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Hello Everyone,
I am facing a problem in the AFE (MC33771) reset from the microcontroller (S32k312, operating at 3.3V). In my architecture, the AFE ground is different from the MCU ground. Since I want to control the Reset of the AFE from the MCU, the safest option (according to me), is to use an optocoupler.
So now, I have made the connections as:
1. The MCU GPIO is connected to the optocoupler anode via a resistor.
2. The cathode of the Optocoupler is connected to the MCU Ground.
3. Collector the Optocoupler should be connected to a 5V, since the maximum voltage that can be supplied to the Reset pin of AFE should not be more than 6.5V (as per datasheet). Now, since I donot have any 5V trails in my Circuit, I initially connected it to the VCOM pin of the AFE (which gives an output of 5V, 5mA from the AFE's internal regulator).
4. The emitter of the optocoupler is connected to the AFE reset pin directly.
5. Additionally, the AFE's VCOM is also connected to the external LED via a 470ohm resistor (with the LED's ground being the AFE's ground).
I observed the following:
1. When the MCU GPIO is high, and the connections are made as mentioned above, the voltage between AFE's VCOM and AFE's ground is equal to the AFE's RESET and AFE's ground, which is 2.2V (which was supposed to be somewhere near 5V). Also the LED that is connected to the AFE VCOM also dims.
2. When the MCU GPIO is high and the collector of the Optocoupler is disconnected from the AFE VCOM, the voltage between AFE VCOM and AFE Ground is 5V; and the voltage between AFE RESET and AFE ground is OV (which makes sense as the collector of the optocoupler is connected nowhere).
3. When the MCU GPIO is high + Collector of the optocoupler is connected to the AFE VCOM ; but the emitter of the optocoupler is now disconnected from the AFE REset pin (lifted the emitter pin of the optocoupler from the PCB pad), then also the voltage between AFE VCOM and AFE Ground is 5V; and the Reset Voltage is 0 (as the reset pin is disconnected).
I am working on a BMS design and want to reset the AFE from the MCU.
Please HELP !!
Thank you in advance.
Please note: It might be because, I am resetting the AFE using its own 5V that its internal regular is supplying. However, I am not sure. Please Help.
Hello @TomasVaverka , Thank you for your response.
I have indeed increased the LED series resistance from 470ohm to 2.2kohm. Then the LED current dropped from 6.4mA to 1.36mA (which I believe is far less than the VCOM maximum current of 5mA; and it should now not overload the VCOM pin).
Please correct me wherever I am wrong.
After the change in resistor as well, the VCOM voltage sort of improved from 2.2V to 2.4V (approximately). Still I believe that is not enough.
Please note: I have in fact disconnected the entire LED branch. In that case, the VCOM voltage was 2.6V.
Please help !!
The main problem here is, the VPWR pin of the AFE has a voltage of somewhere around 30V (the full stack voltage). I can use a voltage divider from that VPWE pin (but I am not quite comfortable in doing so. I am afraid that might cause other problems). Also, there is no 3.3V or 5V trail in my battery side (that's another problem)!!
Hello Ankan,
I think the root cause of your issue is that VCOM is being overloaded. The MC33771 VCOM pin is a current-limited internal regulator (5 mA in TPL mode, 10 mA in SPI mode). It is not designed to power an optocoupler pull-up circuit. When your optocoupler turns ON, two current paths load VCOM simultaneously:
1. LED via 470 Ω — at 5 V this already draws ~6–7 mA, at or above the VCOM limit on its own.
2. Optocoupler CE path — current flows VCOM → opto collector → emitter → RESET pin → internal pull-down → AFE GND, adding further load.
The combined current exceeds VCOM's rating, triggering its undervoltage protection and causing the voltage to droop to ~2.2 V. This also explains the LED dimming.
To fix this:
1. Do not use VCOM as the optocoupler collector supply. Use a separate supply referenced to AFE GND (e.g., 3.3 V or 5 V from your battery-side circuitry) with a current-limiting resistor (1–4.7 kΩ) on the collector. The RESET pin accepts 0–6.5 V, so 3.3 V works fine.
2. Add a 10 kΩ pull-down from RESET to AFE GND to ensure a clean LOW when the optocoupler is off.
3. Remove the LED from VCOM, or increase the series resistor to keep its current well under 1–2 mA. Budget VCOM carefully for its intended loads (NTCs, EEPROM, etc.).
BRs, Tomas