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PoE STARTUP EXAMPLE
3.3V
Output
PoEInput
Voltage
PoEInput
Current
D etection
C l ass
I nrush
C o nverter
Starts
THERMAL PROTECTION
CONVERTER CONTROLLER OVERVIEW
TPS23750
TPS23770
SLVS590B – JULY 2005 – REVISED FEBRUARY 2008
Figure 31 demonstrates detection, classification, and startup. The PSE controls the voltage on the PI, while the
PD controls the current. The waveforms presented are the PI voltage, PI current, and dc/dc converter output
voltage. Testing with different PSEs may result in waveforms that are not exactly the same because the IEEE
802.3af standard allows for different implementations.
The first event is detection. Two voltage levels of about 4 V and 8 V are seen, but the detection current levels are
not seen because of the current scale. The second event is classification. The PD draws about 28 mA while the
PI voltage is about 17 V, indicating it is a class 3 device. The third event is startup. The PI voltage ramps to
about 46 V and the PD draws an inrush current between 120 mA and 140 mA as the downstream bulk capacitor
is charged. The PI current drops once the bulk capacitor is charged, allowing the inrush state to terminate and
the converter to enable. The final event is converter startup into a fixed 1- Ω load. The converter output voltage
ramps to 3.3 V with a corresponding PI current draw. The PI current increases to a steady-state value of 260 mA
with only a small overshoot as the output capacitor is charged. The PD is powered, and the applications circuits
are operational at the end of startup.
Figure 31. Typical Startup Waveforms
The TPS23750 enters a low-power mode if the die temperature exceeds 140 ° C. The pass MOSFET, dc/dc
converter, AUX regulator, and CLASS regulator are turned off when this occurs. Sources of internal dissipation
include bias currents, the pass MOSFET, and the AUX, V
BIAS
, and CLASS regulators. Loading on AUX and V
BIAS
is a dominant contributor when the AUX rail is not externally biased. The TPS23750 automatically restarts when
the die temperature has fallen approximately 17 ° C with the pass MOSFET set in the inrush state, the converter
disabled, and the TMR capacitor discharged.
The TPS23750 is built using a PowerPAD package to provide a low thermal resistance from the junction to the
circuit board. The PowerPAD should be soldered to a large copper area on the circuit board to provide good
thermal performance.
Other sources of local PCB heating should be considered during the thermal design. Typical calculations assume
that the TPS23750 is the only heat source contributing to the PCB temperature rise.
The TPS23750 dc/dc controller implements a typical current-mode control topology reminiscent of the UC3844,
but with a number of enhancements.
A class AB inverting error amplifier, with a 1.5 V fixed reference, connects between input FB and output COMP.
The error amplifier has a 1.5 MHz gain-bandwidth product and can source or sink several milliamps. This
amplifier can be disabled to allow an optocoupler feedback circuit to drive the PWM section
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