LTC4267-1
24
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
The external preregulator has improved efficiency over
the simple resistor-shunt regulator method mentioned
previously. R
B
can be selected so that it provides a small
current necessary to maintain the zener diode voltage and
the maximum possible base current Q1 will encounter. The
actual current needed to power the LTC4267-1 switching
regulator goes through Q1 and P
VCC
sources current on
an “as-needed” basis. The static current is then limited
only to the current through R
B
and D1.
Compensating the Main Loop
In an isolated topology, the compensation point is typically
chosen by the components configured around the external
error amplifier. Shown in Figure 14, a series RC network
is connected from the compare voltage of the error am-
plifier to the error amplifier output. In PD designs where
transient load response is not critical, replace R
Z
with a
short. The product of R2 and C
C
should be sufficiently large
to ensure stability. When fast settling transient response
is critical, introduce a zero set by R
Z
C
C
. The PD designer
must ensure that the faster settling response of the output
voltage does not compromise loop stability.
In a nonisolated design, the LTC4267-1 incorporates an
internal
error amplifier where the I
TH
/RUN pin serves as
a compensation point. In a similar manner, a series RC
network can be connected from I
TH
/RUN to PGND as
shown in Figure 15. C
C
and R
Z
are chosen for optimum
load and line transient response.
Selecting the Switching Transistor
With the N-channel power MOSFET driving the primary of
the transformer, the inductance will cause the drain of the
MOSFET to traverse twice the voltage across V
PORTP
and
PGND. The LTC4267-1 operates with a maximum supply
of – 57V; thus the MOSFET must be rated to handle 114V
or more with sufficient design margin. Typical transis-
tors have 150V ratings while some manufacturers have
developed 120V rated MOSFETs specifically for Power-
over-Ethernet applications.
The NGATE pin of the LTC4267-1 drives the gate of the
N-channel MOSFET. NGATE will traverse a rail-to-rail volt-
age from PGND to P
VCC
. The designer must ensure the
MOSFET provides a low “ON” resistance when switched
to P
VCC
as well as ensure the gate of the MOSFET can
handle the P
VCC
supply voltage.
For high efficiency applications, select an N-channel
MOSFET with low total gate charge. The
lower total gate
charge
improves the efficiency of the NGATE drive circuit
and minimizes the switching current needed to charge
and discharge the gate.
Auxiliary Power Source
In some applications, it may be desirable to power the
PD from an auxiliary power source such as a wall trans-
former. The auxiliary power can be injected into the PD
at several locations and various trade-offs exist. Power
can be injected at the 3.3V or 5V output of the isolated
power supply with the use of a diode ORing circuit. This
method accesses the internal circuits of the PD after the
isolation barrier and therefore meets the 802.3af isola-
tion safety requirements for the wall transformer jack on
the PD. Power can also be injected into the PD interface
portion of the LTC4267-1. In this case, it is necessary to
ensure the user cannot access the terminals of the wall
Figure 14. Main Loop Compensation for an Isolated Design
R1
R2
C
C
R
Z
TO OPTO-
ISOLATOR
42671 F14
V
OUT
LTC4267-1
C
C
R
Z
I
TH
/RUN
PGND
42671 F15
Figure 15. Main Loop Compensation for a Nonisolated Design
