HVLED807PF datasheet(25/36 Pages) STMICROELECTRONICS

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HVLED807PF

Device description

Doc ID 023464 Rev 5

25/36

Equation 13

Finally, the Rdmg resistor can be calculated as follows:

Equation 14

In this case the peak drain current does not depend on input voltage anymore, and as a

consequence the average output current IOUT do not depend from the VIN input voltage.

When high power factor is implemented (see

Section 4.11), the feed-forward current has to

be minimized because the line regulation is assured by the external offset circuitry (see

Figure 1: Application circuit for high power factor LED driver - single range input).

The maximum value is limited by the minimum Idmg internal current needed to guarantee

the correct functionality of the internal circuitry:

Equation 15

4.8

Burst-mode operation at no load or very light load

When the voltage at the COMP pin falls 65 mV is below the internally fixed threshold

VCOMPBM, the IC is disabled with the MOSFET kept in OFF state and its consumption

reduced at a lower value to minimize VCC capacitor discharge.

In this condition the converter operates in burst-mode (one pulse train every TSTART=500

µs), with minimum energy transfer.

As a result of the energy delivery stop, the output voltage decreases: after 500 µs the

controller switches-on the MOSFET again and the sampled voltage on the DMG pin is

compared with the internal reference VREF. If the voltage on the EA output, as a result of the

comparison, exceeds the VCOMPL threshold, the device restarts switching, otherwise it

stays OFF for another 500 µs period.

In this way the converter will work in burst-mode with a nearly constant peak current defined

by the internal disable level. A load decrease will then cause a frequency reduction, which

can go down even to few hundred hertz, thus minimizing all frequency-related losses and

making it easier to comply with energy saving regulations. This kind of operation, shown in

the timing diagrams of

Figure 20 along with the others previously described, is noise-free

since the peak current is low.

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