LED2000DR datasheet(22/40 Pages) STMICROELECTRONICS

22 / 40 page

Application information

LED2000

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DocID023432 Rev 4

The software calculates external components according to the internal database. It is also

possible to define new components and ask the software to use them.

Bode plots, estimated efficiency and thermal performance are provided.

Finally, the user can save the design and print all the information including the bill of material

of the board.

Application information

7.1

Component selection

7.1.1

Sensing resistor

In closed loop operation the LED2000 feedback pin voltage is 100 mV, so the sensing

resistor calculation is expressed as:

Equation 22

Since the main loop (see Section 6.1) regulates the sensing resistor voltage drop, the

average current is regulated into the LEDs. The integration period is at minimum 5 * TSW

since the system bandwidth can be dimensioned up to FSW/5 at maximum.

The system performs the output current regulation over a period which is at least five times

longer than the switching frequency. The output current regulation neglects the ripple

current contribution and its reliance on external parameters like input voltage and output

voltage variations (line transient and LED forward voltage spread). This performance can

not be achieved with simpler regulation loops such as a hysteretic control.

For the same reason, the switching frequency is constant over the application conditions,

which helps to tune the EMI filtering and to guarantee the maximum LED current ripple

specification in the application range. This performance can not be achieved using constant

ON/OFF-time architecture.

7.1.2

Inductor and output capacitor selection

The output capacitor filters the inductor current ripple that, given the application condition,

depends on the inductor value. As a consequence, the LED current ripple, that is the main

specification for a switching current source, depends on the inductor and output capacitor

selection.

100 mV

ILED

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