AN2929 datasheet(21/32 Pages) STMICROELECTRONICS

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AN2929

Testing the board

Doc ID 15333 Rev 2

21/32

2.5

Overload protection

The VIPER27LN has several protections, one of which prevents converter damage in case

of overload or output short-circuit. If the load power demand increases, the output voltage

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

increase of the feedback pin voltage leads to the PWM current set point increase which

increases the power delivered to the output until this power equals the load power. If the

load power demand exceeds the converter’s power capability (which can be adjusted using

RLIM), the voltage on feedback pin continuously rises but the power delivered no longer

increases. When the feedback pin voltage exceeds VFB_lin (3.3 V typ.), VIPER27LN logic

assumes that it is a warning for an overload event. Before shutting down the system, the

device waits for a period of time set by the capacitor present on the feedback pin. In fact if

the voltage on the feedback pin exceeds VFB_lin, the internal pull-up is disconnected and the

pin starts sourcing a 3 A current that charges the capacitor connected to it. As the voltage

on the feedback pin reaches the VFB_olp threshold (4.8 V typ.), VIPER27LN stops switching

and is not allowed to switch again until the VDD voltage goes below VDD_RESTART (4.5 V typ.)

and rises again up to VDD_ON (14 V typ.).

The following waveform shows the behavior of the converter when the output is shorted.

Figure 21.

Output short-circuit

If the short-circuit is not removed, the system starts to work in auto-restart mode. The

behavior, when a short-circuit is permanently applied on the output, is a short period of time

where the MOSFET is switching and the converter tries to deliver to the output as much

power as it can, and a longer period where the device is not switching and no power is

processed.

Ch1: V

OUT

Ch2: V

Ch4: I

DR AIN

IN =115VAC

Fu ll Load

Before the short

AN2929 Datasheet(PDF) 21 Page - STMicroelectronics