LTC4267 LINER [Linear Technology], LTC4267 Datasheet - Page 23

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

APPLICATIO S I FOR ATIO
External Preregulator
The circuit in Figure 13 shows a third way to power the
LTC4267 switching regulator circuit. An external series
preregulator consists of a series pass transistor Q1, zener
diode D1, and a bias resistor R
P
turn-off threshold of 6.8V. Resistor R
charges the P
enabling the switching regulator. The voltage on C
begins to decline as the switching regulator draws its
normal supply current, which exceeds the delivery of
R
the desired value. By this time, the pass transistor Q1
catches the declining voltage on the P
virtually all the supply current required by the LTC4267
switching regulator. C
handle the switching current needed to drive NGATE while
maintaining minimum switching voltage.
The external preregulator has improved effi ciency over
the simple resistor-shunt regulator method mentioned
previously. R
current necessary to maintain the zener diode voltage and
the maximum possible base current Q1 will encounter. The
The shunt regulator can sink up to 5mA through the P
pin to PGND. The values of R
selected for the application to withstand the worst-case
load conditions and drop on P
turn-off threshold is not reached. C
suffi ciently to handle the switching current needed to drive
NGATE while maintaining minimum switching voltage.
VCC
START
at 7.6V nominal, well above the maximum rated P
. After some time, the output voltage approaches
Figure 12. Powering the LTC4267 Switching
Regulator via the Shunt Regulator
FROM
B
–48
PSE
VCC
can be selected so that it provides a small
node up to the P
+
–
U
PVCC
U
V
PORTN
should be sized suffi ciently to
LTC4267
VCC
V
START
B
PORTP
PGND
P
P
. The preregulator holds
VCC
OUT
, ensuring that the P
VCC
W
PVCC
VCC
and C
START
turn-on threshold,
pin, and provides
should be sized
R
PGND
PVCC
START
C
momentarily
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PVCC
U
must be
PVCC
VCC
VCC
VCC
actual current needed to power the LTC4267 switching
regulator goes through Q1 and P
an “as-needed” basis. The static current is then limited
only to the current through R
Compensating the Main Loop
In an isolated topology, the compensation point is typically
chosen by the components confi gured around the external
error amplifi er. Shown in Figure 14, a series RC network
is connected from the compare voltage of the error am-
plifi er to the error amplifi er output. In PD designs where
transient load response is not critical, replace R
short. The product of R2 and C
to ensure stability. When fast settling transient response
is critical, introduce a zero set by R
must ensure that the faster settling response of the output
voltage does not compromise loop stability.
In a nonisolated design, the LTC4267 incorporates an
internal error amplifi er where the I
a compensation point. In a similar manner, a series RC
network can be connected from I
shown in Figure 15. C
load and line transient response.
Figure 14. Main Loop Compensation for an Isolated Design
FROM
–48
PSE
ISOLATOR
Figure 13. Powering the LTC4267 Switching
Regulator with an External Preregulator
TO OPTO-
+
–
V
PORTN
LTC4267
V
PORTP
PGND
P
P
C
OUT
VCC
and R
R
Z
B
C
8.2V
Z
PGND
should be suffi ciently large
C
D1
and D1.
C
R
are chosen for optimum
B
VCC
TH
PGND
Z
TH
C
/RUN pin serves as
sources current on
C
/RUN to PGND as
. The PD designer
V
LTC4267
Q1
OUT
PGND
C
R2
R1
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PVCC
R
START
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Z
23
with a
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