AN2864

Testing the board

Doc ID 15282 Rev 1

21/29

3.5

Overload protection

The VIPER17L has several protection features, one of which protects against overload or

output short-circuit. If the load power demand increases, then the output voltage decreases

and the feedback loop reacts by increasing the voltage on the feedback pin. The feedback

pin voltage rise increases the PWM current set point, augmenting the power delivered to the

output until this power equals the load power. If the load power demand exceeds the

converter power capability (which can be adjusted using the RLIM), the voltage on the

feedback pin keeps rising but the power delivered remains stable. When the feedback pin

overload event. Before shutting down the system, the device waits for a time fixed by the

then the internal pull-up is disconnected and the pin starts sourcing a 3 µA current that

charges the capacitor connected to it. As the voltage on the feedback pin reaches the

Figure 19

shows the behavior of the converter when the output is shorted.

Figure 19.

If the short-circuit is not removed, the system switches into auto-restart mode. When a

short-circuit is permanently applied on the output, the behavior displayed is a shorter

MOSFET switching period, the converter attempts to deliver as much power as possible to

the output and a longer period when the device does not switch and no power is processed.

The duty cycle of the power delivery is very low (around 1.5%) and the average power

throughput is also very low (see Figure 20).