ltc4267cgn-1 Linear Technology Corporation, ltc4267cgn-1 Datasheet - Page 24

• Manufacturer Part Number: ltc4267cgn-1
• Description: Power Over Ethernet Ieee 802.3af Pd Interface With Integrated Switching Regulator
• Manufacturer: Linear Technology Corporation

LTC4267-1
APPLICATIO S I FOR ATIO
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
24
VCC
START
at 7.6V nominal, well above the maximum rated P
. After some time, the output voltage approaches
Figure 13. Powering the LTC4267-1 Switching
Regulator with an External Preregulator
Figure 12. Powering the LTC4267-1 Switching
Regulator via the Shunt Regulator
VCC
node up to the P
U
U
B
. The preregulator holds
VCC
W
VCC
START
turn-on threshold,
pin, and provides
momentarily
U
PVCC
VCC
virtually all the supply current required by the LTC4267-1
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 LTC4267-1 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
B
can be selected so that it provides a small
PVCC
should be sized suffi ciently to
C
B
should be suffi ciently large
and D1.
VCC
Z
C
sources current on
C
. The PD designer
Z
with a
42671f