Datasheet

LTC4267-1
20
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
Maintain Power Signature
In an IEEE 802.3af system, the PSE uses the maintain
power signature (MPS) to determine if a PD continues to
require power. The MPS requires the PD to periodically
draw at least 10mA and also have an AC impedance less
than 26.25kΩ in parallel with 0.05µF. If either the DC
current is less than 10mA or the AC impedance is above
26.25kΩ, the PSE may disconnect power. The DC current
must be less than 5mA and the AC impedance must be
above 2MΩ to guarantee power will be removed.
Selecting Feedback Resistor Values
The regulated output voltage of the switching regulator is
determined by the resistor divider across V
OUT
(R1 and
R2 in Figure 11) and the error amplifier reference voltage
V
REF
. The ratio of R2 to R1 needed to produce the desired
voltage can be calculated as:
R2 = R1 • (V
OUT
– V
REF
)/V
REF
In an isolated power supply application, V
REF
is determined
by the designer’s choice of an external error amplifier.
Commercially available error amplifiers or programmable
shunt regulators may include an internal reference of
1.25V or 2.5V. Since the LTC4267-1 internal reference
and error amplifier are not used
in an isolated design, tie
the V
FB
pin to PGND.
In a nonisolated power supply application, the LTC4267-1
onboard internal reference and error amplifier can be
used. The resistor divider output can be tied directly to
the V
FB
pin. The internal reference of the LTC4267-1 is
0.8V nominal.
Choose resistance values for R1 and R2 to be as large as
possible to minimize any efficiency loss due to the static
current drawn from V
OUT
, but just small enough so that
when V
OUT
is in regulation, the error caused by the nonzero
input current from the output of the resistor divider to the
error amplifier pin is less than 1%.
Error Amplifier and Opto-Isolator Considerations
In an isolated topology, the selection of the external error
amplifier depends on the output voltage of the switching
regulator. Typical error amplifiers include a voltage refer-
ence of either 1.25V or 2.5V. The output of the amplifier
and the amplifier upper supply rail are often tied together
internally. The supply rail is usually specified 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 amplifier supply voltage is not prop-
erly
managed. When the switcher load current decreases
and the output voltage rises, the error amplifier responds
by pulling more current through the LED. The LED voltage
can be as large as 1.5V, and along with R
LIM
, reduces the
supply voltage to the error amplifier. If the error amp does
not have enough headroom, the voltage drop across the
LED and R
LIM
may shut the amplifier off momentarily,
causing a lock-up condition in the main loop. The switcher
will undershoot and not recover until the error amplifier
releases its sink current. Care must be taken to select the
reference voltage and R
LIM
value so that the error amplifier
always has enough headroom. An alternate solution that
avoids these problems is to utilize the LT1431 or LT4430
where the output of the error amplifier and amplifier supply
rail are brought out to separate pins.
The PD designer must also select an opto-isolator such
that its bandwidth is sufficiently wider than the bandwidth
of the main control loop. If this step is overlooked, the
main control loop may be difficult to stabilize. The output
collector resistor of the opto-isolator
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
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