LED6000 datasheet(25/49 Pages) STMICROELECTRONICS

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LED6000

Application notes - step-down conversion

Different capacitors can be considered:



Electrolytic capacitors

These are the most commonly used due to their low cost and wide range of operative

voltage. 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 the very low ESR). The drawback

is their high cost.



Tantalum capacitor

Small, good quality tantalum capacitors with very low ESR are becoming more available.

However, they can occasionally burn if subjected to a very high current, for example when

they are connected to the power supply.

The amount of the input voltage ripple can be roughly overestimated by Equation 9.

Equation 9

In case of MLCC ceramic input capacitors, the equivalent series resistance (RES,IN) is

negligible.

In addition to the above considerations, a ceramic capacitor with an appropriate voltage

rating and with a value 1 µF or higher should always be placed across VIN and power

ground and across VCC and the IC GND pins, as close as possible to the LED6000 device.

This solution is necessary for spike filtering purposes.

5.2

Output capacitor and inductor selection

The output capacitor is very important in order to satisfy the output voltage ripple

requirement. Using a small inductor value is useful to reduce the size of the choke but

increases the current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is

required.

The current in the output capacitor has a triangular waveform which generates a voltage

ripple across it. This ripple is due to the capacitive component (charge and discharge of the

output capacitor) and the resistive component (due to the voltage drop across its ESR). So

the output capacitor must be selected in order to have a voltage ripple compliant with the

application requirements.

The allowed LED current ripple (

ILED,PP) is typically from 2% to 5% of the LED DC current.

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OUT

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