LED5000 datasheet(24/52 Pages) STMICROELECTRONICS

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Application notes - buck conversion

LED5000

24/51

Doc ID 023951 Rev 1

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 27

where the term 8/

π2 represents the main harmonic of the inductor current ripple (which has

a triangular shape) and

ΔI

L is the inductor current ripple.

Equation 28

so L value can be calculated as:

Equation 29

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

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

would be the COUT value to meet the specification.

A general rule to dimension L value is:

Equation 30

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

ripple specification with the module of the Fourier transformer (see

Equation 27) calculated

at fSW frequency.

Equation 31

Example (see

Chapter 5.6):

VIN=48 V, ILED=700 mA,

ILED/ILED=2%, VFW_LED=3.7 V, nLED=10

A lower inductor value maximizes the inductor current slew rate for better dimming

performance.

Equation 30 becomes:

RIPPLE s

()

-----

ΔI

ESR

OUT

⋅⋅

()

⋅⋅

ESR

LED

⋅

() C

OUT

⋅⋅

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

ΔI

OUT

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

OFF

⋅

LED

FW_LED

200mV

⋅

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

OFF

⋅

ΔI

LED

----------- 0.5

≤

RIPPLE s=j

⋅

()

RIPPLE_SPEC

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