LED2000 datasheet(24/40 Pages) STMICROELECTRONICS

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Application information

LED2000

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DocID023432 Rev 4

Equation 27

Example 2(see 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

H inductor value.

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

Finally, given the selected inductor value, a 2.2

F 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

Table 6. Inductor selection

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Würth 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

IRIPPLE s=j 







IRIPPLE_SPEC



I

ILED

-----------

0,5

IRMS





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

–



-------



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