STLD20D datasheet(14/31 Pages) STMICROELECTRONICS

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Components selection

STLD20D

14/31

Components selection

7.1

L, Boost inductor selection

To get a good trade-off thickness/efficiency, an attention must be given on the inductor

choice.

●

The inductance value must be selected to remain in the discontinuous conduction mode.

●

Its saturation current (Isat) must be equal or higher than the programmed current (ILIM).

●

An attention must be taken on the dynamic inductor parameters. Actually, some power

losses can occur in the boost inductor when it works at several hundred KHz and can

reduce the efficiency.

7.1.1

Calculation of the inductor value

The inductor must be dimensioned so that the STLD20D stays running in discontinuous

conduction mode operation in the worst operation condition (VIN = VIN_min= 2.8V). The limit

between continuous and discontinuous mode is called critical mode and characterized by an

uninterrupted current through the inductor (see figure 18).

The formula [1] gives the maximum typical value of the inductor for a discontinuous mode

operation in the worst case condition (critical mode). Figure 19 shows the typical L value

versus the voltage across the LED branch N.VLED. Note that this curve includes the

STLD20D and inductance dispersions (20%).

N.VLED = 4x4V = 16V and ILOAD = 20mA

Figure 18.

3 different conduction modes

Continuous

Discontinuous

Critical

typ

η V

in min

() NVLED

in min

()

–

LED

LDS

⋅

()

⋅

2.4

LED

max

LED

LDS

⋅

()

⋅⋅

⋅

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[]

≤

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