STMicroelectronics EVALHVLED815W10 and EVALHVLED815W15 Evaluation Board for the Offline LED Driver with Primary-Sensing and High Power Fact EVALHVLED815W10 データシート

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DocID023409  Rev 5

Finally, the R

DMG

 resistor can be calculated as follows:

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

OUT

 does not depend from the V

IN

 input voltage.

When high power factor is implemented (see 

), the feedforward current has to 

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

The maximum value is limited by the minimum I

DMG

 internal current needed to guarantee 

the correct functionality of the internal circuitry:

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

COMPBM

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

reduced at a lower value to minimize V

CC

 capacitor discharge. 

In this condition the converter operates in burst mode (one pulse train every 
T

START 

= 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 V

REF

. If the voltage on the EA output, as a result of the 

comparison, exceeds the V

COMPL

 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 

 along with the others previously described, is noise-free 

since the peak current is low.