LED5000 datasheet(22/52 Pages) STMICROELECTRONICS

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Application notes - buck conversion

LED5000

22/51

Doc ID 023951 Rev 1

5.8.1

Dimming frequency vs. dimming depth

As seen in

Chapter 5.8 the LEDs current rising and falling edge time mainly depends on the

system bandwidth (TRISE) and the selected output capacitor value (TRISE and TFALL).

The dimming performance depends on the minimum current pulse shape specification of

the final application. The ideal minimum current pulse has rectangular shape, in any case it

degenerates into a trapezoid or, at worst, into a triangle, depending on the ratio (TRISE +

TFALL)/ TDIM

Equation 23

The small signal response in

Figure 14 is considered as an example.

Equation 24

Assuming the minimum current pulse (TMIN_PULSE) shape specification as:

Equation 25:

where TDIMMING represents the dimming period and DMIN the minimum duty cycle which

gives the TMIN_PULSE charge. In the given example TMIN_PULSE=9µs

Figure 15.

dimming signal

Given TMIN_PULSE it is possible to calculate the maximum dimming depth given the dimming

frequency or vice versa.

For example, assuming a 10 KHz dimming frequency the maximum dimming depth is 9% or

given a 5% dimming depth it follows a 5.5 KHz maximum fDIM.

The LED5000 dimming performance is strictly dependent on the system small signal

response. As a consequence, an optimized compensation network (good phase margin and

bandwidth maximized) and minimized COUT value are crucial for best performance. Once

rec

gle

tan

RISE

FALL

DIM

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

→

trapezoid

RISE

FALL

DIM

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

→

triangle

RISE

FALL

DIM

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

RISE

μs

≅

FALL

μs

≅

⎩

⎨

⎧

RISE

FALL

0.75

MIN_PULSE

⋅

0.75

MIN

DIMMING

⋅⋅

AM13499v1

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