si8405 Silicon Laboratories, si8405 Datasheet - Page 19

• Manufacturer Part Number: si8405
• Description: Bidirectional I 2c Isolators With Unidirectional Digital Channels
• Manufacturer: Silicon Laboratories

5. Typical Application Overview
5.1. I
In many applications, I
elimination. For example, Power over Ethernet (PoE) applications typically use an I
between the PoE power sourcing device (PSE), and the earth ground referenced system controller. Galvanic
isolation is required both by standard and also as a practical matter to prevent ground loops in Ethernet connected
equipment.
The physical interface consists of two wires: serial data (SDA) and serial clock (SCL). These wires are connected
to open collector drivers that serve as both inputs and outputs. At first glance, it appears that SDA and SCL can be
isolated simply by placing two unidirectional isolators in parallel, and in opposite directions. However, this
technique creates feedback that latches the bus line low when a logic low asserted by either master or slave. This
problem can be remedied by adding anti-latch circuits, but results in a larger and more expensive solution. The
Si840x products offer a single-chip, anti-latch solution to the problem of isolating I
require no external components except the I
maximum of 2.5 kV
5.2. I
Without anti-latch protection, bidirectional I
through an adjacent isolator channel creating a stable latched low condition on both sides. Anti-latch protection is
typically added to one side of the isolator to avoid this condition (the "A" side for the Si8400/05).
The following examples illustrate typical circuit configurations using the Si8405.
The "A side" output low (V
isolator V
2
2
I2C/SMBus
C Background
C Isolator Operation
IL
Unit 1
to prevent the latch condition.
RMS
V
V
IL
, support I
2
OL
C, SMBus, and PMBus interfaces require galvanic isolation for safety or ground loop
OL
) and input low (V
2
C clock stretching, and operate to a maximum I
Figure 15. Isolated Bus Overview
2
C isolators latch when an isolator output logic low propagates back
2
IL
C/SMBus pull-up resistors. In addition, they provide isolation to a
Preliminary Rev. 0.1
) levels are designed such that the isolator V
V
OL
Si8400/05
V
ISO2
IL
+
-
ISO1
2
C bus speed of 1.7 Mbps.
2
C interface for communication
2
C/SMBus applications and
OL
is greater than the
I
2
Si840x
C/SMBus
Unit 2
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