2
dc1145bf
DEMO MANUAL DC1145B
OPERATING PRINCIPLES
After detection and classification, the PD is powered up
when the input voltage exceeds the LTC4267 turn-on under-
voltage lock out (UVLO) through a dual-level current-limited
power switch. The 5μF capacitance minimum requirement
after the in-rush circuit is met with C1 and C9. While the
voltage between P
OUT
and VPORTN is above the Power
Good trip point, the amperage through the power switch
is held below the low-level current limit. When the volt-
age between P
OUT
and VPORTN is below the Power Good
trip point, the Power Good signal goes active low and the
amperage through the power switch is held below the
high level current limit.
For the PD to remain powered on, it must present to the
PSE both AC and DC components of the Maintain Power
Signature (MPS). The PD must hold the DC MPS by
drawing at least 10mA or the PSE may disconnect power.
To meet the AC MPS, the PD must present 27kΩ of AC
impedance or less. The LTC4267 will remain on with a PSE
that uses the AC disconnect method since it will always
meet the AC MPS.
The LTC4267 flyback converter operates at a typical
switching frequency of 200kHz, controlled by the current
mode controller portion of the LT4267. Galvanic isolation
is achieved through transformer T1, capacitor C14 and
opto-isolators ISO1 and ISO2.
The primary side power path is comprised of C1, L1, C9,
½ of T1, Q2, and R6. These components should be as
close to each other as possible when laying out the printed
circuit board. In an actual implementation, a snubber
circuit can be added across Q2 to reduce voltage spikes
seen at the FET. The secondary side power path is made
up of the other ½ of T1, D1, C4, C5 and C6. These parts
should also be laid out as close to each other as possible,
without overlapping any of the circuitry or traces of the
primary side.
When a 12V auxiliary supply is connected at connect J2,
which takes over as the supply for the output. ISO2 provides
an isolated signal from the auxiliary supply to disable the
LTC4267. All LTC4267 signature detection, classification
and the internal power MOSFET switch are disabled. A PSE
that uses DC disconnect will remove power. The LTC4267
can resume normal PoE operation when the auxiliary
supply is removed. If power was removed, then the PSE
will perform a new detection and then power the device.
Downloaded from
Downloaded from
