AS3687 datasheet(30/55 Pages) AMSCO

30 / 55 page

AS3687/87XM

Datasheet

www.austriamicrosystems.com/AS3687

1v3-4

29 - 54

Figure

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

currX_current

I/2

I/4

I/4 with up to

100% duty cycle

next step: I/2 with

50% duty cycle

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

Figure

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

PWM

Modulator

currX_current

pwm_code

IDAC

2MHz

CURRX

Dimming

Ramp

Gen

From serial

Interface

adder_currentX

currX_adder

subX_en

if pwm_dim_mode = 01 or 10

Adder Logic

The adder logic (available for CURR30-32, CURR1, CURR2 and CURR6) 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).

If the register subX_en is set, the result from the pwm_modulator is inverted logically. That means for up

dimming the starting current is defined by currX_adder - 1 and the end current is defined by currX_adder -

currX_current - 1. An overflow of the internal bus (8 Bits wide to the IDAC) has to be avoided by the register

settings (currX_adder - currX_current - 1 must not be below zero).

Its purpose is to dim one channel e.g. CURR30 from e.g. 110% to 10% of curr30_current and at the same time

dim another channel e.g. CURR31 from 20% to 120% of curr31_current.

Note:

The adder logic operates independent of the currX_mode setting, but its main purpose is to work

together with the pwm modulator (improved up/down dimming)

If the adder logic is not used anymore, set the bit currX_adder to 0. (Setting adder_currentX to 0 is

not sufficient)

Figure

24 – PWM Dimming Table

Decrease by 1/4th every

step

Decrease by 1/8th every

step

Seconds

Seconds Seconds Seconds

Step

%Dimming

PWM

%Dimming

PWM

50msec/

Step

25msec/

Step

5msec/

Step

2,5msec/

Step

100,0

255

100,0

255

0,00s

0,000s

75,3

192

87,8

224

0,05s

0,03s

0,005s

0,003s

56,5

144

76,9

196

0,10s

0,05s

0,010s

0,005s

42,4

108

67,5

172

0,15s

0,08s

0,015s

0,008s

31,8

59,2

151

0,20s

0,10s

0,020s

0,010s

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