AN2269 - Application Note

Emergency Deactivation/Fault Detection

13/25

3.4

The ST8024 is able to supply the card with current pulses of about 140mA for no longer than

5.5µs, typical (see

Figure 11 and Figure 12 on page 14).

Short circuit protection is an important interface feature that warns the sequencer block if

the output current becomes higher than the short circuit current limit (

≈120mA) for too much

time. This characteristic allows the device to supply the card with current pulses higher than

the maximum allowed, if their lengths are not too long. If the current pulses last for more

than 5.5µs, the deactivation sequence starts to protect the card. The OFF pin goes low so

as to warn the microcontroller about the overcurrent fault. The sequence in

Figure 12 on

page 14 shows how the current pulse becomes long enough to make the short circuit

protection happen.

Figure 11.

ST8024 Current Supply Sequence

Legend:

1.

CH1 = CMDVcc

2.

3.

4.

CH4 = OFF