AN1215 APPLICATION NOTE

2/7

The output current is not limited and may cause PCB traces burning. Converter's power throughput declines

only slightly. Furthermore, power losses get concentrated on some specific parts, e.g. the output rectifier(s),

which may overheat and go towards destruction if not rated for this extreme condition and if heatsinking capa-

bility is not generously provided.

When pulse-by-pulse current limitation cannot maintain the peak primary current under control, hiccup mode

protection takes over and halts the resulting current runaway. This happens when the switch ON-time needed

More details on this "runaway condition" can be found in the appendix.

With reference again to figure 1, in case of current runaway the voltage on the current sense pin will go over 1V.

As it reaches 1.2V a comparator triggers this special functionality [1], [2].

Unlike pulse-by-pulse limitation, hiccup mode operation keeps output current and power throughput very low. It

would be desirable, then, to rely on hiccup operation to conveniently handle a load short circuit. However, it is

quite common that even a dead short at the converter's output cannot activate hiccup protection.

Referring to the appendix for a detailed analysis, high input voltage, high regulated output voltage and high

switching frequency favour current runaway and, therefore, hiccup operation to be tripped further to a load short

circuit. In many applications, such as AC-DC adapters or PC's silver boxes, where the output voltage is not so

high and the switching frequency is quite low (below 100 kHz) it is extremely likely that the runaway condition

will never be met and the hiccup protection will never be activated. Furthermore, in these applications the load

to control peak current needs not be so short.

In other applications, such as monitor SMPS, the runaway condition is likely to be met for the highest output

voltages, especially at high input voltage and if they are synchronized at high frequency.

Actually, hiccup protection serves two different purposes. The first one is related to the capacity of the L5991

and L5993 to be operated at a very high frequency (up to and beyond 500 kHz): to guarantee a safe handling

of a load short circuit under all conditions of operating frequency, hiccup protection must be added to pulse-by-

pulse limitation. The second purpose is to cover some fault conditions that pulse-by-pulse limitation cannot han-

dle. These are basically:

1.

short circuit of the secondary winding (regardless of the topology);

2.

secondary catch diode short circuit (in flyback converters);

3.

secondary freewheeling diode short circuit (in forward converters).

Figure 2. Fault conditions that activate hiccup mode operation.

Vin

Secondary winding

short circuit

(any topology)

Vin

Secondary diode

short circuit

(flyback topology)

Vin

Freewheeling diode

short circuit

(forward topology)

Vin

Equivalent circuit