ISO808 Fault status

[kesavparu]

During a short circuit of the ISO808 channel output (+28V) to ground, the fault status shows a voltage between 0.8V and 1.4V instead of 0V. When the short is removed, the fault status reads 3.3V. A 1k pull-up resistor is connected to the fault open-drain pin. Due to this voltage level, the microcontroller is unable to correctly detect the low state for fault status. how to get 0v on fault pin during the short circuit time.

[Peter BENSCH]

Welcome @kesavparu, to the community!

The Fault output of the ISO808 is an open drain, which is relatively high-impedance and therefore has too high a level if the drain current is too high. If you increase the pull-up at VDD=3.3V from 1k to at least 3.3K, a maximum of 0.4V should be measurable at Fault according to the data sheet.

However, your requirement for 0V is unrealistic and only feasible with an extremely large pull-up and extremely small load. You may want to consider a buffer stage with a Schmitt trigger.

Hope that helps?

Regards
/Peter

[kesavparu]

Hi Peter, thanks for your feedback.

We are using three ISO808 ICs, with their fault pins connected together and connected to a single microcontroller input pin. A 3.92k pull-up resistor is tied to a 3.3V supply, as shown in the attached image. When there is no short, the fault pin voltage is 3.3V. However, when any output is shorted, the fault voltage drops to 1.5V instead of 0.4V and gradually decreases to 0.9V. This fluctuation causes the fault signal to toggle, making it difficult for the microcontroller to reliably detect the fault status. Please suggest a solution to resolve this issue."

[kesavparu]

kindly respond.