Anyone used mosfet RDS as current sense ?

[station240]

Was looking at a design that uses the LTC1871, and uses the Mosfet as a current sense. That is measure the voltage drop across the mosfet source and drain as if it were a current sense resistor.

This is not something I've seen elsewhere, anyone used this idea in their own designs ?
What sort of things should be considered, besides careful selection of mosfet RDS (on resistance) ?

[0xdeadbeef]

I would assume something like this is used in self protected drivers for current limitation/sensing. It's most probably not very accurate though compared to a shunt resistor.

[danadak]

Contact manufacturer but device to device Rdson variation probably exceeds
50%. So if you are looking for absolute accuracy in readings then either you
provide a cal cycle or relay on sense R of your choosing.

There is a method in manufacturing you can turn a poor sensor into high precision,
see attached.

Regards, Dana.

[nctnico]

I have used RDSon current sensing in various designs. As long as you keep in mind it is used to sense inducator saturation where the current will exceed the limit a lot you'll be fine. When designing assume the RDSon to be at least 3 times higher than specified.

[zlymex]

I use RDS as variable over current sense(10A to 500A) in my portable battery  protection circuit. It will be too bulky to use a current shunt for 500A. I don't need that precision.

[Skimask]

Was looking at a design that uses the LTC1871, and uses the Mosfet as a current sense. That is measure the voltage drop across the mosfet source and drain as if it were a current sense resistor.

This is not something I've seen elsewhere, anyone used this idea in their own designs ?
What sort of things should be considered, besides careful selection of mosfet RDS (on resistance) ?

Last night I figured out that the TP4056 PCBs I've been using to charge single 18650 Li-Ion cells use the DW01 for overcharge/overdischarge/overcurrent protection, and that board uses R(ds) to trip the overcurrent shutdown.  At least I think it does...either that or I'm talking out my ass...
Example:
http://www.ebay.com/itm/10PCS-5V-Micro-USB-1A-18650-Lithium-Battery-Charging-Board-Charger-Module-/201039030988?hash=item2ecedc2acc:g:zHgAAOSw1vlUxZ0Q

[station240]

MOSFET RDSon sense are usually used in overload protection or hybrid mode SMPS controllers. In a pure current mode SMPS, you better have a more precise current sensor.

The circuit calls the current sense resistor I wish to remove IMax, description calls it input current sense.
Does seem like an overload detection, or to sense inductor saturation.

There is a second current sense resistor that senses the output current.

[Miyuki]

And what to use HEXSense ? Or are they already deprecated now ?