AN442
S i 11 0 2
AND
S i 112 0 D
E S I G N E R
’
S
G
U I D E
1. Introduction
The Si1102 and Si1120 are low-cost, high-performance, active-optical reflectance-based proximity sensors. Both
drive an LED to illuminate a target, then measure the reflectance from the target to determine its proximity.
Both the Si1102 and Si1120 use short-duration strobe pulses to measure reflectance. This keeps the average
power consumption in the microwatt range. Both devices cancel background dc ambient before making a
reflectance measurement. The reflectance measurement is the difference between the dc ambient and dc ambient
plus reflection from the target illuminated by the LED.
The Si1102 is a stand-alone, dual-port proximity sensor driving a single LED. It uses an internal analog wakeup
controller that is controlled through an external resistor to set the time interval between measurements. The PRX
output is maintained between measurements allowing the Si1102 to behave as a proximity on-off function. The
Si1102 is well suited for applications, such as electronic toys, powering transmitters for RF alarm sensors, and
saving energy in homes or offices.
Although the best performance is achieved with 850 nm LEDs, the Si1102 can also be used in short-range
applications where red visible LEDs are useful, such as hand washers or paper towel dispensers.
The Si1102 sensitivity adjustment sets a fixed proximity threshold, a reflectance level at which it will detect
proximity. This absolute level threshold may drift around 20% or more depending on temperature, LED supply
voltage, LED aging, and other environmental factors. Consequently, although it is possible to set a threshold that is
less than 10% different than the absolute reflection, it may not be consistent. It is, therefore, good system practice
to allow for some system programmability, such as using potentiometers.
The Si1120 is designed to operate with a microcontroller. Rather than an on/off output as in the Si1102, the Si1120
encodes the reflectance measurement as a pulse-width-modulated output where the pulse width is directly
proportional to the measured reflectance.
Silicon Laboratories offers a wide range of microcontrollers that are well-suited for use with the Si1120. Most
Silicon Laboratories microcontrollers offer the PCA (Programmable Counter Array) that can easily be used to
measure the pulse width output. With the addition of a microcontroller, higher level functions can be added.
The microcontroller can be used to control multiple LEDs, enabling position determination through triangulation. In
addition, unique human interface concepts, such as gestures, can be implemented to enhance the user interface
for your product. Such an interface can provide product differentiation, resulting in additional product revenue. The
microcontroller can also be used to control multiple sensors and a single LED for applications that are particularly
power-sensitive.
The Si1120 can be programmed to drive a 400 mA or 50 mA pulse. When used with a microcontroller, it is possible
to dynamically change the LED current drive to either optimize for range or lower overall system power
consumption. With the microcontroller, the reflectance measurement frequency can be customized based on the
current usage state.
Another feature made possible by a microcontroller is the ability to improve the SNR of the reflectance
measurement through pulse averaging. When used with IR filters and lenses, it is possible to use the Si1120 and
microcontroller to detect human-sized objects one meter away.
Rev. 0.1 1/15
Copyright © 2015 by Silicon Laboratories
AN442
AN442
Object in
proximity
Case
Optical
window
Transmit LED
Si1120 or
Si1102
Optical
Barrier
Figure 1. Reflectance-Based Proximity Detection
3.3 V
VDD / DC+
C1
1.0 uF
GND / DC-
P0.0/VREF
P0.1 / AGND
P0.2 / XTAL1
P0.3 / XTAL2
P0.4 / TX
P0.5 / RX
P0.6 / CNVSTR
P0.7 / IREF0
Si1120
PRX
TXGD
TXO
STX
VSS
MD
SC
VDD
C8051F931
DCEN
VBAT
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
XTAL3
XTAL4
RST / C2CK
P2.7 / C2D
TX LED
C4
68.0 uF
R1
30 ohm
C2
10 uF
C3
0.1 uF
GND
Figure 2. Si1120 Circuit
2
Rev. 0.1
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VDD
R3
5
C1
0.1 µF
C2
1
10 µF
PRX
TXGD
TXO
DNC
VSS
FR
SREN
VDD
8
7
6
5
PRX
C3
10 µF
2
3
TxLED
4
R1
100 k
R2
100 k
Si1102
VSS
Figure 3. Si1102 Circuit
2. Electrical Considerations
This section is applicable to both the Si1120 and the Si1102. This section focuses on the electrical properties of the
Si1120 and Si1102. Items covered include:
Choosing the right resistor and capacitor for your system
Choosing the voltage rail used to drive the LED
Estimating system power requirements
PCB layout guidelines
The following sections do not need to be read sequentially; it is best to simply reference topics of particular interest.
2.1. TXO (LED Driver) Characteristics
The LED current is governed by the following parameters:
LED Voltage Rail (V
LED
)
Resistor (R
LED
)
Capacitor (C
LED
)
Unlike the Si1102, the Si1120 has a choice of either driving a peak of 50 mA (PRX50 and PRX50H modes) or
400 mA (PRX400 mode). In this section, only the 400 mA drive option is discussed since this peak LED current is
common to both the Si1120 and the Si1102. The principles apply regardless of the peak LED current choice.
The current through the LED can come either from the C
LED
capacitor or from V
LED
flowing through the resistor,
R
LED
.
Assuming that the C
LED
is in a “fully charged” initial condition, the LED current initially comes from the charge
stored in the capacitor. As the capacitor sources current, the voltage across the capacitor begins to drop. This
voltage drop causes a potential difference across R
LED
. After a sufficient period of time, the current is sourced only
through V
LED
, through the R
LED
resistor.
