Part Number: INA199EVM
Other Parts Discussed in Thread: INA199, , INA190, INA186
Tool/software:
Hello everyone,
I would like to use an INA199 to measure LED current via a shunt. To do this, I purchased the INA199EVM so that I could test the variants and the component itself. Currently, the INA199B2 is connected. Below, I have created a small equivalent circuit diagram to explain the setup and the problem. 
The structure is basically the same for all four LEDs and is for the sake of comparability. VLED is connected to the anode at 4.76V. The cathode leads to the LCx pin of the MLX81118 and is controlled by it with 3.4V in the LED-OFF state and 1.8V in the LED-ON state. This is done using PWM, which is not relevant here, because the LED-ON state and the current measurement are working fine. However, in the LED-OFF state, the following behavior occurs:
R1 = 300 mOhm; R2 = 90 kOhm
Measuring point V1 (red) measures the expected 3.4V. Measuring point V2 (green) does the same. However, measuring point V3 (yellow) now measures 3.18V, which corresponds to a voltage drop of 220mV. LED4 should now serve as a “replacement circuit diagram,” as it can be assumed that the voltage loss is caused by the INA199. With R2 set to approx. 90 kOhm, we obtain the same result at V4 = 3.18V.
Do you have an explanation for this behavior? Is the selected component not suitable for this application?
As mentioned, everything works perfectly in the LED-ON state and the voltage levels are correct. If I move the measuring shunt including INA199 to the high side, it also works just fine.
BR Nico
