Reference Design for a Powered-Device (PD) Module Based on the MAX5941B PWM Controller
Abstract: This application note provides a reference design for an IEEE® 802.3af-compliant, 12.95W adjustable-output powered-device module. Assembled on a 12cm² PCB, the module is based on the MAX5941B PWM controller and includes hot-swap power switching, a DC-DC converter, and a pair of ORing diode bridges for compatibility with an external 12V adapter. This article details the performance of the module and provides a schematic, PCB layout, and components list for the design.
General Description
This application note presents an IEEE 802.3af-compliant, powered-device (PD) module for power-over-Ethernet (PoE) applications. Based on the MAX5941B PoE interface/PWM controller, this module provides the PD with a detection signature, a configurable classification signature (optional), programmable undervoltage lockout (UVLO), and an isolation switch with programmable inrush-current control.
The MAX5941B PD module is assembled on a 12cm² PCB and includes hot-swap power switching, a DC-DC converter, and a pair of ORing diode bridges for compatibility with an external 12V adapter. In short, it provides all the functions necessary to implement a DC-DC, fixed-frequency, isolated power supply for PDs, such as IP phones, wireless access nodes, and security cameras.
Typical Application
The MAX5941B PD module can be used in numerous applications. Figure 1 illustrates a typical application in which the data outputs from the switch are connected to the inputs of a midspan. The midspan then adds power to the data on each output that supports PoE.
In this example, port 1 is connected to an Ethernet camera and port 2 is connected to a wireless access point. When the midspan is switched on (or when the device is connected), the midspan checks each output for a PoE signature. The module identifies the peripherals on ports 1 and 2 as PoE-enabled devices, and the midspan supplies both data and power to these peripherals.
The midspan continuously monitors each output to see if a PoE-enabled device has been added or removed. Since the other ports in this example do not have a PoE signature, the midspan only passes data through to the connected peripherals.
Figure 1. In a typical application, the data outputs from the switch are connected to a midspan, which adds power to the data to provide power over Ethernet.
Features
IEEE 802.3af compliant
36V to 60V input voltage range
12V/1A output
No minimum load requirement
Small SIL package size
Low output ripple and noise
High-efficiency powered device
No external capacitor required
Adjustable output voltage
Low cost
1500V isolation (input to output)
On-board ORing diode used with an external 12V adapter
Pin Description
Figure 2. Pin assignment.
Table 1. Pin Description
Pin Number
Name
Description
1
VA1
Rx Input (1) This input pin is used in conjunction with VA2 and connects to the center tap of the transformer connected to pins 1 and 2 of the RJ45 connector (Rx)—it is not polarity sensitive.
2
VA2
Tx Input (2) This input pin is used in conjunction with VA1 and connects to the center tap of the transformer connected to pins 3 and 6 of the RJ45 connector (Tx)—it is not polarity sensitive.
3
VB1
Direct Input (1) This input pin is used in conjunction with VB2 and connects to pins 4 and 5 of the RJ45 connector—it is not polarity sensitive.
4
VB2
Direct Input (2) This input pin is used in conjunction with VB1 and connects to pins 7 and 8 of the RJ45 connector—it is not polarity sensitive.
5
CP1
Class Programming (1) Connecting an external resistor to CP2 will change the current class of the module. With no resistor fitted the module will default to Class 0.
6
CP2
Class Programming (2) Connecting an external resistor to CP1 will change the current class of the module. With no resistor fitted the module will default to Class 0.
7
GND
Ground The ground return for the output.
8
VOUT
DC Output This pin provides the regulated output voltage from the DC-DC converter.
9
ADJ
Output Adjust The output voltage can be adjusted from its nominal output by connecting an external resistor from this pin to either the VOUT pin or GND pin.
10
N.C.
No Connection This pin is not connected internally.
Power Classification
Power classification is an optional method for the PD to indicate its power requirements to the power-sourcing equipment (PSE). The MAX5941B module allows the current class to be externally programmed by connecting a resistor between the CP1 and CP2 pins, as shown in Figure 3. If no resistor is fitted, the module will default to Class 0. Table 2 provides a full list of programming resistor values.
Figure 3. To set the power classification, connect a resistor between pins CP1 and CP2.
Table 2. Resistor Values for Programming the Power Class
Class
Programming Resistance (Ω)
Minimum Power (W)
Maximum Power (W)
0
Do not fit
0.44
12.95
1
770
0.44
3.84
2
388
3.84
6.49
3
242
6.49
12.95
4
161
Reserved
Reserved
Output Adjustment
The MAX5941B PD module has an ADJ pin to trim the output voltage up or down from its nominal value. To adjust the output voltage, connect a resistor between the ADJ pin and either the GND pin or the VOUT pin (Figure 4). Equations 1 and 2 calculate the resistor values required to achieve the desired trimmed-up and trimmed-down output voltages.
where VTRIM_UP is the desired trimmed-up output voltage and VTRIM_DOWN is the desired trimmed-down output voltage.
Figure 4. To adjust the output voltage, connect a resistor between ADJ and GND (trim up) or ADJ and VOUT (trim down).
Figure 5A. The trimmed-up output voltage curve.
Figure 5. The trimmed-down output voltage curve.
Typical Connections with an External 12V Adapter
Conventionally, the PD is used simultaneously with an adapter, and a diode is connected in series at each output as shown in Figure 6.
Figure 6. In the conventional solution, the powered device is connected to an adapter, with a diode placed in series at each output.
For the MAX5941B PD module, the output diode D1 is assembled internally. If the PD is used independently, replace the diode with a 0Ω resistor to improve efficiency. Figure 7 shows the placement of the ORing diode D1 on the board.
Figure 7. The location of the internal diode D1 on the MAX5941B PD module.
The module only requires one external capacitor, as shown in Figure 8; minimally, a 1µF ceramic capacitor is recommended.
Figure 8. Typical connection diagram showing the external capacitor connected between GND and VOUT.
Electrical Characteristics
Table 3. Absolute Maximum Ratings
Parameter
Min
Typ
Max
Units
DC Supply Voltage
-0.3
60
V
DC Supply Voltage Surge for 1ms
-0.6
80
V
Storage Temperature
-40
+100
°C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the module. These are stress ratings only, and functional operation of the module at these or any other conditions beyond those indicated is not implied. Exposure to absolute maximum ratings conditions for extended periods may affect the module's reliability.
Table 4. Recommended Operating Conditions
Parameter
Min
Typ
Max
Units
Input Supply Voltage
36
48
60
V
Undervoltage Lockout
30
36
V
Operating Temperature
-20
25
+85
°C
Table 5. DC Electrical Characteristics
Parameter
Min
Typ
Max
Units
Nominal Output Voltage (Without the ORing Diode)
11.76
12
12.24
V
Output Current
0
1
A
Output Ripple and Noise
250
mV
Efficiency Without the ORing Diode (48V Input, 1A Output)
85
%
Efficiency with the ORing Diode (48V Input, 1A Output)
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