LM3481: Current sense resistor calculation

Albin Vinu30

Prodigy 210 points

Part Number: LM3481

To find current sense resistance which formula should be used.

a) Rsen = Vsen/Ipeak

b) Rsen = (Vsen - Vsl -Rsl*k) / Ipeak

vsl = 90mV, Rsl=1k, k=40uA

over 7 years ago

0 Garrett Roecker over 7 years ago

TI__Mastermind 21720 points

Albin,

Equation B is the correct but the Vsl and k factors should be scaled by the duty cycle (D) at that operation point. Duty cycle is needed to properly set calculate the peak inductor current.

Rsen = (Vsen - (Vsl +Rsl*k)*D) / Ipeak

Thanks,

Garrett

0 Albin Vinu30 over 7 years ago in reply to Garrett Roecker

Prodigy 210 points

Hi, Thanks for prompt response. I have a doubt to be clarified. For example, if MOSFET's drain current is 3.5A , sense resistance is 10milliOhm and Rsl = 1kOhm. What will be the current seen by controller. Will it see 3.5A or do i need to calculate as (3.5A*10milliOhm)-(10uA*1kOhm)=0.025V -> 0.025V/10milliohm = 2.5A. I like to know this to understand whether controller can sense my entire range of current or not.

0 Garrett Roecker over 7 years ago in reply to Albin Vinu30

TI__Mastermind 21720 points

Albin,

Please use the equation I provided above and rearrange to solve for Ipeak.

IPEAK = (Vsen - (Vsl +Rsl*k)*D) / RSEN

For example if: VIN = 12V, VOUT = 24V, IOUT =2 FSW = 300kHz and Rsen = 9mOHm, Rsl = 1kOHm

IPEAK = (160mV - (90mV+40uA*1kOHm)*(1-(12V/24V))/9mOhm = 10.556A

Hope that clarifies.

Thanks,

Garrett

0 Albin Vinu30 over 7 years ago in reply to Garrett Roecker

Prodigy 210 points

Yes it did.. Thank your for the quick response.

0 Albin Vinu30 over 7 years ago in reply to Garrett Roecker

Prodigy 210 points

Hi Garrett,

Does the formula remain same to find short circuit current limits too ??

Thanks,

Albin

0 Garrett Roecker over 7 years ago in reply to Albin Vinu30

TI__Mastermind 21720 points

Albin,

Yes. The only think that changes is the current limit voltage.

-Garrett

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LM3481: Current sense resistor calculation