VIPER27LD datasheet(23/31 Pages) STMICROELECTRONICS

23 / 31 page

VIPER27

Operation descriptions

Doc ID 15133 Rev 5

23/31

7.11

Burst-mode operation at no load or very light load

When the load decrease the feedback loop reacts lowering the feedback pin voltage. If it

falls down the burst mode threshold, VFBbm, the power MOSFET is not more allowed to be

switched on. After the MOSFET stops, as a result of the feedback reaction to the energy

delivery stop, the feedback pin voltage increases and exceeding the level, VFBbm +

VFBbmhys, the power MOSFET starts switching again. The burst mode thresholds are

reported on

Table 8 and Figure 30 shows this behavior. Systems alternates period of time

where power MOSFET is switching to period of time where power MOSFET is not switching;

this device working mode is the burst mode. The power delivered to output during switching

periods exceeds the load power demands; the excess of power is balanced from not

switching period where no power is processed. The advantage of burst mode operation is

an average switching frequency much lower then the normal operation working frequency,

up to some hundred of hertz, minimizing all frequency related losses. During the burst-mode

the drain current peak is clamped to the level, ID_BM, reported on Table 8.

Figure 30.

Burst mode timing diagram, light load management

7.12

Brown-out protection

Brown-out protection is a not-latched shutdown function activated when a condition of mains

under voltage is detected. The Brown-out comparator is internally referenced to VBRth

threshold, see

Table 8 on page 7, and disables the PWM if the voltage applied at the BR pin

is below this internal reference. Under this condition the power MOSFET is turned off. Until

the Brown out condition is present, the VDD voltage continuously oscillates between the

VDDon and the UVLO thresholds, as shown in the timing diagram of Figure 31 on page 24. A

voltage hysteresis is present to improve the noise immunity.

The switching operation is restarted as the voltage on the pin is above the reference plus the

before said voltage hysteresis. See

Figure 5 on page 9.

The Brown-out comparator is provided also with a current hysteresis, IBRhyst. The designer

has to set the rectified input voltage above which the power MOSFET starts switching after

brown out event, VINon, and the rectified input voltage below which the power MOSFET is

switched off, VINoff. Thanks to the IBRhyst, see Table 8 on page 7, these two thresholds can

be set separately.

IDRAIN

FBbm

VFB

50 mV

hyster.

Burst-mode

Normal-mode

50 mV

hyster.

Burst-mode

Normal-mode

100

VIPER27LD 数据表(PDF) 23 Page - STMicroelectronics

VIPER27LD 数据表(HTML) 23 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

数据表下载