Selecting an Ethernet ICM the easy way
An ICM, or integrated connector module is a type of connector that integrates all the magnetics required to meet the Ethernet standard for xBASE-T connections. While it is possible to achieve this with simple standalone connectors and discrete magnetics, in practice this is only done in very high volume situations where fractions of a cent need to be saved. ICMs provide a quick and easy, integrated solution which effectively guarantees the standards will be met (assuming you choose the right one), while offering convenience, more board space and improved reliability versus discrete solutions.
There are several standards that ICMs have to meet. The first is the IEEE 802.3 Ethernet standard, which provides the guidelines for data communication using the Ethernet protocol. The standard breaks the network down into a stack of layers which have well-defined inputs and outputs in order to allow effective communication. The bottom layer in the stack is called the physical layer (or PHY) – in an Ethernet device this layer is represented by a PHY chip or chips from a supplier such as Broadcom or Marvell. Between the PHY layer and the actual physical medium, the cable, there is a media dependent interface (MDI), which provides the physical and electrical connection. This is where the ICM comes in – it connects the network’s physical layer (PHY chip) to the physical medium (cable).
ICMs also have to meet the TIA-968-A standard. This one is defined by the Administrative Council for Terminal Attachment (ACTA), and it defines the size and shape of the generic RJ45 connector. The idea is to ensure mechanical compatibility between male and female connectors (plugs and ports) from different manufacturers.
There is also the RF radiation standard, FCC Part 15b. This standard limits the amount of RF radiation devices can emit for EMI prevention purposes. While this doesn’t apply to ICMs per se (it is a system-level standard), it’s worth bearing in mind that ICMs should be designed to attenuate RF emissions as much as possible.
It’s the IEEE 802.3 Ethernet standard that defines the properties ICM magnetics need to have. Galvanic isolation is one of the requirements, which is usually met by a 1:1 transformer, which can also provide DC biasing as required (DC biasing is needed when the nodes are powered from different sources with different ground levels). The exact levels are specified in the standard. For example, a transformer for a 100BASE-T connection is required to have an inductance value of at least 350µH when a DC bias of 8mA is applied. Transformers also provide a level of common mode rejection, which helps with noise attenuation.
As well as transformers, there will also be capacitors, which provide additional isolation and decoupling. Resistors are also required to provide effective termination for any unused pairs – usually this is a 75 Ω resistor combined with a 1000pF, 2000V capacitor in a special configuration called the Bob Smith termination.
While ICMs may look the same, and have similar electrical properties to meet the standards discussed above, there are a few factors which distinguish them from each other.
First thing to consider are the system requirements. Some ICMs can support 10, 100 and 1000 BASE-T, others only 10 and 100. Some are suitable for 2.5G PHYs. Single port and multiport configurations are available for all these options. Consider also whether your system uses Power over Ethernet (PoE or PoE+), as some ICMs support this and some don’t.
Application requirements are also important. What operating temperatures do you expect your system to experience? How much space is available? Some manufacturers offer reduced height ICMs for tight spaces. ICMs also come in different configurations for different orientations – vertical or horizontal, and with the tab in either direction. Do you need LEDs in the ICM to provide visible confirmation of a good connection? Modern products have various different colours available. Would a combined RJ45 and USB connector help save space and could this be made to work in your system?
A selection of Bel Magnetic Solutions MagJack ICMs in different configurations
The easy way
The easiest recommended way to select an ICM for a particular PHY is to take a look at the manufacturer’s reference design for the PHY chip and pick something that is as similar to the ICM they used as possible, noting the application speed and how many cores are required, and taking your applications mechanical (size and orientation) requirements into account.
There may also be ‘cheat sheets’ available in the form of selection guides. For example, Avnet Abacus has a handy selection guide available for Bel Magnetic Solutions MagJack ICMs that meet the requirements of a range of the most popular Broadcom or Marvell PHYs. This guide is fairly foolproof, but feel free to contact our technical specialists via Ask an Expert for help or advice at any time.
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