AN-1540| Application Note

Power Measurement of Ethernet Physical Layer Products

Power Measurement of Ethernet Physical Layer Products
1.0 Introduction
System designers require accurate component power consumption specifications, for the purposes of thermal management, component selection, and power distribution planning. National Semiconductor Ethernet product datasheets provide data which accurately reflects power consumption in typical network applications. This application note details the key factors that influence power consumption, and explains how they affect this important parameter. The degree to which these factors affect power consumption is demonstrated by presenting power consumption data for the DP83848 under a variety of operating conditions. This application note is applicable to the following products: DP83848C DP83848I DP83848YB DP83848M DP83848T DP83848H DP83848J DP83848K DP83849C DP83849I DP83849ID DP83849IF

National Semiconductor Application Note 1540 Anthony Davis, David Miller, Devin Seely November 2006

during normal operation. Component specifications that do not include termination and bias current fail to provide designers with adequate operating information for power budget planning. 3.2 OPERATIONAL CONFIGURATION Operating configurations that influence power consumption include data rate, disconnected configuration, and power down configuration. Data rate influences power demand through variations in the signaling (data coding, voltage and operating frequency) used for 10 Base-T and 100 Base-Tx operation. 10Mb data is comprised of +/-2.5 Volt pulses, operating at frequencies of 5 MHz and 10 MHz. 100Mb data is comprised of +/-1 Volt signals, operating at a frequency of 125 MHz. Refer to Figure 1 for typical waveforms. A physical layer device is disconnected when the receive signals are not connected or linked to a transmitting network signal source. In typical applications, Auto-Negotiation is enabled when a device is disconnected. When the network interface is not active, the physical layer component ca
n be placed in a power down mode either manually or automatically (using the Energy Detect feature), which results in minimal current demand. (See individual product datasheets for details regarding Power Down and Energy Detect features.) 3.3 TRAFFIC PAYLOAD Network traffic payload is also a factor, in both traffic density and data content. Traffic density is maximized when maximum size packets are transmitted and received, utilizing minimum interframe gaps between packets. Traffic content that maximizes I/O data transitions also increases power consumption. 3.4 DIGITAL I/O PIN LOADING Digital output pins include clock output pins, general-purpose output pins, and MII digital output pins. Load impedance on digital I/O's can have a dramatic influence on power consumption. For example, 6 digital outputs driving 5 pF loads at 25 MHz can result in a current demand of 15 mA in a typical application. Power demand can be reduced by making MII signal traces as short as possible, and by adding series termination to t
he MII output signals. Some PHYTER products include integrated digital output series resistance; refer to specific datasheets for details. 3.5 ENVIRONMENTAL CONDITIONS Ambient temperature is also an environmental factor that influences power consumption. High ambient temperatures increase current demand, while low temperatures decrease current demand when compared to typical 25 C. ambient conditions. Temperature has the least amount of influence on total power consumption because PHYTER products are designed to internally compensate temperature variations.

Although the data presented in this document is specific to the DP83848 device, the methodologies and general conclusions also apply to other PHYTER products listed above.

2.0 Recommendations
The principal factor influencing power consumption is the current demand from the signal termination and center-tap biasing on the component side of the isolation transformer. In this test the termination and bias current represents up to 65% of the total current required. The data also demonstrates that environmental variations can result in power demand changes of up to +/-19% from typical power consumption specifications. Among environmental factors that affect power consumption, supply voltage has the most dramatic effect (about +/-11%), while temperature variation has the least effect (less than +/-1%).

3.0 Power Consumption Factors
Factors that influence power consumption measurement include signal termination and isolation transformer center-tap bias current, the operational mode configured, packet data payload, I/O pin loading, and environmental conditions. 3.1 TERMINATION AND BIAS CURRENT An isolation transformer is commonly used at the Ethernet physical layer to provide electrical isolation between the cable and the device and significantly reduce the common mode voltage seen by the receiver. The resistive termination and transformer center-tap bias current is important because it comprises a large percentage of the total current demand

AN-1540

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2006 National Semiconductor Corporation

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