MP8005 – IEEE 802.3af PoE POWERED DEVICE CONTROLLER
MP8005 Rev. 1.0 www.MonolithicPower.com 9
9/26/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
OPERATION
The MP8005 operates in the manner described
here and in the IEEE 802.3af Powered Device
(PD) Specification. This device (along with the
power sourcing element (PSE)) operates as a
safety device to supply potentially lethal voltages
only when the power sourcing element
recognizes a unique, tightly specified resistance
at the end of an unknown length of Ethernet
cable.
A 25k resistance is presented as a load to the
PSE in Detection Mode, when the PSE applies
two “safe” voltages of less than 10.1V each while
measuring the change in current drawn in order
to determine the load resistance. If the PSE
“sees” the correct load, then it may either further
increase the applied voltage to enter the
“classification” range of operation or switch on
the nominal 48V power to the load.
The classification mode can further specify to the
PSE the expected load range of the device under
power so that the PSE can intelligently distribute
power to as many loads as possible (within its
maximum current capabilities). If a classification
resistance is not present, the PD load is
assumed to be the maximum of approximately 13
Watts. The classification mode is active between
14.5V and 20.5V.
The main power switch will pass a limited current
above 31V, charging the external DC-to-DC
converter’s input capacitor in a controlled manner.
The charging will continue until the controlled
current drops below either an externally
programmed limiting level or 450mA, depending
upon the Rlim current setting resistor. The main
power switch is internally thermally protected to
100V by reducing the output current using a
foldback technique. The required power
dissipation of the IC drops from the allowed peak
value of 26W (450mA x 57V) to 0.16W ((450mA)
2
x R
ON
) during the normal operation at turn-on.
The minimum allowed capacitance of 5µF will
charge in 500µs. A larger capacitor will take a
proportionally longer time to charge due to the
constant current charging method. A capacitor
that is too large will overheat the part and force it
into thermal shutdown. The IC will reattempt
charging for a number of cycles but ultimately will
be shut down until the input voltage from the PSE
is recycled. This is the way the IC protects itself
under overload and/or shorted conditions.
Once the capacitor tied between Vdd, Ethernet
input positive supply and the RTN pin is charged
to within a volt of the applied Ethernet input
supply potential, the PG output is driven low to
enable the switching regulator controller. This
delay prior to turning on the switching regulator is
required in order to minimize the power
dissipation incurred during the startup of the
controller. The capacitance tied across the Vdd
and RTN lines provide the inductor ripple current
while maintaining a small voltage differential such
that the average current flowing in the RTN line is
under the current limit level. Without a storage
capacitance at this point, instantaneous current
peaks, required by the switching regulator during
normal operation, would send the main PD
switch (at the RTN pin) in and out of current limit
resulting in high power dissipation due to the
resultant voltage across the “switch”. A current
greater than the current limit value would
increase the voltage on the RTN pin to the
Ehternet Input supply voltage immediately
resulting in power dissipation levels that would
immediately shut down the PD switch without the
required storage capacitor.
The soft-start capacitor, CSS, slows the rate of
current delivery to the switching regulator to a
value that can be delivered by the storage
capacitor/PD switch.
A resistor tied between the Rdelay pin and
AGND sets a time delay between the falling edge
of the synchronous gate and the rising edge of
the main gate. This delay can be set to
compensate for system delays caused by
architectural structures of the circuit being used.
An isolated secondary type power supply having
a transformer in the feedback loop may require
extra time due to the electrical path incurred. The
delay time is adjustable from as little as 40nSec
to over 200nSec through the choice of resistance
value. A smaller resistance will result in a shorter
delay. Nominal values range from 0 - 200 k for
time delays from 40nSec to 200nSec. The delay
