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ISL29501-ST-EV1Z Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL29501-ST-EV1Z Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 23 page ISL29501 11 FN8681.3 June 29, 2016 Submit Document Feedback of a lens. PDs in a traditional LED package have a built in lens so the effective active area can be 20 times more the silicon area would suggest. Large area diodes are accompanied with larger intrinsic capacitances leading to slow rise and fall times. There is a trade off between detector area and capacitance that need to be considered for system performance. The fully differential front-end converts the photo current into voltage and allows for common-mode noise/crosstalk to be rejected. An effective capacitance of less than 15pF is recommended for robust performance, for applications where distance measurement is required. Using larger capacitance will cause increase in noise and not functional failure. The decision to use small or large photodiode (i.e., capacitance) has to be made by the system engineer based on the application. Emitter Selection The ISL29501 supports the use of light sources such as LEDs, VCSELS and lasers. The sensor will drive any emitter within the maximum current range supported by the emitter DAC. The sensor working principle is wavelength agnostic and determination of wavelength can be made based on application. The emitter wavelength should be an NIR or MWIR (i.e., 800nm to 1300nm) to minimize the influence of ambient light on the precision. The selection between an LED or laser depends on the user application. Some general system considerations are distance, field of view and precision requirements. While an LED is a reliable light source, it might not be the best suited for long distance due to its dispersion characteristics. However, it is good for short range and large area coverage. For higher optical power lasers/VCSEL may offer an advantage. Lasers are more efficient but are more complicated to implement due to eye safety requirements and higher forward voltages. Ambient Light Rejection Ambient light results in a DC current in the TIA. A feedback loop supplies negates this current to prevent impact to the signal path. Subsequent stages of the analog signal chain are AC coupled and are not susceptible to DC shifts at AFE. Ambient light will alter the photon to current delay in the photodiode. This is not an issue if the ambient light is constant but if it changes, the delay in the photodiode changes, which could result in distance error. To minimize the effect of ambient on the system distance measurements, the sensor enables correction algorithms (linear and second order polynomial to correct for any diode related behaviors). Once coefficients are determined and programmed, the ambient induced delay (distance error) is subtracted real time in the chip DSP. Ambient current value can be found by reading Register 0xE3. Power Consumption In a “Time of Flight” application power consumption has two components; the power consumed within the ISL29501 device and the power consumed by the emitter LED or VSCEL. While the emitter current is load current and not part of the ISL29501 power dissipation, it is included in this discussion to help the user understand the entire “Time of Flight” contribution to the total application power budget (see Equation 2). IC POWER CONSUMPTION The power consumed in the ISL29501 has two components. The first is the standby current, which is present whenever the chip is not integrating (making a measurement). The second is the current consumed during a measurement. Chip current is calculated by multiplying the overall duty cycle by 102mA and adding the standby current (~2mA). The overall duty cycle is defined as (integration time/sampling period/2) in continuous mode or the (integration time/user measurement repetition rate/2) in single sample mode see Equation 3. Typical values for IStandby can be found in the "Electrical Specification Table" on page 5. Total Time of Flight Power Consumption To calculate the total “Time of Flight” module current, the load current contribution must be added to the chip current. As with the chip current, the measurement duty cycle has a large effect on the load current. The load current is defined as the product of the emitter current and the overall measurement duty cycle (see Equation 4). The emitter current is calculated using Equation 5: The duty cycle for this calculation is the same as described in the IC power consumption section. In the application, the best emitter current setting is a balance of the required optical power and the acceptable power consumption. Similarly, the duty cycle is a balance between the precision of a measurement and power consumption. It should be noted that choosing high duty cycles can cause heating of the emitter introducing drift in distance measurements. For additional details refer to “Emitter Selection” on page 11 and “Integration Time” on page 12. Shutdown Shutdown disables all the individual components that actively consume power, with the exception of the I2C interface. There are multiple options for the system designer based on the time to bring up the system. IDDToF IDDIC IDDLoad + = (EQ. 2) IDDIC 102mA DCOverall IS dby tan + = (EQ. 3) Load I DCOverall IEmitter = (EQ. 4) IEmitter reg0x90 15 reg0x91 255 255mA = (EQ. 5) |
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