Datasheet
LTC4267-3
20
42673fa
The load capacitor can store significant energy when fully
charged. The design of a PD must ensure that this energy
is not inadvertently dissipated in the LTC4267-3. The
polarity-protection diode(s) prevent an accidental short
on the cable from causing damage. However, if the V
PORTN
pin is shorted to V
PORTP
inside the PD while the capacitor
is charged, current will flow through the parasitic body
diode of the internal MOSFET and may cause permanent
damage to the LTC4267-3.
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 26.25k in parallel with 0.05µF. If either the DC current
is less than 10mA or the AC impedance is above 26.25k,
the PSE may disconnect power. The DC current must be
less than 5mA and the AC impedance must be above 2M
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 LTC4267-3 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 LTC4267-3
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 LTC4267-3 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 Optoisolator 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 optoisolator 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 optoisolator can be selected for
an increase in bandwidth at the cost of a reduction in gain
of this stage.
APPLICATIONS INFORMATION