Electronics > Projects, Designs, and Technical Stuff
MOSFET Rds(on) as current shunt - it's not stupid if it works, right?
HwAoRrDk:
Can anyone advise me whether there are any flaws or 'gotchas' in what I'm doing here?
I'm using a high-side load switch to switch power to a load, but I also want to measure the current that the load is drawing. However, I'm only interested in the relative current draw between an 'idle' and 'active' state. I don't really care what the absolute amount of current is, only what one state is in relation to the other. So, given that, I thought rather than add a separate series shunt resistor, why not use the switch's RDS(on) as the shunt resistance? The RDS(on) of the switch I plan to use is approx. 80mΩ, so should serve my needs (I was thinking otherwise of using a 100mΩ shunt) when combined with an amplifier gain of 100V/V.
Now, before you say anything, yes I know using a MOSFET's on-resistance as a measurement shunt isn't typically a good idea, due to imprecision of resistance specification, temperature coefficients, etc, etc. :blah: But given my scenario - I only care about relative current measurement, not what the actual values are - I think it should actually be suitable.
But is this still a good idea? Are there any reasons I haven't thought of why it won't work?
These are the concerns that I've had come to mind that I think are adequately addressed:
- Temperature variation: even if the RDS(on) of the switch does increase at higher temperatures (e.g. maybe it goes up to 100mΩ), it doesn't really matter for my application. The 'idle' and 'active' current readings will both increase by the same amount. And if it's the 'active' current draw itself making the RDS(on) increase only in that state, then it'll just mean there's a slightly higher difference between the two states, which is totally fine.
- Current sense amplifier Vsense voltage when the load switch is off: the INA180 is rated for ±26V differential between VP and VM and common-mode of 0-26V, so if the load side of the switch is at ground when off, and the other is at the supply voltage of +5V, then that's well within the specification, right? The switch I'm considering right now, the MT9700 doesn't have output discharge, though, so I guess technically the load side would be floating? So I don't know what impact that would have on the differential Vsense voltage of the amp. I'm thinking maybe I should change to one with output discharge when off (i.e. load side is switched to ground through a resistance), just to be sure.
- ESD protection diode: the load is off-board, so I want to protect the load output from ESD, so have added TVS diodes that should serve dual duty to protect both current-sense amplifier and switch. I've chosen diodes with minimal leakage current (80nA), as I assume anything of any significance will affect the amp input, in the same way that input offset currents do? Diode capacitance isn't really something I think I need to be concerned with, though, because it'll just form an RC low-pass filter on the amp inputs with the series 10Ω resistors; I'm already low-pass filtering the amplifier output, so some much, much higher-frequency LP filtering on the input is of no importance.
Any other things I should be concerned about? Or do I need to go back to the drawing board? :)
PCB.Wiz:
Capacitive touch buttons work fine on change of value.
You have not said how much or how quickly the current changes ?
One issue is immediately after your circuit starts, or resets, is the load state unknown?
langwadt:
here's a gate driver that shows using the Rdson for current sense, https://www.infineon.com/dgdl/Infineon-ir2127-DS-v01_00-EN.pdf?fileId=5546d462533600a4015355c868861696
Greybeard:
https://www.planetanalog.com/low-side-rdson-current-sensing/
HwAoRrDk:
--- Quote from: PCB.Wiz on January 23, 2025, 06:53:30 pm ---Capacitive touch buttons work fine on change of value.
--- End quote ---
Huh? Where did I mention capacitive touch buttons? :-//
--- Quote from: PCB.Wiz on January 23, 2025, 06:53:30 pm ---You have not said how much or how quickly the current changes ?
One issue is immediately after your circuit starts, or resets, is the load state unknown?
--- End quote ---
When turned on, the load will pulse between 'idle' and 'active' current draw a few times a second.
The load will be off at all times unless the microcontroller in charge specifically turns it on with the high-side load switch (i.e. LOAD_EN goes high).
--- Quote from: langwadt on January 23, 2025, 07:23:07 pm ---here's a gate driver that shows using the Rdson for current sense, https://www.infineon.com/dgdl/Infineon-ir2127-DS-v01_00-EN.pdf?fileId=5546d462533600a4015355c868861696
--- End quote ---
I'm not sure how that's relevant. I'm not driving the gate of a MOSFET. And that datasheet doesn't explain how it operates.
--- Quote from: Greybeard on January 23, 2025, 07:42:41 pm ---https://www.planetanalog.com/low-side-rdson-current-sensing/
--- End quote ---
While that's interesting, I'm doing high-side switch and current sensing.
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