LED2001PUR datasheet(24/42 Pages) STMICROELECTRONICS

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Application information

LED2001

24/42

DocID024346 Rev 1

Equation 27

Example (see Section Example 1):

VIN=12 V, ILED=700 mA,

ILED/ILED=2%, VFW_LED=3.5 V, nLED=2.

A lower inductor value maximizes the inductor current slew rate for better dimming

performance. Equation 26 becomes:

Equation 28

which is satisfied selecting a10

μH inductor value.

The output capacitor value must be dimensioned according to Equation 27.

Finally, given the selected inductor value, a 2.2

μF ceramic capacitor value keeps the LED

current ripple ratio lower than the 2% of the nominal current. An output ceramic capacitor

type (negligible ESR) is suggested to minimize the ripple contribution given a fixed capacitor

value.

7.1.3

Input capacitor

The input capacitor must be able to support the maximum input operating voltage and the

maximum RMS input current.

Since step-down converters draw current from the input in pulses, the input current is

squared and the height of each pulse is equal to the output current. The input capacitor

must absorb all this switching current, whose RMS value can be up to the load current

divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these

capacitors must be very high to minimize the power dissipation generated by the internal

ESR, thereby improving system reliability and efficiency. The critical parameter is usually the

RMS current rating, which must be higher than the RMS current flowing through the

capacitor. The maximum RMS input current (flowing through the input capacitor) is:

Equation 29

where

η is the expected system efficiency, D is the duty cycle and IO is the output DC

current. Considering

η = 1 this function reaches its maximum value at D = 0.5 and the

Table 6. Inductor selection

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Wurth Elektronik

WE-HCI 7040

1 to 4.7

20 to 7

WE-HCI 7050

4.9 to 10

20 to 4.0

Coilcraft

XPL 7030

2.2 to 10

29 to 7.2

RIPPLE s=j

⋅

()

RIPPLE_SPEC

ΔI

LED

-----------

0.5

RMS

⋅

---------------

–

-------

⋅

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