LM3533 Datasheet, PDF (22/53 Pages) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

LM3533
ALS Functional Blocks
Figure 8 shows the functional block diagram of the LM3533’s ambient light sensor (ALS) interface.
FIGURE 8. Ambient Light Sensing Block Diagram
30135707
AMBIENT LIGHT SENSOR INPUT
The ALS input is designed to connect to an analog or PWM output ambient light sensor. The ALS Configuration Register Bit [1]
selects which type of sensor interface will be used at the ALS input (see Table 22).
ANALOG OUTPUT AMBIENT LIGHT SENSORS (ALS GAIN SETTING RESISTORS)
With ALS Cnfiguration Register bit [1] = 0, the ALS input is set for Analog Sensor mode. In this mode the LM3533 offers 128
programmable internal resistors at the ALS input (including a high-impedance option); see Table 21. These resistors are designed
to take the output of an analog ambient light sensor and convert it into a voltage. The value of the resistor selected is typically
chosen such that the ALS input voltage is 2V at the maximum ambient light (LUX) value. The sensed voltage at the ALS input is
digitized by the LM3533’s internal 8-bit ADC with a full-scale value (0xFF) corresponding to 2V.
PWM OUTPUT AMBIENT LIGHT SENSORS (INTERNAL FILTERING)
With the ALS Configuration Register bit [1] = 1, the ALS input is set for PWM-Sensor mode. In this mode the LM3533 offers an
internal level shifter and low-pass filter (ALS PWM Input mode). With this mode enabled the ALS input accepts logic level PWM
signals and converts them into a 0-to-2V analog voltage which is then filtered. This 0-to-2V analog representation of the PWM
signal is then applied to the internal 8-bit ADC, where 2V is the full scale (code 0xFF). The internal filter has a corner frequency of
540Hz and provides 51dB of attenuation (355x) at a 10kHz input frequency.
Since the internal ADC for the ambient light sensor utilizes an 8-bit ADC, the attenuation of the ALS input signal needs to be greater
than 1/255 (1 LSB = 7.843mV) in order to realize the full 8-bit range. This forces the frequency for the PWM signal at the ALS input
to be around 6kHz or greater. For slower moving signals an external RC filter may need to be combined with the Analog Sensor
Mode (see Applications Information section).
When the ALS input is set for ALS PWM Input Mode the internal ALS resistor setting is automatically set for high impedance, no
matter what the setting in the ALS Select Register.
INTERNAL 8-BIT ADC
The LM3533 digitizes the ALS voltage using an internal 8-bit ADC. The ADC is active as long as the ALS enable bit is set. Once
set, the ADC begins sampling and converting the voltage at the ALS input at 7.142ksps. The ADC output can be read back via the
ADC register (address 0x37). With the ALS enable bit set, the ADC register is updated every 140µs. Figure 9 details the timing of
the ADC.
22
Copyright © 1999-2012, Texas Instruments Incorporated