STLD20D datasheet(16/31 Pages) STMICROELECTRONICS

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Components selection

STLD20D

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Figure 20 shows the maximum peak current Ip(max) through the coil for L=10µH versus the

voltage across the LED branch, N.VLED and the LED current ILED for VIN > 2.8V. As N.VLED

= 16V and ILED = 20mA, then Ip(max) = 0.45A. The curve below ends when the converter

reaches the critical mode operation.

Therefore, the saturation current (Isat) of the inductor must be higher than 0.45A. To

conclude:

7.1.3

Choice of the RSET resistor

The resistor RSET fixes the maximum peak current flowing through the inductor whatever the

operating conditions. Thus, current saturation (Isat) is never reached. If the height constraint

is important, this function allows using low profile inductor with a small saturation current.

The

Figure 12. on page 12 gives the corresponding typical value of the external resistor

RSET versus the ILIM value. This curve is slightly dependent of the temperature and the input

voltage.

To prevent the coil saturation RSET must be equal to 56kΩ, see Figure 12. Thus:

7.1.4

Reference selection

The table below gives some coil references suitable for the STLD20D versus L, DCR, Isat

value and sizing requirements.

Figure 20.

Maximum peak current (Ip(max)) versus the white LEDs voltages for 3 outputs current

- VIN > 2.8V

10µH

0.2

0.25

0.3

0.35

0.4

0.45

0.5

15mA

20mA

25mA

N.V

(V)

LED

I (max) (A)

sat

pmax

()

≥

sat

0.4A

≥

sat

LIM

pmax

()

≥≥

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