TPS23750
TPS23770
www.ti.com
SLVS590A–JULY 2005–REVISED AUGUST 2005
Option 1 inserts power before the TPS23750. A 48-V adapter that meets the TPS23750 UVLO is needed. If the
adapter supply applies power to the PD before the PSE, it prevents the PSE from detecting the PD. This occurs
because the input bridges are reverse biased and the Ethernet power path looks open. If the PSE is already
powering the PD when the auxiliary source is plugged in, priority is given to the higher supply voltage.
Option 2 has the benefit that the adapter voltage may be lower than the PoE operating range. The TPS23750
was designed to operate from 24-V adapters. The bulk capacitor connected from VDD to RTN should be large
enough to control voltage transients that occur when plugging an adapter in. Usually at least several microfarads
minimum are required. Once the PD is powered from the adapter, the PSE does not successfully detect the PD.
This occurs because the internal MOSFET body diode establishes reverse bias voltage across the input bridges.
Once the PD is powered from the PSE, the auxiliary source only takes over if its voltage is higher than the PSE
output.
In some cases it may be desirable to make one power input dominant when using option 1 or 2. This can be
accomplished for some configurations by using switches in the appropriate power path to allow one source to
turn the other one off. These solutions require a number of additional components. In order for the PSE to
disconnect, the dc current must be dropped below 5 mA, and the ac impedance must be greater than 2 MΩ at
500 Hz.
Option 3 consists of ORing power to the output of the PoE dc/dc converter. This option is often used in cases
where PoE is added to an existing design that uses a low-voltage wall adapter. The relatively large PD output
capacitance reduces the potential for transients when the adapter is plugged in. The adapter output may be
grounded if the PD incorporates an isolated converter. The highest voltage source will dominate. Simple circuits
using P-channel MOSFETs can be designed that force the auxiliary power source off. The auxiliary source can
force the TPS23750 converter to stop switching by forcing the feedback node above its regulated value. This
results in the converter shutting down.
ESD
The TPS23750 has been tested to EN61000-4-2 using the circuit of Figure 40. The levels used were 8 kV
contact discharge and 15 kV air discharge. Surges were applied between the RJ-45 and the dc EVM outputs,
and between an auxiliary power input jack and the dc outputs. No failures were observed.
ESD requirements for a unit that incorporates the TPS23750 have a much broader scope and operational
implications than are used in TI’s testing. Unit-level requirements should not be confused with reference design
testing that only validates the ruggedness of the TPS23750.
COMPONENT SELECTION
Converter Section Bypass Capacitors
The converter section’s AUX and VBIAS pins should be bypassed with high-quality ceramic capacitors. The VBIAS
supply provides a quiet source of power for internal and external circuits. The VBIAS regulator is stable for output
capacitances of 0.08 µF to 1.5 µF. A 1 µF capacitor is recommended, and this should be located as close to the
TPS23750 as practical. The AUX supply is the source of the GATE drive current pulses. It requires a minimum
0.8 µF of ceramic bypass capacitor. The AUX capacitor must be located as close as possible to the TPS23750.
The AUX regulator can be loaded with much larger capacitance.
PoE Data Transformer
Ethernet interfaces running on twisted pair commonly use an isolation transformer (see Figure 1) per
long-standing IEEE 802.3 requirements. The transformer must include a center tap on the media (cable) side and
be rated to handle the dc current and imbalance of IEEE 802.3af.
Input Diodes or Diode Bridges
IEEE 802.3af requires the PD to accept power on either set of input pairs with either polarity. This requirement is
satisfied by using two full-wave input bridge rectifiers as shown in Figure 1. Silicon p-n diodes with a 1 A or 1.5 A
rating and a minimum breakdown of 100 V are recommended. Diodes exhibit large dynamic resistance under
low-current operating conditions such as in detection. The diodes should be tested for their behavior under this
condition. The diode forward drops must be less than 1.5 V at 500 µA at the lowest operating temperature.
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