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ISL29044AIROMZ-T7 Arkusz danych(PDF) 8 Page - Intersil Corporation |
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ISL29044AIROMZ-T7 Arkusz danych(HTML) 8 Page - Intersil Corporation |
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8 / 19 page  ISL29044A 8 FN8419.1 April 19, 2013 Principles of Operation Photodiodes and ADCs The ISL29044A contains two photodiode arrays, which convert photons (light) into current. The ALS photodiodes are constructed to mimic the human eye’s wavelength response curve to visible light (see Figure 5). The ALS photodiodes’ current output is digitized by a 12-bit ADC in 100ms. These 12 bits can be accessed by reading from I2C registers 0x9 and 0xA when the ADC conversion is completed. The ALS converter is a charge-balancing, integrating 12-bit ADC. Charge-balancing is best for converting small current signals in the presence of periodic AC noise. Integrating over 100ms highly rejects both 50Hz and 60Hz light flicker by picking the lowest integer number of cycles for both 50Hz/60Hz frequencies. The proximity sensor is an 8-bit ADC that operates in a similar fashion. When proximity sensing is enabled, the IRDR pin will drive the internal infrared LED, the emitted IR reflects off an object (e.g., a human head) back into the ISL29044A, and a sensor converts the reflected IR wave to a current signal in 0.54ms. The ADC subtracts the IR reading before and after the LED is driven (to remove ambient IR such as sunlight), and converts this value to a digital count stored in Register 0x8. The ISL29044A is designed to run two conversions concurrently: a proximity conversion and an ALS (or IR) conversion. Please note that because of the conversion times, the user must let the ADCs perform one full conversion first before reading from I2C Registers PROX_DATA (wait 0.54ms) or ALSIR_DT1/2 (wait 100ms). The timing between ALS and Prox conversions is arbitrary, as shown in Figure 15. The ALS runs continuously with new data available every 100ms. The proximity sensor runs continuously with a time between conversions decided by PROX_SLP (Register 1 Bits [6:4]). Ambient Light and IR Sensing The ISL29044A is set for ambient light sensing when Register bit ALSIR_MODE = 0 and ALR_EN = 1. The light-wavelength response of the ALS appears, as shown in Figure 5. ALS measuring mode (as opposed to IR measuring mode) is set by default. When the part is programmed for infrared (IR) sensing (ALSIR_MODE = 1; ALS_EN = 1), infrared light is converted into a current and digitized by the same ALS ADC. The result of an IR conversion is strongly related to the amount of IR energy incident on our sensor, but is unitless and is referred to in digital counts. Proximity Sensing When proximity sensing is enabled (PROX_EN = 1), the internal IR LED is driven for 0.1ms by the built-in IR LED driver through the IRDR pin. The amplitude of the IR LED current depends on Register 1 bit 3: PROX_DR. If this bit is low, the load will see a fixed 110mA current pulse. If this bit is high, the load on IRDR will see a fixed 220mA current pulse, as seen in Figure 16. When the IR from the LED reaches an object and gets reflected back into the ISL29044A, the reflected IR light is converted into current as per the IR spectral response shown in Figure 5. One entire proximity measurement takes 0.54ms for one conversion (which includes 0.1ms spent driving the LED), and the period between proximity measurements is decided by PROX_SLP (sleep time) in Register 1 Bits 6:4. Average LED driving current consumption is given by Equation 1. A typical IRDR scheme is 220mA amplitude pulses every 800ms, which yields 28μA DC. Total Current Consumption Total current consumption is the sum of IDD and IIRDR. The IRDR pin sinks current (see Figure 16) and the average IRDR current can be calculated using Equation 1. IDD depends on voltage and the mode-of-operation, as seen in Figure 9. Interrupt Function The ISL29044A has an intelligent interrupt scheme designed to shift some logic processing away from the intensive microcontroller I2C polling routines (which consume power) and towards a more independent light sensor, which can instruct a system to “wake up” or “go to sleep”. An ALS interrupt event (ALS_FLAG) is governed by Registers 5 through 7. The user writes a high and low threshold value to these registers and the ISL29044A will issue an ALS interrupt flag if the actual count stored in Registers 0x9 and 0xA are outside the user’s programmed window. The user must write 0 to clear the ALS_FLAG. FIGURE 15. TIMING DIAGRAM FOR PROX/ALS EVENTS - NOT TO SCALE ALS ACTIVE 100ms 100ms 100ms PROX SENSOR ACTIVE IRDR (CURRENT DRIVER) ALS CONVERSION TIME = 100ms (FIXED) 0.54ms FOR PROX CONVERSION SERIES OF CURRENT PULSES TOTALING 0.1ms SLEEP TIME (PROX_SLP) 100ms 100ms SEVERAL µs BETWEEN CONVERSIONS TIME TIME TIME FIGURE 16. CURRENT DRIVE MODE OPTIONS IRDR 220mA (PROX_DR = 1) (IRDR IS HI-Z WHEN NOT DRIVING) 110mA (PROX_DR = 0) LED+ INTERNAL IR-LED LED- PCB TRACE I lRDR AVG ; I lRDR PEAK ; 100 μs × 540 μst + SLEEP -------------------------------------------------------- = (EQ. 1) |
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