STLD20D_06 datasheet(15/31 Pages) STMICROELECTRONICS

15 / 31 page

STLD20D

Components selection

15/31

Where:

●

ηis the efficiency (80%)

●

N is the number of the white LEDs in series

●

VLED is the forward voltage of the LED for the ILED current (VLED = 4V in our example)

●

VIN(min) is the minimum input voltage (2.8V)

●

VFB is the error amplifier reference (0.3V)

●

RLDS is the internal resistance of the Load Disconnect Switch power MosFET (6Ω)

●

Fmax is the maximum frequency of the STLD20D (600kHz)

●

ILED_MAX is the current through the LED

For example, the case with 4 white LEDs can be considered in order to evaluate L value in

the worst case conduction.

From figure 19, typical inductance must be lower than 11µH. By minimizing the inductance

to ensure the discontinuous mode operation, the standard coil value is equal to 10µH. Then:

L=10µH

7.1.2

Calculation of the saturation current I(sat)

The maximum peak current (Ip(max)) during steady state can be estimated by the formula [2]:

Where:

●

Ltyp is the typical inductance value

●

Fmin is the minimum frequency due to the STLD20D spread-off (400kHz)

Figure 19.

Typical inductance value versus the white LED voltage for three IOUT

Ltyp=f(nV

)

LED

7.0E-06

9.0E-06

1.1E-05

1.3E-05

1.5E-05

1.7E-05

1.9E-05

2.1E-05

15mA

20mA

25mA

11µH

N.V

(V)

LED

L typ (H)

pmax

()

LED

IN min

()

–

LED

LDS

⋅

()

⋅⋅

⋅

η F

min

0.8

typ

LED

LDS

⋅

()

⋅⋅

⋅

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