STLD20D_06 Datasheet, PDF (14/31 Pages) STMicroelectronics – White LED power supply

Components selection
7
Components selection
STLD20D
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 (I_sat) must be equal or higher than the programmed current (I_LIM).
● 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 (V_IN = V_{IN_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).
Figure 18. 3 different conduction modes
I_L
Continuous
Discontinuous
Critical
t
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.V_LED. Note that this curve includes the
STLD20D and inductance dispersions (20%).
N.V_LED = 4x4V = 16V and I_LOAD = 20mA
L_typ ≤₂----_.-^η-₄---^â‹-_â‹-----^V-_I--_L-²--_E---ⁱ-_Dn---⁽-_â‹-^m----ⁱ-_N-ⁿ--⁾---^(â‹--^N----_V--^â‹--_L---_E-^V--_D--^L---^E_â‹---^D---_F--–--_m---^V-_a---^ix-ⁿ---_â‹⁽--^m----₍ⁱ--ⁿ_N---⁾---+_â‹-----^V-_V---^F-_L--^B-_E----+_D----+-^I-^L---^E_I--_L--^D-_E----_D^â‹-----_â‹^R-----^L_R---^D--_L--^S-_D--⁾--_S---₎ [1]
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