ISL29501 Datasheet, PDF (11/23 Pages) Intersil Corporation – Auto gain control mechanism

ISL29501
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).
IDD_ToF = IDD_IC + IDD_Load
(EQ. 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.
IDD_IC = 102mA  DC_Overall + I_{S tandby}
(EQ. 3)
Typical values for I_Standby 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).
I_{Load = DCOverall  IEmitter}
(EQ. 4)
The emitter current is calculated using Equation 5:
I_Emitter = reg0x90  15   reg0x91  255   255mA
(EQ. 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.
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FN8681.3
June 29, 2016