LED2000PUR datasheet(23/40 Pages) STMICROELECTRONICS

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DocID023432 Rev 4

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LED2000

Application information

Figure 15. Equivalent circuit

The LED ripple current can be calculated as the inductor ripple current ratio flowing into the

output impedance using the Laplace transform (see Figure 11):

Equation 23

where the term 8/

2 represents the main harmonic of the inductor current ripple (which has

a triangular shape) and

I

L is the inductor current ripple.

Equation 24

so L value can be calculated as:

Equation 25

where TOFF is the OFF-time of the embedded high switch, given by 1-D.

As a consequence, the lower the inductor value (so the higher the current ripple), the higher

the COUT value would be to meet the specification.

A general rule to dimension L value is:

Equation 26

Finally, the required output capacitor value can be calculated equalizing the LED current

ripple specification with the module of the Fourier transformer (see Equation 23) calculated

at FSW frequency.

IRIPPLE s







------

I

1 s ESR COUT







RS ESR nLED RLED



 C

OUT



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

I

VOUT

-------------- TOFF



nLED VFW_LED 100mV



------------------------------------------------------------------ TOFF



nLED VFW_LED 100mV



I

------------------------------------------------------------------ TOFF



nLED VFW_LED 100mV



I

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

nLED VFW_LED 100mV



VIN

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

–







I

ILED

-----------

0,5



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