Power And Reset; Vcc12V; Vcc5V_Stb; Vcc_Rtc - Emerson COM Express Carrier Design Manual

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18 Power and Reset

COM Express calls for the module to be powered by a single 12 V power rail, with a ±5% tolerance. For

Type 2 modules, there are 33 pins dedicated to +12 V power in, and there are 64 ground return pins.

This ensures that adequate power (up to 188 W) for very high-performance systems can be brought

into the module.Direct connection of a COM Express module to a low impedance supply such as a

battery pack may result in excessive inrush currents. The supply to the COM Express module should be

slew limited to limit the input voltage ramp rate. A typical ATX supply ramps at about 2.5 V/ms.

If suspend functions such as Suspend-to-RAM, Suspend-to-Disk, wake on power button press, wake on

USB activity, etc., are to be supported, then a 5 V suspend power source must also be provided to the

module. If suspend functions are not used, the module VCC_5V_SBY pins should be left open.

The VCC_RTC is for the real time clock (RTC) module. It is responsible for maintaining the time and

date even when the COM Express module is not connected to a main power supply. It is provided by

double power rails. When the system is in the G3 (mechanical off) status, it is provided by a +3 V

lithium battery cell. When the system is already powered on, it can be provided by the 3.3 V standby

power rail. For more information, see Section 16.5, RTC Battery.

The current capacity of a PCB trace is proportional to the trace's cross-sectional area–the product of

the trace width and thickness. The trace thickness is proportional to the weight of copper used. The

weight of the copper is expressed in ounces per square foot in the United States. Usually people will

omit the per square foot and just use ounce to describe the copper. Copper weights of 0.5 ounce and

sometimes 1 oz are common for inner layer traces. A copper weight of 1 oz is common for power

planes. A copper weight of 0.5 oz results in a thickness of approximately 0.0007 in, and 1 oz copper

yields approximately 0.0014 in. Outer layer traces are usually built with 0.5 oz copper, but are then

plated up with additional conductive material, often yielding an effective copper weight of about 1 oz.

The effective weight of outer layer traces may vary with different PCB processes. Check with your PCB

vendor, or play it safe and make conservative assumptions.

Consult sources such as the IPC-2221 (see Appendix G for reference) for charts that relate copper

weight, trace width and trace-current capacity at a given temperature rise to the current capability. It is

best to assume a conservative trace temperature rise, such as 10°C maximum, when making trace-

width decisions. Per the IPC charts, external layer traces can carry significantly more current than

Page 86 of 103

COM Express Carrier Type 2

Design Guide

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