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while using MCP1630 in switch mode power supply design, in the datasheet they gave the input bias current of the "current sense pin"

what are the considerations the designer should take while choosing the sense resistor and the ramp generator circuit (in current mode control)??

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For almost any normal design, that bias current is so low it means "stop worrying, have a nice weekend".

The design consideration is that when you put 100nA into your current sensing network from that pin, that the voltage rise is insignificant compared to the \$\pm\$50mV of uncertainty in the current limit threshold voltage of the chip. That means that in a normal design your current sense resistor should be no more than 500k\$\Omega\$ -- which, since most current sense resistors are in the milliohm range, should be a pretty easy condition to satisfy.

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