LTC4267CDHC datasheet(12/32 Pages) LINEAR

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LTC4267

4267fc

VPORTP

VPORTN

LTC4267

4267 F03

25k SIGNATURE

RESISTOR

SIGNATURE DISABLE

SIGDISA

16k

PSE

Figure 3. 25k Signature Resistor with Disable

Classiﬁcation

Once the PSE has detected a PD, the PSE may option-

ally classify the PD. Classiﬁcation provides a method for

more efﬁcient allocation of power by allowing the PSE

to identify lower power PDs and allocate less power for

these devices. The IEEE 802.3af speciﬁcation deﬁnes ﬁve

classes (Table 2) with varying power levels. The designer

selects the appropriate classiﬁcation based on the power

consumption of the PD. For each class, there is an as-

sociated load current that the PD asserts onto the line

during classiﬁcation probing. The PSE measures the PD

load current to determine the proper classiﬁcation and

PD power requirements.

During classiﬁcation (Figure 4), the PSE presents a ﬁxed

voltage between – 15.5V and – 20.5V to the PD. With the

input voltage in this range, the LTC4267 asserts a load

current from the VPORTP pin through the RCLASS resistor.

The magnitude of the load current is set by the RCLASS

resistor. The resistor values associated with each class

are shown in Table 2. Note that the switching regulator

will not interfere with the classiﬁcation measurement since

the LTC4267 has not passed power to the regulator.

Table 2. Summary of IEEE 802.3af Power Classiﬁcations and

LTC4267 RCLASS Resistor Selection

Maximum

Nominal

LTC4267

Power Levels

Classiﬁcation

RCLASS

at Input of PD

Load Current

Resistor

Class

Usage

(W)

(mA)

(Ω, 1%)

Default

0.44 to 12.95

Open

Optional

0.44 to 3.84

10.5

124

Optional

3.84 to 6.49

18.5

68.1

Optional

6.49 to 12.95

45.3

Reserved

Reserved*

30.9

*Class 4 is currently reserved and should not be used.

The IEEE 802.3af speciﬁcation limits the classiﬁcation

time to 75ms because a signiﬁcant amount of power is

dissipated in the PD. The LTC4267 is designed to handle the

power dissipation for this time period. If the PSE probing

exceeds 75ms, the LTC4267 may overheat. In this situation,

the thermal protection circuit will engage and disable the

classiﬁcation current source in order to protect the part.

The LTC4267 stays in classiﬁcation mode until the input

voltage rises above the UVLO turn-on voltage.

VPORTN Undervoltage Lockout

The IEEE speciﬁcation dictates a maximum turn-on voltage

of 42V and a minimum turn-off voltage of 30V for the PD.

In addition, the PD must maintain large on-off hysteresis

to prevent resistive losses in the wiring between the PSE

and the PD from causing start-up oscillation. The LTC4267

incorporates an undervoltage lockout (UVLO) circuit that

monitors the line voltage at VPORTN to determine when

to apply power to the integrated switching regulator

(Figure 5). Before the power is applied to the switching

regulator, the POUT pin is high impedance and sitting at

the ground potential since there is no charge on capacitor

C1. When the input voltage rises above the UVLO turn-on

threshold, the LTC4267 removes the detection and clas-

siﬁcation loads and turns on the internal power MOSFET.

C1 charges up under the LTC4267 current limit control

and the POUT pin transitions from 0V to VPORTN. This

sequence is shown in Figure 1. The LTC4267 includes

a hysteretic UVLO circuit on VPORTN that keeps power

applied to the load until the input voltage falls below the

UVLO turn-off threshold. Once the input voltage drops

below –30V, the internal power MOSFET is turned off and

Figure 4. IEEE 802.3af Classiﬁcation Probing

APPLICATIO S I FOR ATIO

VPORTP

RCLASS

VPORTN

LTC4267

CONSTANT

LOAD

CURRENT

INTERNAL

TO LTC4267

4267 F04

RCLASS

CURRENT PATH

PSE

PSE CURRENT MONITOR

PSE

PROBING

VOLTAGE

SOURCE

–15.5V TO –20.5V

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