1
\$\begingroup\$

I'm looking for a simple circuit that uses only a comparator (or an opamp) to detect the presence of a small current through a high-side current resistor. The idea is for a microcontroller input to go high when the current through the resistor exceeds a predefined value (of let's say 200mA).

This is to detect when a phone is actually pulling power from a +5V source. The circuit below illustrates my intent, by I feel that it is missing something, mainly that the comparator may not reset when the voltage across the sense resistor reaches zero. I'm not aiming for precision, but rather a rough "there is current" and "there is no current" type of circuit.

Draft Comparator current detector

\$\endgroup\$
1
\$\begingroup\$

TC75S58' spec
- common mode input voltage range : VSS to VDD− 0.9 V .

So this IC as a high side current detector will not work.
But you might use it on the low side. (Or find a high side Comparator.)

Choose shunt R to read 50mV at full scale. ( most common)

schematic

simulate this circuit – Schematic created using CircuitLab

You must be careful on matched R tolerances, input offset since the current sense is very low mV

THis puts the high side in the acceptable CM range. enter image description here

There are many advantages to low side sensing .

\$\endgroup\$
  • 1
    \$\begingroup\$ The OP wants a high side current monitor not a low side. \$\endgroup\$ – Jack Creasey May 8 at 14:25
  • \$\begingroup\$ Yes but more important, he wants it to work. Wants vs needs ought to be in spec, the difference on hi/lo sense being? Nothing so far... @JackCreasey \$\endgroup\$ – Sunnyskyguy EE75 May 8 at 14:54
  • \$\begingroup\$ It's typically much more challenging to lay out a PCB for low side monitoring, which I would guess id why the OP asked for high side. \$\endgroup\$ – Jack Creasey May 8 at 14:55
  • \$\begingroup\$ For LiPo chargers , low side is also common. Usually for any floating load, it doesn't matter \$\endgroup\$ – Sunnyskyguy EE75 May 8 at 14:56
  • 1
    \$\begingroup\$ I've accepted SunnyskyguyEE75's answer because it covers the basics of the circuit. My main question did not justify the high-side choice, but @JackCreasey's document from Linear and SunnyskyguyEE75's comments made me understand the challenges of high-side current sensing. SunnyskyguyEE75's drawings made me understand how to properly use a comparator in this particular purpose and the clever use of the feedback resistor to add hysteresis to the circuit and prevent unstable behavior of the comparator. In the end I'm going to use the modified version of my circuit that was provided. \$\endgroup\$ – Charles Gervais May 13 at 18:07
1
\$\begingroup\$

Looking for a simple circuit that uses only a comparator (or an opamp) to detect the presence of a small current through a high-side current resistor.

By far the easiest way to accurately detect current levels for an application like yours is to use a High side current monitor such as the TI INA series.

I suggested in the comments the INA186 which is a simple current/voltage translation (and you could use an A/D input on your MCU to measure the current). It may be however that something such as the INA200/201 would be a better fit for your application. This devices includes the current/voltage translator AND a comparator to give you a logic level threshold.

enter image description here

The translator G produces a voltage representative of the current flowing in the sense resistor, and the divider R1/R2 sets the threshold level for detection. CMPout provides a simple logic level to the MCU. In your case *RESET would be left floating to enable transparent mode.

\$\endgroup\$
  • \$\begingroup\$ I had a look at the INA series before posting, but I haven't found the versions with the included comparator. The INA381 is actually the version that makes the most sense (and cheapest) for this particular application. I'm still going to use the generic comparator circuit provided to limit dependencies on specific ICs. \$\endgroup\$ – Charles Gervais May 13 at 18:11
  • \$\begingroup\$ @CharlesGervais There are plenty of INA current sense with comparators available, they are high stock items on Digikey and Mouser. Any of the INA200, INA380 or INA381 would be suitable candidates. The problem with using a low CM capable opamp is the loss of accuracy due to the dividers dropping the signal into the offset range of the opamp. Very hard to set up for accurate results without expensive high tolerance resistors. But of course the solution choice is yours. \$\endgroup\$ – Jack Creasey May 13 at 19:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.