AN1978 - APPLICATION NOTE

10/14

Figure 13: Vth2 threshold.

5.2. Vcc short-circuit protection

An important feature of the voltage regulator is the short-circuit protection that warns the sequencer

block (see Figure 1) if the output current grows more than the short circuit current limit (about 90mA). In

this case a deactivation sequence starts to protect the card and the OFF pin falls down warning the

occurred fault as shown in Figure18. The following figures (Figure 14 and Figure 15) show the load curve

of the ST8004 Step-up converter and regulator.

As specified in the NDS electrical requirements the ST8004 is able to maintain the Vcc voltage included

in the range 4.75V÷5.25V for the Class-A and 2.80V÷3.20V for the Class-B with current load from 0 to

the short-circuit current.

An example of short circuit protection intervention is shown in Figure 16, in which the activation

sequence starts in presence of a Vcc short circuit and the protection acts at the end of the activation

sequence.

If the short circuit condition happens when the device is already activated, the protection acts

immediately as shown in Figure 17.

Conditions:

V

V

V

5/3V=H.

CH1: Vcc

CH2: V

CH3: RST pin.

When the V

below

the

Vth2

value

(about 2.1V) a deactivation

sequence starts; the RST

pin (CH3) is the first to go

low after about 6µs.

Conditions:

V

V

V

5/3V=H.

CH1: Vcc

CH2: V

CH3: RST pin.

When the V

below

the

Vth2

value

(about 2.1V) a deactivation

sequence starts; the RST

pin (CH3) is the first to go

low after about 6µs.