VIPER35 datasheet(26/44 Pages) STMICROELECTRONICS

26 / 44 page

Operation description

VIPer35

26/44

DocID026980 Rev 4

When the system operates in valley-skipping mode, uneven switching cycles may be

observed under some line/load conditions, due to the fact that the off-time of the power

MOSFET changes its discrete steps one ringing cycle, while the off-time needed for cycle-

by-cycle energy balance could fall in between. Therefore one or even longer switching

cycles are compensated by one or more shorter cycles and vice versa. This mechanism is

natural and any effect on the converter performance and on its output voltage appears.

This operation does not consider the blanking time tBLANK after power MOSFET turn-off.

Actually tBLANK is not taken into account as long as the following condition is met:

Equation 2

where D is the MOSFET duty cycle. If this condition is not met, the time during which

MOSFET turn-on is inhibited is extended beyond tOSClim by a fraction of tBLANK. As a

consequence, the maximum switching frequency is a little bit lower than the internal limit set

by the oscillator and valley-skipping mode takes place slightly earlier than expected.

8.8

Blanking time

The blanking time, tBLANK, can have two different values: the lower one is 2.5 seconds

(typical value) and the higher one is 6.3 seconds (typical value). The value is linked to the

voltage VZCD, sampled during the time tSTROBE. The time tBLANK has the lower value if VZCD

> 1 V or it has the higher value if VZCD < 1 V, refer to Table 7 and Figure 35.

The higher value of the blanking time is active during the start-up phase or in case of output

short-circuit, when the output voltage of the converter is quite lower than the regulated

value. In this condition, during the demagnetization of the transformer, VZCD can be very

close to the arming and triggering thresholds (VZCDAth and VZCDTth) and ZCD circuit can be

erroneously trigged, leading the system to work with higher frequency and in continuous

mode. This false trigger is inhibited by the selection of tBLANK higher value when VZCD is

lower than 1 V.

During the normal operation, in steady-state condition, the voltage VZCD during the

demagnetization is higher than 1 V and the selected tBLANK value is the lower one.

Figure 35 shows the typical waveforms during the power-up and the linked tBLANK selection.

BLANK

osclim

------------------

BLANK

osclim

⋅

–

≤

VIPER35 数据表(PDF) 26 Page - STMicroelectronics

VIPER35 数据表(HTML) 26 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

数据表下载