LTC4292 datasheet(28/44 Pages) AD

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LTC9101-2/LTC9102

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

disruption to the PDs during extreme power transients.

Furthermore, PDs that do not meet minimum bulk capaci-

tance requirements are particularly vulnerable to power

brownouts with traditional PSE solutions. The LTC9101-2/

LTC9102 provides an improved hot swap responsiveness

with excellent recovery from surge events.

During a surge event, the LTC9102 GATE pin quickly turns

off the external MOSFET current flow to protect the PSE, the

MOSFET, and downstream circuitry. As the surge dissipates,

the LTC9102 quickly turns the MOSFET back on in a safe,

current limited manner while minimizing power disruption

to the PD. The LTC9102 fast MOSFET turn off and power

recovery better support both IEEE compliant PDs and PDs

with lower bulk capacitance in high reliability applications.

Autoclass

IEEE 802.3bt introduces a new optional feature, Autoclass.

Autoclass enables the PSE to reclaim power budget from

single-signature PDs requesting more power than needed

under worst-case operating conditions. 802.3bt does not

specify Autoclass for dual-signature PDs. The LTC9101-2/

LTC9102 fully supports Autoclass.

Prior versions of the 802.3 PoE standard specify mini-

mum PSE output power for worst-case IR drop across

the Ethernet cable and minimum PSE output voltage.

However, a method for the PSE to reclaim over-allocated

power is not specified. When a shorter Ethernet cable is

used, or when the guaranteed PSE output voltage is above

the specified minimum, the specified minimum PSE out-

put power substantially over-allocates power to the PD.

An example PoE system is shown in two versions.

Figure 22 shows a 100W four port PSE servicing three

25.5W PDs over 100meter cables. Such a system requires

the PSE to allocate 25.5W per PD and a further 4.5W for

each 100m cable’s IR drop.

The total power allocation is:

3 Ports • (4.5W + 25.5W) = 90W

Disconnect

The LTC9101-2/LTC9102 monitors powered channels to

ensure the PD continues to draw the minimum speci-

fied current. The IHOLD-2P threshold, monitored as the

VHOLD-2P threshold across the 0.1Ω sense resistor, is

used to determine if a PD has been disconnected.

The IHOLD-2P (DC Disconnect) threshold is set automati-

cally based on the number of classification events pre-

sented to the PD. See Table 6 and Table 7 for details.

A disconnect timer (tDIS) counts up whenever channel

current is below the IHOLD-2P threshold, indicating that the

PD has been disconnected. If the appropriate tDIS timer(s)

expire, the port or channel (Table 15) will be turned off and

the corresponding disconnect faults are set. If the current

increases above IHOLD-2P before the tDIS timer expires, the

timer(s) reset. As long as the PD exceeds the minimum

current level before tDIS expires, it will remain powered.

Although not recommended, the DC disconnect fea-

ture can be disabled by clearing the corresponding DC

Disconnect Enable bits. Disabling the DC disconnect fea-

ture forces the LTC9101-2/LTC9102 out of compliance

with the IEEE standard. A powered port will stay powered

after the PD is removed; the still-powered port may be

subsequently connected to a non-PoE data device, poten-

tially causing damage.

The LTC9101-2/LTC9102 does not include AC discon-

nect circuitry. AC disconnect is not a supported feature

of 802.3bt.

Table 15. DC Disconnect Effect on Port/Channel State

PD CONFIGURATION

FAULT RESULT:

TURN OFF PORT OR CHANNEL

tDIS

Single

Port*

Dual

Channel

*If tDIS expires on both channels

Fast Surge Recovery

High reliability systems demand excellent surge recovery.

It is increasingly important for a PSE to minimize power

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