TPS23750
TPS23770
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SLVS590A–JULY 2005–REVISED AUGUST 2005
APPLICATION INFORMATION (continued)
PoE THRESHOLDS
The TPS23750 has a number of internal comparators with hysteresis for stable switching between the various
states as shown in Figure 29. Figure 30 relates the parameters in the Electrical Characteristics section to the
PoE states. The mode labeled idle between classification and operation implies that the DET, CLASS, and RTN
pins are all high impedance.
PD Powered
Idle
Classification
Detection
V
DD
V
V
CL_H
CU_H
V
V
UVLO_R
UVLO_F
1.4V
V
CU_OFF
V
CL_ON
Figure 30. Threshold Voltages
DETECTION
This feature of IEEE 802.3af reduces the risk of damaging Ethernet devices not intended for application of 48 V.
When a voltage in the range of 2.7 V to 10.1 V is applied to the PI, an incremental resistance of 25 kΩ signals
the PSE that the PD is both capable of, and ready to, accept power. The incremental resistance is measured by
applying at least two different voltages to the PI and measuring the current it draws. These two test voltages
must be within the specified range and be at least 1 V apart. The incremental resistance equals the difference
between the voltages divided by the difference between the currents. The allowed range of resistance is 23.75
kΩ to 26.25 kΩ.
The TPS23750 is in detection mode whenever the supply voltage is below the lower classification threshold. The
TPS23750 draws a minimum of bias power in this condition, while RTN is high impedance and almost all the
internal circuits are disabled. The DET pin is pulled to ground during detection, so a 24.9 kΩ, 1% resistor from
VDD to DET presents the correct signature. RDET may be a small, low-power resistor since it only sees a stress of
about 5 mW. When the input voltage rises above the 11.3 V upper detection comparator threshold, the DET pin
goes to an open-drain condition to conserve power.
The input diode bridge’s incremental resistance may be hundreds of Ohms at the very low currents seen at 2.7 V
on the PI. The bridge’s resistance is in series with RDET and increases the total resistance seen by the PSE. The
nonlinearity in the detection signature of Figure 29 is caused by the diode bridge. This varies with the type of
diode selected by the designer, and it is not usually specified on the diode data sheet. The value of RDET may be
adjusted downwards to accommodate a particular diode type.
CLASSIFICATION
Once the PSE has detected a PD, it may optionally classify the PD. Classification allows a PSE to determine a
PD’s power requirements rather than assuming every PD requires 15.4 W, which allows the PSE to power the
maximum number of PDs from its 48-V power supply. This step is optional because some PSEs can afford to
allot the full power to every powered port.
The classification process applies a voltage between 14.5 V and 20.5 V to the input of the PD, which in turn
draws a fixed current set by RCLASS. The PSE measures the PD current to determine which of the five available
classes (see Table 2) that the PD falls into. The total current drawn from the PSE during classification is the sum
of bias currents and current through RCLASS. The TPS23750 disconnects RCLASS at voltages above the
classification range to avoid excessive power dissipation (see Figure 29 and Figure 30).
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