AN4496 datasheet(15/18 Pages) STMICROELECTRONICS

tên linh kiện	AN4496

Giải thích chi tiết về linh kiện	How to get a high power factor with the HVLED815PF device

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DocID026370 Rev 1

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AN4496

Designing a high PF wide range LED driver with the HVLED815PF

2.7

Secondary/auxiliary turn ratio

The operating range of VCC is between 11.5 V and 23 V. The mid voltage is around 17 V.

The drop voltage on the limiting resistor Rlim (R9) and the auxiliary rectifier diode (D2) at the

nominal operating point is set to 1 V.

Equation 31

Vdrop AUX = VFaux + VRlim = VFaux + ICC * Rlim

Then is defined the secondary/auxiliary turn ratio.

Equation 32

Two conditions must be checked:

In case of an open circuit the output voltage is a bit upper at the threshold VOUT OVP, in

this situation Rlim must limit the current to 25 mA.

Equation 33

The higher value of Rlim must be used in the design, if greater Rlim is determined for the

open circuit conditions, the secondary/auxiliary turn ratio to compensate the drop on

Rlim must be recalculated.

If requested, wide range output voltage at the constant current VCC must be increased

to 21 V.

To demonstrate the high voltage startup functionality in this design VCC is lower, then:

2.8

Feed forward

Equation 34

Real value used:

NS / NAUX = 1.75

VCC = 12 V

Real value used:

Rdmg = 91 k

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