AS3675 datasheet(41/81 Pages) AMSCO

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AS3675

Datasheet - D e ta i l e d D e s c r i p t i o n

The PWM generator can be used for any current sink. The setting applies for all current sinks, which are controlled by

the pwm generator (e.g. CURR1 is pwm controlled if curr1_mode = 10). The pwm modulated signal can switch on/off

the current sinks and therefore depending on its duty cycle change the brightness of an attached LED.

Internal PWM Generator

The internal PWM generator uses the 2MHz internal clock as input frequency and its dimming range is 6 bits digital

(2MHz / 2^6 = 31.3kHz pwm frequency) and 2 bits analog. Depending on the actual code in the register pwm_code the

following algorithm is used:

If pwm_code bit 7 = 1

Then the upper 6 bits (Bits 7:2) of pwm_code are used for the 6 bits PWM generation, which controls the selected cur-

rents sinks directly

If pwm_code bit 7 =0 and bit 6 = 1

Then bits 6:1 of pwm_code are used for the 6 bits PWM generation. This signal controls the selected current sinks, but

the analog current of these sinks is divided by 2

If pwm_code bit 7 and bit 6 = 0

Then bits 5:0 of pwm_code are used for the 6 bits PWM generation. This signal controls the selected current sinks, but

the analog current of these sinks is divided by 4

Figure 21. 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

exceeded. It is used to smoothly and automatically dim the brightness of the LEDs connected 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 22. 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:

AS3675 Datasheet(PDF) 41 Page - ams AG