AN442

Rev. 0.1

3

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:

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:

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.

stored in the capacitor. As the capacitor sources current, the voltage across the capacitor begins to drop. This

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.

PRX

PRX

1

TXGD

2

3

TXO

4

DNC

VSS

FR

SREN

VDD

8

7

6

5

Si1102

VDD

VSS

C1

C2

C3

R3

TxLED

R1

R2

0.1 µF

10 µF

10 µF

5



100 k



100 k

