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Help me choose a current sense resistor for DC-DC converter

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iscmos

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Anyone here did dc-dc converter, since I want to know how to choose current sense resistor? This resistor series with MOSFET which is paralleled to PowerMOSFET. One thing is we need to make it small to get a accurate current ratio, but on the other hand, small resistor leads to small voltage, which also means a low offset current sense amplifier will be required. So how to make a tradeoff between them. Thanks:D
 

mister_rf

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Re: DC-DC converter

''For maximizing energy conversion efficiency of the DC/DC converter, the value of the current sensing resistor is calculated according to the worst-case low current- limiting threshold voltage of the control IC and the peak inductor current by the following equation:

Rs = VL/I peak

Rs= Sense Resistor (Ω)
I peak = Peak Inductor Current = (Maximum DC Load Current) x 1.15
VL = Worst-case-low current-limiting threshold voltage

The key factor in determining the resolution (current limiting threshold voltage) of the control IC is its semiconductor noise.''

Some other examples and details:

http://www.irf.com/technical-info/designtp/dt92-6.pdf
http://www.nxp.com/acrobat_download/applicationnotes/AN10322_1.pdf
http://pdfserv.maxim-ic.com/en/an/AN3883.pdf
 

iscmos

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Re: DC-DC converter

Thank you mister_rf. I think your information is available for me and I aslo checked the paper you refered. I guess you had some experiences with designing DC-DC Converter and I got some other problems, for example, How to choose a better current sense amplifier, maybe we have two choices, one is using common source structure, the other is using common gate configuration, what is the difference between them? One engineer told me should use the latter, but I don't know why?
 

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