So here's the scenario. There'a a battery charging IC whose charge current is limited to 10A if a a 10mohm current sense resistor is used. But I want to make this charge current adjustable from ~0A to 20A.
To me, the only way I can come up with is to have have ten 50ohm current sense resistors in parallel that can be switched in. This would allow 2 to 20A in 2A increments.
To have kelvin connections on a bunch of switchable, parallel current sense resistors might be a problem both electrically and for routing on a PCB unless I took up a lot of space on a second layer. But I figure that just a kelvin connection sense across only one resistor that will always be connected should be good enough- the other resistors should be sharing the same current anyways so the voltage drops should be the same. In that case only 9 switches would be used.
But anyways, its pretty cumbersome with all the switches and expensive at $1 per current sense resistor. Any alternative approaches would be appreciated (there is a resistive divider on the IC that sets current sense voltage that must be seen to limit charge current. I lose a lot of sensitivity though with this method since the one current sense resistor used must be the smallest value to support the highest level of current.
There's also the issue of switching all the paralle resistors. A switch per resistor is kind of a major PITA especially since it's so redundant- any n-resistors can be switched in to set a particular current level. It doesn't matter which resistors as long as n of them have been switched into the circuit. All methods to enable the use of only one switch either require a rather odd high current switches, or a relay bank with the associated drive transistors, 10 pole switch, and a strange control IC similar to a one-hot input priority encoder (rather than a typical binary input priority encoder). It's a crapload of parts though to reduce things down to a single switch.
To me, the only way I can come up with is to have have ten 50ohm current sense resistors in parallel that can be switched in. This would allow 2 to 20A in 2A increments.
To have kelvin connections on a bunch of switchable, parallel current sense resistors might be a problem both electrically and for routing on a PCB unless I took up a lot of space on a second layer. But I figure that just a kelvin connection sense across only one resistor that will always be connected should be good enough- the other resistors should be sharing the same current anyways so the voltage drops should be the same. In that case only 9 switches would be used.
But anyways, its pretty cumbersome with all the switches and expensive at $1 per current sense resistor. Any alternative approaches would be appreciated (there is a resistive divider on the IC that sets current sense voltage that must be seen to limit charge current. I lose a lot of sensitivity though with this method since the one current sense resistor used must be the smallest value to support the highest level of current.
There's also the issue of switching all the paralle resistors. A switch per resistor is kind of a major PITA especially since it's so redundant- any n-resistors can be switched in to set a particular current level. It doesn't matter which resistors as long as n of them have been switched into the circuit. All methods to enable the use of only one switch either require a rather odd high current switches, or a relay bank with the associated drive transistors, 10 pole switch, and a strange control IC similar to a one-hot input priority encoder (rather than a typical binary input priority encoder). It's a crapload of parts though to reduce things down to a single switch.
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