參數(shù)資料
型號(hào)： LTC1737
廠(chǎng)商： Linear Technology Corporation
元件分類(lèi)： 基準(zhǔn)電壓源/電流源
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 抗輻射高效，5安培開(kāi)關(guān)穩(wěn)壓器
文件頁(yè)數(shù)： 17/32頁(yè)
文件大小： 1975K
代理商： LTC1737
LTC4267
17
4267f
Diode Bridge
IEEE 802.3af allows power wiring in either of two configu-
rations: on the TX/RX wires or via the spare wire pairs in
the RJ45 connector. The PD is required to accept power in
either polarity on either the main or spare inputs; therefore
it is common to install diode bridges on both inputs in
order to accommodate the different wiring configurations.
Figure 9 demonstrates an implementation of these diode
bridges. The IEEE 802.3af specification also mandates
that the leakage back through the unused bridge be less
than 28μA when the PD is powered with 57V.
The IEEE standard includes an AC impedance requirement
in order to implement the AC disconnect function. Capaci-
tor C14 in Figure 9 is used to meet this AC impedance
requirement. A 0.1μF capacitor is recommended for this
application.
The LTC4267 has several different modes of operation
based on the voltage present between V
PORTN
and V
PORTP
pins. The forward voltage drop of the input diodes in a PD
design subtracts from the input voltage and will affect the
transition point between modes. When using the LTC4267,
it is necessary to pay close attention to this forward voltage
drop. Selection of oversized diodes will help keep the PD
thresholds from exceeding IEEE specifications.
The input diode bridge of a PD can consume over 4%
of the available power in some applications. It may be
desirable to use Schottky diodes in order to reduce power
loss. However, if the standard diode bridge is replaced
with a Schottky bridge, the transition points between the
modes will be affected. Figure 10 shows a technique for
using Schottky diodes while maintaining proper threshold
points to meet IEEE 802.3af compliance. D13 is added to
compensate for the change in UVLO turn-on voltage caused
by the Schottky diodes and consumes little power.
Classification Resistor Selection (R
CLASS
)
The IEEE specification allows classifying PDs into four
distinct classes with class 4 being reserved for future use
(Table 2). An external resistor connected from R
CLASS
to
V
PORTN
(Figure 4) sets the value of the load current. The
designer should determine which power category the PD
falls into and then select the appropriate value of R
CLASS
from Table 2. If a unique load current is required, the value
of R
CLASS
can be calculated as:
R
CLASS
= 1.237V/(I
DESIRED
– I
IN_CLASS
)
where I
IN_CLASS
is the LTC4267 IC supply current during
classification and is given in the electrical specifications.
The R
CLASS
resistor must be 1% or better to avoid degrading
the overall accuracy of the classification circuit. Resistor
power dissipation will be 50mW maximum and is transient
so heating is typically not a concern. In order to maintain
loop stability, the layout should minimize capacitance at
the R
CLASS
node. The classification circuit can be disabled
by floating the R
CLASS
pin. The R
CLASS
pin should not be
shorted to V
PORTN
as this would force the LTC4267 clas-
sification circuit to attempt to source very large currents
and quickly go into thermal shutdown.
Power Good Interface
The
P
W
R
G
D signal is controlled by a high voltage, open-
drain transistor. The designer has the option of using this
signal to enable the onboard switching regulator through
the I
TH
/RUN or the P
VCC
pins. Examples of active-high
interface circuits for controlling the switching regulator
are shown in Figure 7.
In some applications, it is desirable to ignore intermittent
power bad conditions. This can be accomplished by in-
cluding capacitor C15 in Figure 7 to form a lowpass filter.
With the components shown, power bad conditions less
than about 200μs will be ignored. Conversely, in other
applications it may be desirable to delay assertion of
P
W
R
G
D to the switching regulator using C
PVCC
or C17
as shown in Figure 7.
It is recommended that the designer use the power
good signal to enable the switching regulator. Using
P
W
R
G
D ensures the capacitor C1 has reached within
1.5V of the final value and is ready to accept a load. The
LTC4267 is designed with wide power good hysteresis
to handle sudden fluctuations in the load voltage and
current without prematurely shutting off the switching
regulator. Please refer to the Power-Up Sequencing of the
Application Information section.
APPLICATIU
W
U
U
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