Hello!
There is such a button with a backlight, when connecting the backlight to a 24V power supply, it consumes 6-7 mA.
After trying to control this backlight using stm32f407, the pin in open-drain mode burns out. Now in open-drain mode, in a high state, there is a potential of 1.2 V on the leg, and it should hang in the air. After that, in push-pull mode, the pin works fine, although I did not measure the consumption without a load.
The question is, why is the pin lit? According to the datasheet, the maximum load on the pin is 20 mA, in output mode, the pin resistance is close to zero, so the current should be the same as when connecting the backlight to the power supply, i.e. 6-7 mA. Why is the pin lit? If I made a mistake in the calculations, then how can I correctly calculate the permissible load for operation in this mode?
I want to understand how exactly the currents flowed and where the malfunction occurred.
Here is the driver diagram.
As I understand it, the port burns out because the open drive mode is not turned on immediately after power is supplied, but after the initialization of the gpio. Before initialization, the output resistance is very high, so when 24V is supplied through the LED and resistor, the current will flow through the protective diode to VDD_FT. The pin will have a voltage of VDD_FT + voltage drop on the protective diode, which does not exceed the permissible value of VDD+4.
The current is 6 mA. The datasheet states that positive leakage current is impossible, the main thing is not to exceed the voltage VDD+4 on the pin.
What happens next, how do currents flow in the driver, what elements are destroyed and why? Why is there now a voltage of 1.2 V on the pin in the high state in open drain mode? What resistor value would not burn out the controller pin at 24 volts?
So it turns out that we supply 24V to a de-energized microcontroller if the 24V power supply starts faster than 3.3? Why then does VDD_FT rise to 24V? It should then be in a high-resistance state. I drew a diagram, did I understand you correctly? Sorry for the repeated questions, I use Google Translate, some words can distort the meaning of your message.
