LTC4267CDHC datasheet(15/32 Pages) LINEAR

15 / 32 page

LTC4267

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 ﬂowing 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 classiﬁcation, 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 classiﬁcation current if the die temperature exceeds

the overtemperature trip point. When the die cools down

below the trip point, classiﬁcation 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 200µF 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 ampliﬁer that can be enabled for this

speciﬁc 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 ampliﬁer. The error ampliﬁer 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. Speciﬁcally, 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 ﬁrst. In this way, the peak

current levels through the ﬂyback 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 ampliﬁer 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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