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Keywords:
Power over Ethernet, PoE, Current Sense Amplifier, Current sensing, Sense resistor, Ethernet cables, Power sourcing Equipment,
Powered Device
APPLICATION NOTE 6220
40W SINGLE PORT MAX5971A CURRENT SENSING
Abstract: Many PoE power sourcing equipment (PSE) single port applications require real time knowledge of the powered device (PD) current used
after startup. This application note details a method to obtain real time current monitoring and reporting of the actual current sent to the PD. It uses a
MAX4080 high-side, current-sense amplifier and a current sense resistor on the PSE high-side power path.
Overview
Many Power-over-Ethernet (PoE) power sourcing equipment (PSE) single port applications require real-time knowledge of the powered device (PD)
current used after startup. The MAX5971A PSE controller with an Integrated
MOSFET is capable of delivering up to 40W in to a single port using
endpoint or midspan modes. The MAX5971A features current foldback during startup and normal operation, an internal sense resistor for monitoring
continuous overcurrent and short
-circuit conditions. The MAX5971A powers down the port for continuous overstress. However, no mechanism is
available for real-time monitoring and reporting of the actual current sent to the PD during normal operation.
This application note details a method to obtain real-time current monitoring and reporting of the actual current sent to the PD. It uses a
MAX4080
high-side, current-sense amplifier, and a current sense resistor on the PSE high-side power path.
Current Sensing Methodology
Typical of a PSE, the MAX5971A internal MOSFET connects and disconnects the low-side negative 48V rail from the input 48V source negative
low-side rail (V typically). The MAX4080 high-side, unidirectional current-sense amplifier enables current monitoring without interfering with the low
side (V for Telecom) return currents or adding additional resistance to the low-side path. For a PoE application, this requires placing a current
sense resistor high-side, ideally after the positive input 48V source rail and prior to the PSE Ethernet magnetics. Typically, circuitry associated with
a
high-side
resistor must cope with large common-mode signals; however, the MAX4080 76V input voltage range applies independently to both
supply voltage (V ) and common-mode input voltage (V ). Additionally, the high-side current monitoring does not interfere with the low-side
return current path of the load being measured or the MAX5971A IC's internal current monitoring. Refer to "application note 746"
for additional
information on
high-side current sensing applications.
System and Ethernet Cable Delivery Resistance
The current sense resistor is typically chosen so that maximum load current develops full-scale voltage across the resistor. However, from a system
perspective we also need to consider other resistance in the power path such as cabling and the PSE controller. Cabling wise, minimizing the
EtherNet cabling system resistance is critical to delivering all available PSE power to the PD. Cat 5e EtherNet cabling has approximately 12.5Ω
resistance per 100m (328ft). This resistance is for a single EtherNet cable with 8 wires, grouped into 4 pairs.
See
Figure 1 below detailing an Ethernet
wire and a PoE power pair configuration using 2 pairs of 4 Ethernet wires.
Figure 1.
Therefore the total PoE power pair loop resistance is 12.5Ω (6.25Ω + 6.25Ω) when considering the PSE supply and return lines to the PD. See
Figure 1.
The MAX5971A's internal
MOSFET switch has a typical and max R of 0.6Ω and 1.3Ω respectively at 85°C. Thus, from a power-loss
perspective the current-sense resistor should be significantly less than these values and not add much to the overall cabling and system resistance.
From
a current-sensing
resistor perspective, higher resistance values allows lower currents to be accurately measured. Additionally, the sense
resistor I R loss at high current can become significant, increase the resistor temperature and introduce measurement drift.
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