AS3688 datasheet(44/72 Pages) AMSOSRAM

44 / 72 page

AS3688

austriami crosyst ems

Datasheet, Confidential

www.austriamicrosystems.com (ptr,tje)

Revision 1.1.1 / 20060707

42 - 70

Figure

25 – PWM Control

Automatic Up/Down Dimming

If the register pwm_dim_mode is set to 01 (up dimming) or 10 (down dimming) the value within the register

pwm_code is increased (up dimming) or decreased (down dimming) every time and amount (either 1/4

th or 1/8th)

defined by the register pwm_dim_speed. The maximum value of 255 (completely on) and the minimum value of

0 (off) is never exceeded. It is used to smoothly and automatically dim the brightness of the LEDs connceted to

any of the current sinks. The PWM code is readable all the time (Also during up and down dimming)

The waveform for up dimming looks as follows (cycles omitted for simplicity):

Figure

26 – PWM Dimming Waveform for up dimming (pwm_dim_mode = 01); currX_mode = PWM controlled (not all steps shown)

The internal pwm modulator circuit controls the current sinks as shown in the following figure:

Figure

27 – PWM Control Circuit (currX_mode = 10b (PWM controlled)); X = any current sink

The adder logic (available for RGB1, RGB2, RGB3, CURR41, CURR42, CURR43, CURR1 and CURR2) is

intended to allow dimming not only from 0% to 100% (or 100% to 0%) of currX_current, but also e.g. from 10% to

110% (or 110% to 10%) of currX_current. That means for up dimming the starting current is defined by 0 +

currX_adder and the end current is defined by currX_current + currX_adder.

An overflow of the internal bus (8 Bits wide to the IDAC) has to be avoided by the register settings (currX_current

+ currX_adder must not exceed 255).

pwm_code

7 6 5 4 3 2 1 0

6 bit PWM

to current sink(s)

6 bit PWM

to current sink(s) but analog

currents are divided by 2

6 bit PWM

to current sink(s) but analog

currents are divided by 4

0 0

PWM

Modulator

currX_current

pwm_code

IDAC

2MHz

CURRX

Dimming

Ramp

Gen

From serial

Interface

adder_currentX

Only for RGB1,2,3

CURR41,42,43

CURR1, CURR2

currX_adder

subX_en

if pwm_dim_mode = 01 or 10

Adder Logic

currX_current

I/2

I/4

I/4 with up to

100% duty cycle

next step: I/2 with

50% duty cycle

AS3688 Datasheet(PDF) 44 Page - ams-OSRAM AG

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