Si8220/21
Rev. 1.2 13
4. Functional Description
4.1. Theory of Operation
The Si8220/21 is a functional upgrade for popular opto-isolated drivers, such as the Avago HPCL-3120, HPCL-
0302, Toshiba TLP350, and others. The operation of an Si8220/21 channel is analogous to that of an opto coupler,
except an RF carrier is modulated instead of light. This simple architecture provides a robust isolated data path and
requires no special considerations or initialization at start-up. A simplified block diagram for the Si8220/21 is shown
in Figure 6.
Figure 6. Simplified Channel Diagram
A channel consists of an RF Transmitter and RF Receiver separated by a semiconductor-based isolation barrier.
Referring to the Transmitter, input A modulates the carrier provided by an RF oscillator using on/off keying. The
Receiver contains a demodulator that decodes the input state according to its RF energy content and applies the
result to output B via the output driver. This RF on/off keying scheme is superior to pulse code schemes as it
provides best-in-class noise immunity, low power consumption, and better immunity to magnetic fields. See
Figure 7 for more details.
Figure 7. Modulation Scheme
RF
OSCILLATOR
MODULATOR DEMODULATOR
A
B
Semiconductor-
Based Isolation
Barrier
Transmitter
Receiver
LED
Emulator
0.5 to 2.5 A
peak
Gnd
V
DD
Input Signal
Output Signal
Modulation Signal
