Clarification on EN/FAULT Pin Usage and MCU Software Handling – STEF12S eFuse

[Sandiri]

Dear STMicroelectronics Support Team,

We are currently using the STEF12S eFuse in a 12 V power distribution design and would like clarification on the correct usage of the EN/FAULT shared pin, specifically from a microcontroller software perspective.

Application Context

• Nominal input voltage: 12 V

• Load current: up to 5 A

• MCU-controlled power path

• Requirement to detect output short-to-GND, overcurrent, and thermal faults in firmware

Understanding So Far
From the datasheet and functional diagrams, we understand that:

• The EN/FAULT pin serves as an enable input during normal operation.

• The same pin is pulled LOW by the device (open-drain) to indicate fault conditions such as overcurrent, short-circuit, or thermal shutdown.

• During thermal shutdown, the pin voltage may be at an intermediate level due to internal pull-up and pull-down circuitry.

• The output is disabled during fault conditions.

Clarifications Requested

1. Is the recommended firmware approach to treat any LOW or non-HIGH level on EN/FAULT as a fault condition, regardless of the exact voltage level?

2. During a fault condition, should the MCU:

   • Release the GPIO (set as input/high-Z), or

   • Actively drive the pin LOW to disable the device?

3. What is the recommended sequence and timing for clearing a fault and re-enabling the device using the EN/FAULT pin?

4. Are there any reference designs, application notes, or example firmware flow diagrams that demonstrate correct MCU handling of the EN/FAULT pin?

5. Is an external pull-up resistor on the EN/FAULT pin recommended when interfacing with an MCU GPIO, or is the internal pull-up sufficient for reliable operation?

Our primary goal is to ensure robust and unambiguous fault detection in firmware when the output is shorted to ground, while avoiding contention on the shared EN/FAULT pin.

[Jiri HLADIK]

Hello,

I don’t fully understand the use case. Are there more eFuses with EN/FAULT connected together or just one? Which version do you use Auto-retry (STEF12SA) or Latch (STEF12S)?

1. There is no specific recommendation for software however the pin should be set to open drain or input configuration.
2. If Latch version is used. The fault condition is signalized by eFuse pulling the signal down to around 1,3V. This can be detected in MCU by ADC or by internal comparator. Based on requested behavior MCU should pull the EN/FUALT down to reset power supply. There can be some logic depending on application needs. Typically, it can try several times to restart to see if failure is persistent or not and then turn off and indicate the error to the supervising system. In case auto-retry version is used MCU can monitor EN/FAULT pin and if the periodical restart is detected (MCU pin as input) it can still disable STEF for a while and try later and/or disable permanently.
3. There is no specific timing requested. EN/FAULT pin is ok to be pulled low at least for few microseconds for proper reset. Then 250us delay time takes place and after that the output voltage starts to ramp-up with slew-rate defined by Cdv/dt capacitor
4. In the datasheet you can find PCB layout example for SOT23 package. However, due to thermal characteristics I better recommend using DFN10 package with exposed pad which better dissipates heat into PCB. The reference PCB design can be found here: 5 V and 12 V power line protection based on STEF05 and STEF12 electronic fuses
5. External pull up is not needed as there is 20uA current source to drive pin high. But if the EN/FAULT pin is traced outside PCB or if shared with other functionalities which might induce some noise or transients then we recommend putting PCB layout for pull up if needed.

Best regards

Jiri