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参数资料
| 型号： | DC859A |
| 厂商： | Linear Technology |
| 文件页数： | 7/32页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR LTC4267 |
| 设计资源： | DC859A Design File DC859A Schematic |
| 标准包装： | 1 |
| 主要目的： | 电源管理，以太网供电（POE） |
| 嵌入式： | 否 |
| 已用 IC / 零件： | LTC4267 |
| 已供物品： | 板 |
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LTC4267
15
4267fc
temperature exceeds the overtemperature trip point, the
current is reduced to zero and very little power is dissi-
pated in the part until it cools below the overtemperature
set point. Once the LTC4267 has charged up the load
capacitor and the PD is powered and running, there will
be minor residual heating due to the DC load current of
the PD owing through the internal MOSFET. The DHC
package offers superior thermal performance by including
an exposed pad that is soldered to an electrically isolated
heat sink on the printed circuit board.
During classication, excessive heating of the LTC4267
can occur if the PSE violates the 75ms probing time limit.
To protect the LTC4267, thermal overload circuitry will dis-
able classication current if the die temperature exceeds
the overtemperature trip point. When the die cools down
below the trip point, classication current is reenabled.
The PD is designed to operate at a high ambient tem-
perature and with the maximum allowable supply (57V).
However, there is a limit to the size of the load capacitor
that can be charged up before the LTC4267 reaches the
overtemperature trip point. Hitting the overtemperature
trip point intermittently does not harm the LTC4267, but it
will delay the completion of capacitor charging. Capacitors
up to 200F can be charged without a problem over the
full operating temperature range.
Switching Regulator Main Control Loop
Due to space limitations, the basics of current mode
DC/DC conversion will not be discussed here. The reader
is referred to the detail treatment in Application Note 19
or in texts such as Abraham Pressman’s Switching Power
Supply Design.
In a Power over Ethernet System, the majority of ap-
plications involve an isolated power supply design. This
means that the output power supply does not have any
DC electrical path to the PD interface or the switching
regulator primary. The DC isolation is achieved typically
through a transformer in the forward path and an op-
toisolator in the feedback path or a third winding in the
transformer. The typical application circuit shown on the
front page of the datasheet represents an isolated design
using an optoisolator. In applications where a nonisolated
topology is desired, the LTC4267 features a feedback port
and an internal error amplier that can be enabled for this
specic application.
In the typical application circuit (Figure 11), the isolated
topology employs an external resistive voltage divider
to present a fraction of the output voltage to an external
error amplier. The error amplier responds by pulling
an analog current through the input LED on an optoiso-
lator. The collector of the optoisolator output presents a
corresponding current into the ITH/RUN pin via a series
diode. This method generates a feedback voltage on the
ITH/RUN pin while maintaining isolation.
The voltage on the ITH/RUN pin controls the pulse-width
modulator formed by the oscillator, current comparator,
and RS latch. Specically, the voltage at the ITH/RUN pin
sets the current comparator’s trip threshold. The current
comparator monitors the voltage across a sense resistor
in series with the source terminal of the external N-Chan-
nel MOSFET. The LTC4267 turns on the external power
MOSFET when the internal free-running 200kHz oscillator
sets the RS latch. It turns off the MOSFET when the cur-
rent comparator resets the latch or when 80% duty cycle
is reached, whichever happens rst. In this way, the peak
current levels through the yback transformer’s primary
and secondary are controlled by the ITH/RUN voltage.
In applications where a nonisolated topology is desirable
(Figure 11), an external resistive voltage divider can present
a fraction of the output voltage directly to the VFB pin of
the LTC4267. The divider must be designed so when the
output is at its desired voltage, the VFB pin voltage will
equal the 800mV onboard internal reference. The internal
error amplier responds by driving the ITH/RUN pin. The
LTC4267 switching regulator performs in a similar manner
as described previously.
Regulator Start-Up/Shutdown
The LTC4267 switching regulator has two shutdown
mechanisms to enable and disable operation: an un-
dervoltage lockout on the PVCC supply pin and a forced
shutdown whenever external circuitry drives the ITH/RUN
pin low. The LTC4267 switcher transitions into and out of
shutdown according to the state diagram (Figure 8). It is
important not to confuse the undervoltage lockout of the
PD interface at VPORTN with that of the switching regulator
at PVCC. They are independent functions.
APPLICATIO S I FOR ATIO
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