AN4376 datasheet(9/37 Pages) STMICROELECTRONICS

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AN4376

EVLHVLED815W10A - main characteristics and circuit description

2.1

LED current definition

The sense resistors R2 = 1.2

 ± 1% and R3 = 1.8  ± 1% sense the current flowing into the

inductor primary side and fix the output LED current according to Equation 1:

Equation 1

Where VCLED = [0.192 - 0.2 - 0.208] V is the equivalent internal voltage that includes Gi, Iref,

R parameters. and it is an internal parameters of the controller (see the HVLED815PF

device datasheet for more details [See 1. in Section 8: Supporting material on page 36] ).

2.2

LED current ripple definition

The output capacitor size and the LED current ripple definition have been calculated with

Equation 2:

Equation 2

For this demonstration board 3 parallel capacitor of 68

F-100 V have been selected to

have a current ripple of less than 100 mA pk-pk with 23 LEDs each with a dynamic

resistance of 0.8

.

2.3

DMG pin and OVP setting

Due to the topology the LED string load is connected to a floating output and the loop is

closed through a primary sensing regulation. From auxiliary winding, an accurate image of

the output voltage is fed to the DMG pin that is the inverting input of the internal, error

amplifier. Then R8 is connected to the auxiliary winding providing the inductor

demagnetization signal to the DMG pin and turning on the internal MOSFET at any

switching cycle. R8 value impacts on the voltage feed forward function and a 270 k



resistor has been selected for improving the line regulation (see the HVLED815PF

datasheet - section Voltage feed-forward block [1. in Section 8: Supporting material on

page 36]).

The divider composed by the R8 and R5 fixes the maximum output voltage VOVP at no load

(in case of LED -load disconnection) with Equation 3:

Equation 3

N = 3.8 ± 2% is turn-ratio between primary and auxiliary winding.

R s e n s e

C L E D

L E D

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8 8 V

3 . 8

2 . 5 1 V

3 3 k Ω

2 . 5 1 V

2 7 0 k Ω

3 . 8

2 . 5 1 V

R 5

2 . 5 1 V

R 8

O V P

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