STCMB1 datasheet(47/59 Pages) STMICROELECTRONICS

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DocID028958 Rev 4

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STCMB1

Application information

The operation of the converter in the overload is limited in time by a mechanism based on a

pair of internal counters: one is a modulo 128 cyclical counter, clocked by the switching

frequency of the LLC, and the other is a modulo 20 K counter, clocked by the internal

reference at about 1 MHz. The first OCP1 event (first time Visen_hb



800 mV, typ.) turns on

the two counters.

If a second OCP1 event occurs within 128 cycles of the LLC from the first OCP1 event, then

the cyclical counter is reset and restarted, while the modulo 20 K is left free running on its

clock.

If no OCP1 event occurs within 128 cycles of the LLC from the last OCP1 events, that is the

cyclical, the counter reaches its EoC, then the value of the modulo 20 K counter is

decreased of 32 and frozen.

The value of the modulo 20 K counter is decreased of 32 and frozen, each time the modulo

128 cyclical counter reaches its EoC (no OCP1 event within 128 cycles of the LLC from the

last EoC). When the value of the modulo 20 K

counter reaches 0 (because of consecutive

decrements), the modulo 128 cyclical counter is reset and stopped.

Conversely, when an OCP1 event occurs while the modulo 20 K counter is frozen (OCP1

event occurring within 128 cycles of the LLC from the last EoC), its counting is restarted and

the modulo 128 cyclical counter is reset. When the modulo 20 K counter reaches its EoC, a

soft-stop procedure is activated (and the counters are reset and stopped).

The soft-stop procedure consists of a controlled turn-off of the LLC stage in 128 cycles: the

soft-start capacitor is completely discharged to increase the switching frequency so that the

energy in the resonant tank is minimized. After the shutdown, the adapter will be driven by

the natural discharge and recharge by the HVSU of the VCC pin. If the overload condition is

permanent, then the adapter will operate in the hiccup mode.

In practice, the modulo 20 K counter will reach its EoC in 20 ms (18 ms min.) in case of the

output short-circuit, if the latter does not trigger the OCP2, or when the overload is above a

certain current level and/or time length.

Waveforms of the converter operation at the first shutdown by the soft-stop procedure and

during the hiccup mode are given in Figure 29. Detailed operation is described below

Figure 29.

Figure 29. Hiccup mode operation during OVL and detail of current limiting

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