LED5000 datasheet(25/52 Pages) STMICROELECTRONICS

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LED5000

Application notes - buck conversion

Doc ID 023951 Rev 1

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Equation 32

which is satisfied selecting a10

μH inductor value.

The output capacitor value has to be dimensioned according to

Equation 31

Finally, given the selected inductor value, a 1

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.

5.9.3

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 has

to 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

has to 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 33

Where

ηis 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

equivalent RMS current is equal to IO divided by 2. The maximum and minimum duty cycles

are:

Equation 34

and

Table 7.

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

ΔI

LED

-----------

0.5

RMS

⋅

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

–

-------

⋅

MAX

OUT

INMIN

–

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

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