AN3981 datasheet(45/54 Pages) STMICROELECTRONICS

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AN3981

Common application information

Doc ID 022217 Rev 1

45/54

Figure 55.

Example showing how to perform a PWM dimming separately for each

channel

For 20 PWM steps (resulting in 80 clock pulses), all outputs are on. Then the blue is

switched off and for a further 25 PWM steps only green and red are kept on.

From the 46th step on, the green is the only one to remain on for 20 PWM steps. After that

all outputs are switched off for the remaining 35 PWM steps, which complete the PWM

dimming period.

This way to perform the PWM dimming requires the proper choice of data clock frequency in

order to achieve a correct PWM frequency.

A correct PWM frequency means a frequency higher than the frequency perceived by the

human eye:

Equation 17

where fDIMM is the PWM dimming frequency (inverse of the PWM dimming period) and fDIM

is the minimum frequency to prevent the human eye from perceiving the switching of the

light.

The PWM dimming frequency depends on the desired number of PWM steps (n_steps), the

number of channels of the considered LED driver (n_ch) and the data clock frequency

(fCLK):

Equation 18

Therefore the choice of the data clock frequency should comply with the following condition:

LIM

DIMM

≥

steps

CLK

DIMM

⋅

AN3981 Datasheet(PDF) 45 Page - STMicroelectronics