Simple current detection

[JJalling]

Hello,

I have a stm32 running at 3V3. I want to monitor a USB VBUS (5V) line and detect if the current draw is higher than say 50-100mA (the number doesn't need to be exact - just ball park).

How would you do this with as few external components as possible? A sense resistor and a voltage divider on the VBUS line, to the ADC of the STM32?

Thanks in advance.

BR Jonas

[Terry Bites]

Use a current monitor amp and connect its output to your ADC input. You can tie the ADC ref in as well.... FAB
eg https://www.onsemi.com/products/amplifiers-comparators/current-sense-amplifiers/ncs199a3rsqt2g
Cheaper than an Aussie Dollar. Happy days!

[mvs]

How would you do this with as few external components as possible?

current shunt + integrated high side CSA.
If you are space constrained, there is quite small CSA in sot23-3 package with current output - ZXCT1009.
https://www.diodes.com/assets/Datasheets/ZXCT1009.pdf

A sense resistor and a voltage divider on the VBUS line, to the ADC of the STM32?

VBUS line is not exact 5V, so you would need to measure VBUS before and after current shunt and then substract values. shunt + 2 divider = 5 parts

If the rest of your circuit is also 3.3V, you may monitor current after LDO.   

[JJalling]

Thank you, both of you.

I'll use a CSA, but I have one question; won't I have problems, since my circuit is 3.3v and I want to measure on the 5V circuit?
Or should I just supply the CSA from the 3.3v line, and use the voltage reference in the STM32 to supply the CSA reference?

If the rest of your circuit is also 3.3V, you may monitor current after LDO.   

It's not the 3.3v circuit I want to measure, but whatever is connected to the VBUS, but thanks.

BR Jonas

[JJalling]

Thank you, both of you.

I'll use a CSA, but I have one question; won't I have problems, since my circuit is 3.3v and I want to measure on the 5V circuit?
Or should I just supply the CSA from the 3.3v line, and use the voltage reference in the STM32 to supply the CSA reference?

If the rest of your circuit is also 3.3V, you may monitor current after LDO.   

It's not the 3.3v circuit I want to measure, but whatever is connected to the VBUS, but thanks.

BR Jonas

Well, reading the datasheet answers my question

BR Jonas

[Terry Bites]

isn't that part discontinued?

[JJalling]

Hello Terry,

I have picked INA180 - it does what I need

Thanks again,

BR Jonas