LTC4267 LINER [Linear Technology], LTC4267 Datasheet - Page 20

• Manufacturer Part Number: LTC4267
• Description: Power over Ethernet IEEE 802.3af PD Interface with Integrated Switching Regulator
• Manufacturer: LINER [Linear Technology]

LTC4267
APPLICATIO S I FOR ATIO
20
Choose resistance values for R1 and R2 to be as large as
possible to minimize any effi ciency loss due to the static
current drawn from V
when V
input current from the output of the resistor divider to the
error amplifi er pin is less that 1%.
Error Amplifi er and Optoisolator Considerations
In an isolated topology, the selection of the external error
amplifi er depends on the output voltage of the switching
regulator. Typical error amplifi ers include a voltage refer-
ence of either 1.25V or 2.5V. The output of the amplifi er
and the amplifi er upper supply rail are often tied together
internally. The supply rail is usually specifi ed with a wide
upper voltage range, but it is not allowed to fall below the
reference voltage. This can be a problem in an isolated
switcher design if the amplifi er supply voltage is not prop-
erly managed. When the switcher load current decreases
and the output voltage rises, the error amplifi er responds
by pulling more current through the LED. The LED voltage
can be as large as 1.5V, and along with R
supply voltage to the error amplifi er. If the error amp does
not have enough headroom, the voltage drop across the
LED and R
causing a lock-up condition in the main loop. The switcher
will undershoot and not recover until the error amplifi er
releases its sink current. Care must be taken to select the
reference voltage and R
always has enough headroom. An alternate solution that
avoids these problems is to utilize the LT1431 or LTC4430
where the output of the error amplifi er and amplifi er supply
rail are brought out to separate pins.
The PD designer must also select an optoisolator such
that its bandwidth is suffi ciently wider than the bandwidth
of the main control loop. If this step is overlooked, the
main control loop may be diffi cult to stabilize. The output
collector resistor of the optoisolator can be selected for
an increase in bandwidth at the cost of a reduction in gain
of this stage.
Output Transformer Design Considerations
Since the external feedback resistor divider sets the
output voltage, the PD designer has relative freedom in
OUT
is in regulation, the error caused by the nonzero
LIM
may shut the amplifi er off momentarily,
U
OUT
LIM
, but just small enough so that
U
value so that the error amplifi er
W
LIM
, reduces the
U
peak swing current goes from a fraction of an ampere to
several amperes. Care must be taken to ensure proper
circuit operation, especially for small current sense resis-
tor values.
Choose R
the entire range of the I
range is 0.7V to 1.9V and R
experiment. The main loop can be temporarily stabilized
by connecting a large capacitor on the power supply. Apply
the maximum load current allowable at the power sup-
ply output based on the class of the PD. Choose R
such that I
output load current over the entire operating range and
ensure that I
1.9V range. Layout is critical around the R
For example, a 0.020Ω sense resistor, with one milliohm
(0.001Ω) of parasitic resistance will cause a 5% reduction
in peak switch current. The resistance of printed circuit
copper traces cannot necessarily be ignored and good
layout techniques are mandatory.
selecting the transformer turns ratio. The PD designer
can use simple ratios of small integers (i.e. 1:1, 2:1, 3:2)
which yields more freedom in setting the total turns and
mutual inductance and may allow the use of an off the
shelf transformer.
Transformer leakage inductance on either the primary or
secondary causes a voltage spike to occur after the output
switch (Q1 in Figure 11) turns off. The input supply volt-
age plus the secondary-to-primary referred voltage of the
fl yback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating. This spike
is increasingly prominent at higher load currents, where
more stored energy must be dissipated. In some cases,
a “snubber” circuit will be required to avoid overvoltage
breakdown at the MOSFET’s drain node. Application
Note 19 is a good reference for snubber design.
Current Sense Resistor Consideration
The external current sense resistor (R
allows the designer to optimize the current limit behavior
for a particular application. As the current sense resistor
is varied from several ohms down to tens of milliohms,
SENSE
TH
/RUN approaches 1.9V. Finally, exercise the
TH
/RUN voltage remains within the 0.7V to
such that the switching current exercises
TH
/RUN voltage. The nominal voltage
SENSE
can be determined by
SENSE
SENSE
in Figure 11)
resistor.
SENSE
4267f