ltc4267-3 Linear Technology Corporation, ltc4267-3 Datasheet - Page 24

• Manufacturer Part Number: ltc4267-3
• Description: Power Over Ethernet Ieee 802.3af Pd Interface With Integrated 300khz Switching Regulator
• Manufacturer: Linear Technology Corporation

LTC4267-3
APPLICATIO S I FOR ATIO
External Preregulator
The circuit in Figure 13 shows a third way to power the
LTC4267-3 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
24
VCC
START
at 7.6V nominal, well above the maximum rated P
FROM
. After some time, the output voltage approaches
PSE
– 48
Figure 13. Powering the LTC4267-3 Switching
Regulator with an External Preregulator
Figure 12. Powering the LTC4267-3 Switching
Regulator via the Shunt Regulator
FROM
+
–
PSE
– 48
VCC
V
+
–
PORTN
node up to the P
LTC4267-3
U
V
PORTP
PGND
P
P
VCC
OUT
V
PORTN
U
LTC4267-3
V
8.2V
PORTP
PGND
P
P
PGND
D1
B
OUT
VCC
R
. The preregulator holds
B
VCC
W
PGND
START
turn-on threshold,
Q1
R
PGND
PGND
START
C
C
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PVCC
PVCC
momentarily
U
R
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START
PVCC
VCC
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 LTC4267-3
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
actual current needed to power the LTC4267-3 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.
Figure 14. Main Loop Compensation for an Isolated Design
ISOLATOR
TO OPTO-
B
can be selected so that it provides a small
PVCC
R
Z
should be sized suffi ciently to
C
B
C
should be suffi ciently large
C
and D1.
VCC
Z
VCC
C
sources current on
C
V
. The PD designer
OUT
pin, and provides
R2
R1
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Z
with a
42673f