AN3339 datasheet(23/44 Pages) STMICROELECTRONICS

23 / 44 page

AN3339

Functional check

Doc ID 18449 Rev 2

23/44

Figure 40.

130 V load transition

The L6599A is equipped with a current sensing input (pin #6, ISEN) and a dedicated

overcurrent management system. The current flowing in the resonant tank is detected and

the signal is fed into the ISEN pin. It is internally connected to a first comparator referenced

to 0.8 V, and to a second comparator referenced to 1.5 V. Figure 41 shows an example of

overload occurrence. If the voltage externally applied to ISEN exceeds 0.8 V, an internal

switch is turned on and discharges the soft-start capacitor CSS. This operation results in a

nearly constant peak primary current limiting the output power. At the same time, an internal

150 µA current generator, which, via the DELAY pin, charges C37, is activated. Because the

DELAY and DIS pins short each other, if the voltage on C37 reaches 1.85 V, the L6599A

stops switching and the PFC_STOP pin is pulled low disabling also the PFC.

An off-on recycle is necessary to restart the converter. By dimensioning C37 and R54, the

designer can set the maximum time that the converter is allowed to run overloaded or under

short-circuit conditions. Overloads or short-circuits lasting less than the set time do not

Figure 38.

12 V load transition

Figure 39.

24 V load transition

CH3: +12 V

CH1: +130 V

CH2: +24 V

CH4: +12 V o/p current

CH3: +12 V

CH1: +130 V

CH2: +24 V

CH4: +24 V o/p current

CH3: +12 V

CH1: +130 V

CH2: +24 V

CH4: +130 V o/p current

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