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.

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

The LED circuit does not necessarily need to be powered from the same VDD used to power the Si1102 and

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.

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.

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%