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Original question:

LM3478: Current Sense resistor

LM3478: current sense sensor

Juan Cai
Prodigy 10 points
Part Number: LM3478

Hallo I have two questions by selection of Rsen.

Question 1:

I want to use the Rsen to set the current limit, just like the application in  datasheet (page 20).

According to the data sheet, I need parameters Vsense, Vsl or Vsl ratio to determine the value of Rsen.(please see the picture below).

However the range of the parameters are very big (see the table below). When I calculate with the limits of Vsense and Vsl ratio, the Rsen changes in a very big range.

 

For Example, with D = 70% ISWlimit = 5A

When Vsense = 0,125V and Vsl ratio =0,7

VSSmax= Rsen*ISWlimit = 0,125-(0,7*0,125*0,7) = 0,063V -->Rsen = 12,6mohm

 

When Vsense = 0,190V and Vsl ratio =0,3

VCSmax = Rsen*ISWlimit =0,19-(0,7*0,19*0,3) = 0,1501V -->Rsen = 30mohm

So the VCSmax has a range from 63mV to 150mV depending on values of Vsense and Vsl ratio.

 

My question is, whether this calculation realistic is? If not, how do the VCSmax and Rsen be calculated?

Question 2: 

in the data sheet the parameters Vsl, Vsc, Vsense are given for Vin=5V. Do Vin has a big influence on the parameters?

If yes, is there any relationship between those parameters and Vin

 Any help will be appreciated.

  • 0
    TI__Guru* 78270 points
    Hi Juan Cai,

    The datsheet equation is correct. There are tolerances for each parameters and you need to design for worst case to make sure you circuit will not hit the current limit at full load. In this case, I would recommend you to chose the smaller Rsen.

    Regarding the Vin effects, these parameters are basically performances of internal circuits that are biased by internal regulated voltage rails, so they do not vary with Vin once Vin is adequate to enable the IC.

    Hope these clarifies.

    Thanks,
    Youhao Xi