STLD20D datasheet(21/31 Pages) STMICROELECTRONICS

21 / 31 page

STLD20D

Analog dimming control

21/31

Where:

●

VIN(max): is the maximum input voltage

●

DCMIN: is the typical minimum duty cycle of the STLD20D (18%)

●

Ltyp: is the typical vale of the inductance

●

Ftyp: is the typical switching frequency

Figure 23. gives the ILED versus the LED branch voltage N.VLED. This curve is calculated

with:

Ltyp = 10µH, Ftyp = 500kHz, DCMIN = 18% and VIN(max) = 4.2VDC.

Higher the voltage across the branch LEDs, higher the range current control. After these

considerations, it is described here the basics rule to help the designer to choose the

external components such as Rd1, Rdim and RLED versus Vdim and brightness control

current ILED.

9.2

Rd1 Calculation

To avoid significant shifting of the cross over frequency and to keep enough high the

corrector network gain of the error amplifier, it is recommended to dimension the resistor

Rd1 below 10kΩ (10% of R1).

Dimension RLED for full brightness operating mode

RLED is dimensioned to get the nominal current ILED for the full brightness of the LED.

It is recommended to fix Vdim = VFB during the full brightness operating mode so that the

LED current correspond to the programmed value ILED. Thus:

Figure 23.

ILEDC current vs N.VLED corresponding at the DCMIN

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

910

N.V

(V)

LED

(A)

LEDC

= f (

LEDC

LED

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