LED5000 datasheet(26/52 Pages) STMICROELECTRONICS

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Application notes - buck conversion

LED5000

26/51

Doc ID 023951 Rev 1

Equation 35

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 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 could be stressed by an high surge current when connected to the power supply.

In case the selected capacitor is ceramic (so neglecting the ESR contribution), the input

voltage ripple can be calculated as:

Equation 36

5.10

Layout considerations

The layout of switching DC-DC converters is very important to minimize noise and

interference. Power-generating portions of the layout are the main cause of noise and so

high switching current loop areas should be kept as small as possible and lead lengths as

short as possible.

High impedance paths (in particular the feedback connections) are susceptible to

interference, so they should be as far as possible from the high current paths. A layout

example is provided in

Figure 17.

Table 8.

List of ceramic capacitors for the LED5000

Manufacturer

Series

Capacitor value (µ)

Rated voltage (V)

Taiyo Yuden

UMK325BJ106MM-T

Murata

GRM42-2 X7R 475K 50

4.7

MIN

OUT

INMAX

–

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

IN PP

⋅

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

----

–

⎝⎠

⎛⎞ D

⋅

----

–

()

⋅

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