Ethernet physical layer

1 Introduction Texas Instruments Ethernet product data sheets provide data that accurately reflects power consumption in typical network applications. This application report 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 report is applicable to the following products: DP83848C DP83849C DP83848I DP83849I DP83848YB DP83849ID DP83848M DP83849IF DP83848T DP83640 DP83848H DP83848VYB DP83848J DP83848Q-Q1 DP83848K DP83630 DP83620 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 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 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 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. 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 V signals, operating at a frequency of 125 MHz. For typical waveforms, see Figure 1 and Figure 2. 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 can be placed in a power down mode either manually or automatically (using the Energy Detect feature), which results in minimal current demand. (For details regarding Power Down and Energy Detect features, see the device-specific data sheet) 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 the MII output signals. Some PHYTER products include integrated digital output series resistance. For more details, see the device-specific datas heets. 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. 4 Power Consumption Data Power consumption data was measured on several DP83848 devices that were populated in a system representing a typical end-user application. Data was measured under a variety of conditions. The average results of these measurements are presented below. Data is presented based on four basic configuration modes: power down mode (no data), auto-negotiation enabled disconnect mode (no data), 10 MBit full duplex data mode, and 100 Mbit full duplex data mode. For each mode, the first row of data represents current demand for a typical baseline configuration using nominal environmental conditions and a medium size packet (787 bytes) with an incrementing byte count data payload, if applicable. The second and third rows of data represent highest and lowest current consumption conditions. Following these worst and best case condition data points, individual conditions are varied against the baseline condition. The first set of five columns detail scenario and environmental conditions, including VCC supply voltage, temperature, packet length, and packet payload data pattern. The next four columns provide actual current measurement data. These measurements are chip current excluding termination and center-tap current (ICT), termination and center-tap bias current (ICT), total current (ITot), and the percentage of the total current utilized by the termination and center-tap bias current. The next column shows the total system power utilized (ITot * VCC). The final four columns illustrate how much the current and power demand varies when compared to the initial baseline conditions. 4.1 Power Down Data Table 1 shows typical DP83848 power down mode current consumption. Notice that even under power down conditions, center-tap current demand can represent up to ~6% of total current demand for the device. Also, notice that VCC supply variation has the largest impact on overall current consumption. 4.2 Disconnect Data With Auto-Negotiation Enabled Table 2 shows typical current consumption required for the DP83848 in disconnect mode with Autonegotiation enabled. Notice that center-tap current demand represents ~50% of the total current demand for the device in this mode. Also, notice that VCC variation continues to have the largest impact on overall 4.1 Power Down Data Table 1 shows typical DP83848 power down mode current consumption. Notice that even under power down conditions, center-tap current demand can represent up to ~6% of total current demand for the device. Also, notice that VCC supply variation has the largest impact on overall current consumption. 4.2 Disconnect Data With Auto-Negotiation Enabled Table 2 shows typical current consumption required for the DP83848 in disconnect mode with Autonegotiation enabled. Notice that center-tap current demand represents ~50% of the total current demand for the device in this mode. Also, notice that VCC variation continues to have the largest impact on overall current consumption. 4.3 10 Base-T Full Duplex Operating Mode Data Table 3 shows DP83848 10Mb full duplex operating mode current consumption. Notice that center-tap current demand represents up to 66% of the total current demand for the device. Notice that VCC variation continues to have a large impact on overall current consumption. Data payload content and packet size also have more impact on current consumption than temperature variation. 4.4 100 Base-TX Full Duplex Operating Mode Data Table 4 shows DP83848 100 Base-Tx full duplex operating mode current consumption requirements. Notice that this mode requires less current when compared to 10 Base-T mode, predominantly due to decreased center-tap current demand. The use of data pattern scrambling in 100 Base-Tx mode requires less center-tap current. Notice that data payload content and VCC variation have the largest impact on overall current consumption, while packet size and temperature variation have a lesser impact. 4.5 Transmit Loading Condition Data MII digital output pin loading is not typically a parameter that varies during normal operation. However, it can have a significant effect of power consumption. For example, when operating in a baseline 100Mb data mode, a device that has 50 Ω series digital output termination and is connected to a data source through a 0.3 meter shielded twisted pair MII cable requires 96.3 mA current. A typical condition where these same 50 Ω terminated digital pins are directly connected to a digital data device requires 89.29 mA. This represents a 7.9% increase in current demand when the MII cable is used. 5 Summary The data presented above clearly illustrates the effect operating conditions can have on device current demand. It has been shown that signal termination and transformer center-tap bias current demand is an integral part of the overall current demand for the physical layer interface, and represents a substantial percentage of the total current demand. It has also been demonstrated that 10 Base-T mode represents the most demanding operational mode from a current consumption standpoint. In all modes, VCC supply voltage has the most dramatic effect on power demand, followed by data payload content, data packet length, and temperature. It is also important to note that digital output pin loading can have a considerable effect on current consumption.