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SI1102 Arkusz danych(PDF) 5 Page - Silicon Laboratories

Numer części SI1102
Szczegółowy opis  DESIGNERS GUIDE
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Producent  SILABS [Silicon Laboratories]
Strona internetowa  http://www.silabs.com
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SI1102 Arkusz danych(HTML) 5 Page - Silicon Laboratories

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AN442
Rev. 0.1
5
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 CLED and RLED 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 CLED capacitors or excessively small RLED resistors should be avoided.
2.2. Choosing VLED
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 VLED 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 VLED 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.
Table 1. Recommended RLED and CLED vs. VLED
VLED
RLED
CLED
3.3 V
2
 ±5%, 1/16 W
10 µF ±20%
5.0 V
5
 ±5%, 1/4 W
10 µF ±20%
7.0 V
10
 ±5%, 1/2 W
10 µF ±20%


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