參數(shù)資料
型號(hào)： LTC4257IDD-1
廠商： LINEAR TECHNOLOGY CORP
元件分類： 電源管理
英文描述： Power over Ethernet Interface Controller with Dual Current Limit
中文描述： 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO8
封裝： 3 X 3 MM, PLASTIC, M0-229WEED-1, DFN-8
文件頁數(shù)： 23/32頁
文件大小： 1975K
代理商： LTC4257IDD-1
LTC4267
23
4267f
The shunt regulator can sink up to 5mA through the P
VCC
pin to PGND. The values of R
START
and C
PVCC
must be
selected for the application to withstand the worst-case
load conditions and drop on P
VCC
, ensuring that the P
VCC
turn-off threshold is not reached. C
PVCC
should be sized
sufficiently to handle the switching current needed to drive
NGATE while maintaining minimum switching voltage.
actual current needed to power the LTC4267 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.
APPLICATIOU
W
U
U
Figure 12. Powering the LTC4267 Switching
Regulator via the Shunt Regulator
Figure 13. Powering the LTC4267 Switching
Regulator with an External Preregulator
V
PORTP
P
VCC
PGND
P
OUT
V
PORTN
LTC4267
–48
FROM
PSE
R
START
C
PVCC
+
–
PGND
4267 F14
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
B
. The preregulator holds
P
VCC
at 7.6V nominal, well above the maximum rated P
VCC
turn-off threshold of 6.8V. Resistor R
START
momentarily
charges the P
VCC
node up to the P
VCC
turn-on threshold,
enabling the switching regulator. The voltage on C
PVCC
begins to decline as the switching regulator draws its
normal supply current, which exceeds the delivery of
R
START
. After some time, the output voltage approaches
the desired value. By this time, the pass transistor Q1
catches the declining voltage on the P
VCC
pin, and provides
virtually all the supply current required by the LTC4267
switching regulator. C
PVCC
should be sized sufficiently to
handle the switching current needed to drive NGATE while
maintaining minimum switching voltage.
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
V
PORTP
P
VCC
PGND
P
OUT
V
PORTN
LTC4267
–48
FROM
PSE
R
START
C
PVCC
PGND
+
–
PGND
PGND
Q1
D1
8.2V
R
B
4267 F15
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 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.
Figure 14. Main Loop Compensation for an Isolated Design
R1
R2
C
C
R
Z
TO OPTO-
ISOLATOR
4267 F14
V
OUT
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