ZXCT1082E5TA current detection circuit

Question

I am trying to create a USB inrush detection circuit using ZXCT1082E5TA shunt current sensor. I want to control a 3.3 V signal, when new USB device is inserted. I did some research about inrush current and now am trying to design some sort of simple circuit to detect this current using transistors and current sensor. I will try to use voltage on OUTPUT pin of a sensor as my control signal for transistors, when current is present on shunt resistor, OUT pin voltage will rise and open NPN transistor which would close P mosfet and microcontroller get 3.3 V.

I have imported LTspice model, simulated schematic and got desired results on OUT pin of a sensor (I scaled current to better visualize conditions and output):

10 mA is threshold value, which I hope to capture. But when I connect transistors circuit, I get quite a big voltage drop on OUT pin, from 1 V I get around 770 mV:

And NPN opens on 0.7 V, so it is very uncertain, whether in real conditions transistor would open. What could cause this drop? Sensor's datasheet does not include Max OUT source current, but could it be the problem, that resistor is too large and it can't source this much current?

Answer 1

Whenever Vout of U2 is over some voltage, some current will flow through Q2 base. In this condition, R6 is connected in parallel with R5 , so gain is modified. I think you need a buffer (op-amp) just to no influence this gain, or use a comparator with high input impedance; try with a rail to rail type, (voltage is close to ground) and low supply voltage.