LM3549

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SNVS640A – AUGUST 2010 – REVISED MAY 2013

BRIGHTNESS CONTROL

Output current of all drivers can be adjusted using PWM input or FADER register. This can be used to easilly

adjust the total brightness of the LEDs. Brightness control function can be enabled from the CTRL register as

seen in table below. In case of PWM input brightness control (BRC) is the positive duty cycle of the input signal.

In case of FADER register brightness is MASTER FADER[7:0]/255.

MFE

PWM

Brightness Control

0

0

No brightness control

0

1

PWM input

1

0

FADER register

1

1

PWM input

The maximum currents of the drivers are set in the current setting registers. Brightness control keeps the ratio of

the driver currents constant and adjusts the output currents based on the highest current setting. Driver currents

can be adjusted between 100 mA to the maximum current set in the registers (see figures PWM Output Current

Control and Master Fader Register Current Control in Typical Performance Characteristics).

R1

R2

BRC =brightness control

I2 = I1 x R1

I3 = I1x R2

PWM TIMING

Figure 18 shows example of PWM brightness control. PWM input can be change at any given time but control

takes effect when next enable is pulled high. To ensure that control takes effect for the next color time from PWM

change to next enable needs to be greater than timeout time (300 µs typical).

At the beginning of the example frame PWM input is changed from 100% to 80% while green driver is enabled.

Brightness level is not changed in the middle of the green frame but at the beginning of the next color which in

this example is blue. During next green PWM is set back to 100%. This is done at least 300 µs before next

enable is pulled high and control takes effect then. During next green PWM is changed to 0%. Time from PWM

change to next enable (blue) is less than 300 µs and control don’t take effect when blue starts but one color later,

what in this example is red.

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