Si8440/41/42/45

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2.4. Layout Recommendations

To ensure safety in the end user application, high voltage circuits (i.e., circuits with >30 VAC) must be physically

separated from the safety extra-low voltage circuits (SELV is a circuit with <30 VAC) by a certain distance

(creepage/clearance). If a component, such as a digital isolator, straddles this isolation barrier, it must meet those

creepage/clearance requirements and also provide a sufficiently large high-voltage breakdown protection rating

(commonly referred to as working voltage protection). Table 6 on page 14 and Table 7 on page 14 detail the

working voltage and creepage/clearance capabilities of the Si84xx. These tables also detail the component

standards (UL1577, IEC60747, CSA 5A), which are readily accepted by certification bodies to provide proof for

end-system specifications requirements. Refer to the end-system specification (61010-1, 60950-1, etc.)

requirements before starting any design that uses a digital isolator.

The following sections detail the recommended bypass and decoupling components necessary to ensure robust

overall performance and reliability for systems using the Si84xx digital isolators.

2.4.1. Supply Bypass

Digital integrated circuit components typically require 0.1 µF (100 nF) bypass capacitors when used in electrically

quiet environments. However, digital isolators are commonly used in hazardous environments with excessively

noisy power supplies. To counteract these harsh conditions, it is recommended that an additional 1 µF bypass

capacitor be added between VDD and GND on both sides of the package. The capacitors should be placed as

close as possible to the package to minimize stray inductance. If the system is excessively noisy, it is

recommended that the designer add 50 to 100

 resistors in series with the VDD supply voltage source and 50 to

300

 resistors in series with the digital inputs/outputs (see Figure 8). For more details, see "3. Errata and Design

Migration Guidelines" on page 24.

All components upstream or downstream of the isolator should be properly decoupled as well. If these components

are not properly decoupled, their supply noise can couple to the isolator inputs and outputs, potentially causing

damage if spikes exceed the maximum ratings of the isolator (6 V). In this case, the 50 to 300

 resistors protect

the isolator's inputs/outputs (note that permanent device damage may occur if the absolute maximum ratings are

exceeded). Functional operation should be restricted to the conditions specified in Table 1, “Recommended

Operating Conditions,” on page 3.

2.4.2. Pin Connections

No connect pins are not internally connected. They can be left floating, tied to VDD, or tied to GND.

2.4.3. Output Pin Termination

The nominal output impedance of an isolator driver channel is approximately 85

, ±40%, which is a combination

of the value of the on-chip series termination resistor and channel resistance of the output driver FET. When driving

loads where transmission line effects will be a factor, output pins should be appropriately terminated with controlled

impedance PCB traces. The series termination resistor values should be scaled appropriately while keeping in

mind the recommendations described in “2.4.1. Supply Bypass” above.

Figure 8. Recommended Bypass Components for the Si84xx Digital Isolator Family

VDD1

VDD2

GND1

GND2

50 – 300



50 – 300



Input/Output

Input/Output

C2

C3

1

F

1

F

R1 (50 – 100

)

V Source 1

A1

B1

50 – 300



50 – 300



Ax

Bx

C1

R2 (50 – 100

)

V Source 2

C4

0.1

F

0.1

F