when using the voltage drop of the ideal bridge at 30 mV or less?

#3  I calculate RDSON by calculating data sheet.
Is it OK to select RDSON of calculation result so that minimum value or typical value of RDSON of FET exceeds

calculated value?Or should the maximum value exceed the calculated value?

I would like to use as small a value as possible.

Best regards,

Yuya

• Hello Yuya,

LT4320 regulates the Top FET as an ideal diode with a forward drop of 20mV, typical. Decreasing RDSON to lower than (30mV / IAVG) will not yield much better performance. The Bottom FET’s forward drop is not regulated, and this FET is treated as a switch. I’ve addressed each of your questions below.

#1   A lower RDSON FET will be more expensive without much improvement, and may have a larger Qg so the FET will transition slower between on and off states.

#2   The voltage drop of the Top FET is regulated to 20mV, typical. The Bottom FET's voltage drop is not regulated, so it can be lower than the Top FET.

#3   It is alright if the maximum RDSON is equal to or less than this calculated value. The minimum RDSON can be lower than this calculated value. This RDSON = 30mV / IAVG equation is a starting point for selecting a FET. You can use a smaller RDSON value than calculated, as long as VGS(th) is equal to or greater than 2V and Qg isn't too big. For a 24V / 1A application, I would recommend the PSMN011-60MS (11.3mΩ), PSMN040-100MSE (36mΩ), or similar FETs.

Best Regards,

Eric