A typical Si1102 and Si1120 transistor VI curve is shown in Figure 5. Above one volt, the transistor is in its active
region and drives a current of approximately 400 mA. When the TXO voltage is below 0.5 V, the transistor is in its
saturation region and no longer sinks the peak 400 mA.
Rev. 0.1
3
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V
LED
R
LED
LED
TXO
Si11xx
+
C
LED
V
TXO
I
TXO
-
Figure 4. Simplified TXO Circuit
500
450
400
350
ITXO (mA)
300
250
200
150
100
50
0
0
0.5
1
1.5
VTXO (V)
2
2.5
3
Figure 5. TXO Voltage-Current Characteristic
It is important to touch on the internal operation of the Si1102 and Si1120; it is essential to understanding why the
first 20 µsec of the TXO drive is so critical.
The Si1120 and Si1102 contain an ambient IR circuit designed to reach an equilibrium state in which its current is
equal to the photodetector current prior to the start of the LED drive. When the photo detector current increases
due to the IR light (indirectly sourced by the LED through reflection), the Ambient IR circuit does not
instantaneously adjust its current to match that of the photodetector. Therefore, at that moment, the photodetector
current is higher than that of the Ambient IR circuit. This difference in current is the initial detect state.
In the Si1102, this information is fed into an internal circuit. In the Si1120, this event causes the Si1120 to keep the
PRX asserted instead of negating. A consequence of this is that the LED continues to emit light and reach the
photodiode to keep the photodetector operating at this state with increased current.
The Ambient IR circuit is a servo feedback circuit that then tries to match the current to that of the photodetector
current. At the point at which the ambient IR reestablishes equilibrium with the photodetector current, the TXO
drive is shut off. Given that the ambient IR circuit takes time to reestablish this equilibrium with the photodiode
current, this time frame is proportional to the reflectance.
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Rev. 0.1
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For the Si1102, the ambient IR circuit correction is nonlinear; correction is faster for high ambients and the longer
the correction has taken. For the Si1120, the ambient correction circuit slews at a constant, highly linear rate
depending on the gain setting; consequently, the pulse width is a linear function of the reflection.
For the Si1102, the pulse width (proportional to the reflection) is compared with an internal pulse generator (one
shot) whose width is controlled by the resistor value on the SREN pin. If the ambient IR circuit matches the
increase in ambient from reflection before the SREN circuit times out, then the reflection is below the set threshold.
If the SREN pulse generator times out before the IR ambient circuit matches the increase in ambient, then the
reflection is above threshold, and PRX goes low. Whichever decision occurs first, either the IR ambient circuit
matches the reflection increase or the one shot times out; then, the LED is turned off since there is no reason to
leave the it on. This behavior can be seen if you put a scope on the TXO pin. TXO pulse width will be constant for
out-of-range objects, but, when detection threshold is reached, the pulse width will decrease as the reflection
increases.
For the Si1120, the PRX pulse width is kept asserted until the equilibrium state has been reached (as long as STX
stays high). The time frame is the basis of the PRX pulse width. Nearby objects reflect more of the LED IR, which
results in more photodiode current. More photodiode current means that the internal ambient IR circuit needs more
time to overcome the photodiode current. Thus the Si1120 PRX pulse width increases with higher reflectance.
For best linearity, LED current should be consistent throughout the TXO drive. Table 1 summarizes the component
selection for best linearity. Although the Si1102 and Si1120 drivers are both constant current above 0.5 V TXO
voltage (eliminating the need for a current-limiting resistor), the Si1120 has a high-impedance driver that varies
less than 1% per volt on either TXO or VDD (while the Si1102 driver may vary more than 20% per volt). For Si1102
absolute reflection operation, TXO current variation of up to 10% from battery fluctuations is usually not critical.
However, for motion detection (where detection of changes in reflection of less than 1% is important), the low
variation in TXO current with fluctuations in LED or VDD supplies becomes critical.
Finally, the last consideration in choosing C
LED
and R
LED
is the power drawn through TXO. Since the
instantaneous power into any node is governed by the voltage-current product, the TXO pin heats up the Si1102
and Si1120 much more if it is drawing 400 mA at a lower, as opposed to higher, TXO voltage. This means that
excessively large C
LED
capacitors or excessively small R
LED
resistors should be avoided.
Table 1. Recommended R
LED
and C
LED
vs. V
LED
V
LED
3.3 V
5.0 V
7.0 V
R
LED
2
±5%, 1/16 W
5
±5%, 1/4 W
10
±5%, 1/2 W
C
LED
10 µF ±20%
10 µF ±20%
10 µF ±20%
2.2. Choosing V
LED
The LED circuit does not necessarily need to be powered from the same VDD used to power the Si1102 and
Si1120. The decision on which voltage rail to use for V
LED
must be considered. In general, the simplest option is to
use an unregulated voltage rail. Using an unregulated voltage rail (must be < 7 V) is generally the best option.
In any system, there are “regulated” and “unregulated” voltage rails. The Si1102 and Si1120 are designed so that
the LED can be powered from either.
The fundamental issue governing the choice of V
LED
voltage rails is the manner in which the instantaneous LED
current affects the entire system. It is important that the Si1102 and Si1120 LED circuits do not adversely affect
other system components.
2.2.1. LED Current Tapped from Unregulated Voltage Rails
The advantage of using an unregulated power supply is that the effect on other system components is mitigated by
the regulator. The regulator is already expected to regulate the voltage to the rest of the system, and any voltage
ripple introduced by the sourcing of the LED current does not affect the rest of the system.
Rev. 0.1
5
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