LTC4267-3
15
42673fb
For more information www.linear.com/LTC4267-3
applicaTions inForMaTion
temperature exceeds the overtemperature trip point, the
current is reduced to zero and very little power is dissi-
pated in the part until it cools below the overtemperature
set point. Once the LTC4267-3 has charged up the load
capacitor and the PD is powered and running, there will
be minor residual heating due to the DC load current of
the PD flowing through the internal MOSFET.
During classification, excessive heating of the LTC4267-3
can occur if the PSE violates the 75ms probing time limit.
To protect the LTC4267-3, thermal overload circuitry will
disable classification current if the die temperature exceeds
the overtemperature trip point. When the die cools down
below the trip point, classification current is re-enabled.
The PD is designed to operate at a high ambient tem-
perature and with the maximum allowable supply (57V).
However,
there is a limit to the size of the load capacitor
that can be charged up before the LTC4267-3 reaches the
overtemperature trip point. Hitting the overtemperature trip
point intermittently does not harm the LTC4267-3, but it
will delay the completion of capacitor charging. Capacitors
up to 200µF can be charged without a problem over the
full operating temperature range.
Switching Regulator Main Control Loop
Due to space limitations, the basics of current mode
DC/DC conversion will not be discussed here. The reader
is referred to the detail treatment in Application Note 19
or in texts such as Abraham Pressman’s Switching Power
Supply Design.
In a Power over Ethernet System, the majority of applica-
tions involve an isolated power supply design. This means
that the output power supply does not have any DC elec-
trical path to the PD interface or the switching regulator
primary. The DC isolation is achieved typically through a
transformer in the forward path and an opto-isolator in
the feedback path or a third winding in the transformer.
The typical application circuit shown on the front page
of the data sheet represents an isolated design using an
opto-isolator. In applications where a nonisolated topol-
ogy is desired, the LTC4267-3 features a feedback port
and an internal error amplifier that can be enabled for this
specific application.
In the typical application circuit (Figure 11), the isolated
topology employs an external resistive voltage divider to
present a fraction of the output voltage to an external er
-
ror amplifier. The error amplifier responds by pulling an
analog current through the input LED on an opto-isolator.
The collector of the opto-isolator output presents a cor
-
responding current into the I
TH
/RUN pin via a series diode.
This method generates a feedback voltage on the I
TH
/RUN
pin while maintaining isolation.
The voltage on the I
TH
/RUN pin controls the pulse-width
modulator formed by the oscillator, current comparator,
and RS latch. Specifically, the voltage at the I
TH
/RUN pin
sets the current comparator’s trip threshold. The current
comparator monitors the voltage across a sense resistor
in series with the source terminal of the external N-Channel
MOSFET. The LTC4267-3 turns on the external power
MOSFET when the internal free-running 300kHz oscillator
sets the RS latch. It turns off the MOSFET when the cur
-
rent comparator resets the latch or when 80%
duty cycle
is reached, whichever happens first. In this way, the peak
current levels through the flyback transformer’s primary
and secondary are controlled by the I
TH
/RUN voltage.
In applications where a nonisolated topology is desir-
able (
Figure 11), an external resistive voltage divider can
present a fraction of the output voltage directly to the
V
FB
pin of the LTC4267-3. The divider must be designed
so when the output is at its desired voltage, the V
FB
pin
voltage will equal the 800mV onboard internal reference.
The internal error amplifier responds by driving the I
TH
/
RUN pin. The LTC4267-3 switching regulator performs in
a similar manner as described previously.
Regulator Start-Up/Shutdown
The LTC4267-3 switching regulator has two shutdown
mechanisms to enable and disable operation: an un
-
dervoltage lockout on the P
VCC
supply pin and a forced
shutdown whenever external circuitry drives the I
TH
/RUN
pin low. The LTC4267-3 switcher transitions into and out
of shutdown according to the state diagram (Figure 8).
It is important not to confuse the undervoltage lockout
of the PD interface at V
PORTN
with that of the switching
regulator at P
VCC
. They are independent functions.
