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Transient Suppressor
Layout
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
APPLICATION INFORMATION (continued)
Voltage transients on the TPS2375-1 can be caused by connecting or disconnecting the PD, or by other
environmental conditions like ESD. The TPS2375-1 is specified to operate with absolute maximum voltages
V
(VDD-VSS)
and V
(RTN-VSS)
of 100 V. A transient voltage suppressor, such as the SMAJ58A, should be installed
after the bridge and across the TPS2375-1 input as shown in Figure 1. Various configurations of output filters
and the insertion of local power sources across either the TPS2375-1 input or output have the potential to cause
stresses outside the absolute maximum ratings of the device. Designers should be aware of this possibility and
account for it in their circuit designs. For example, use adequate capacitance across the output to limit the
magnitude of voltage ringing caused by downstream filters. Plugging an external power source across the output
may cause ESD-like events. Some form of protection should be considered based on a study of the specific
waveforms seen in an application circuit.
The layout of the PoE front end must use good practices for power and EMI/ESD. A basic set of
recommendations include:
1. The parts placement must be driven by the power flow in a point-to-point manner such as RJ-45
→
Ethernet
transformer
→
diode bridges
→
TVS and 0.1-
μ
F capacitor
→
TPS2375-1
→
output capacitor.
2. There should not be any crossovers of signals from one part of the flow to another.
3. All leads should be as short as possible with wide power traces and paired signal and return.
4. Spacing consistent with safety standards like IEC60950 must be observed between the 48-V input voltage
rails and between the input and an isolated converter output.
5. The TPS2375-1 should be over a local ground plane or fill area referenced to VSS to aid high-speed
operation.
6. Large SMT component pads should be used on power dissipating devices such as the diodes and the
TPS2375-1.
Use of added copper on local power and ground to help the PCB spread and dissipate the heat is recommended.
Pin 4 of the TPS2375-1 has the lowest thermal resistance to the die.
17