LED2000 datasheet(25/40 Pages) STMICROELECTRONICS

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DocID023432 Rev 4

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LED2000

Application information

where

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

current. Considering

 = 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 30

and

Equation 31

where VF is the free-wheeling diode forward voltage and VSW the voltage drop across the

internal PDMOS. Considering the range DMIN to DMAX, it is possible to determine the max.

IRMS going through the input capacitor. Capacitors that can be considered are:

Electrolytic capacitors:

These are widely used due to their low price and their availability in a wide range of

RMS current ratings.

The only drawback is that, considering ripple current rating requirements, they are

physically larger than other capacitors.

Ceramic capacitors:

If available for the required value and voltage rating, these capacitors usually have

a higher RMS current rating for a given physical dimension (due to very low ESR).

The drawback is the considerably high cost.

Tantalum capacitors:

Small tantalum capacitors with very low ESR are becoming more widely available.

However, they can occasionally burn if subjected to very high current during charge.

Therefore, it is suggested to avoid this type of capacitor for the input filter of the device

as they may be stressed by a high surge current when connected to the power supply.

If the selected capacitor is ceramic (so neglecting the ESR contribution), the input voltage

ripple can be calculated as:

Equation 32

Table 7. List of ceramic capacitors for the LED2000

Manufacturer

Series

Capacitor value (µC)

Rated voltage (V)

TAIYO YUDEN

UMK325BJ106MM-T

MURATA

GRM42-2 X7R 475K 50

4.7

DMAX

VOUT VF

VINMIN VSW

–

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

DMIN

VOUT VF

VINMAX VSW

–

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

VIN PP

CIN fSW



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



----

–



 D





----

–





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