LT4295 datasheet(41/50 Pages) AD

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LTC9101-1/

LTC9102/LTC9103

Rev. 0

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APPLICATIONS INFORMATION

Surge Protection

Ethernet ports can be subject to significant cable surge

events. To keep PoE voltages below a safe level and protect

the application against damage, protection components,

as shown in Figure 30, are required at the main supply,

at the LTC9102/LTC9103 supply pins, and at each port.

Bulk transient voltage suppression (TVSBULK) and bulk

capacitance (CBULK) are required across the main PoE

supply and should be sized to accommodate system level

surge requirements.

Across each LTC9102/LTC9103 AGND pin and VEE pin is a

SMAJ58A 58V TVS (D1) and a 1µF, 100V bypass capaci-

tor (C1). These components must be placed close to the

LTC9102/LTC9103 pins.

Each port requires an S1B clamp diode from OUTn to

supply AGND. This diode steers harmful surges into the

supply rails where they are absorbed by the surge sup-

pressors and the VEE bypass capacitance. The layout of

these paths must be low impedance.

Sense Resistors

The LTC9101-1/LTC9102/LTC9103 is designed for a low

0.1Ω current sense resistance per channel, laid out as

shown in the Layout Requirements section, Figure 31. In

order to meet the IHOLD-2P, ICUT-2P, and ILIM-2P accuracy

required by the IEEE specification, the sense resistors

should have ±1% tolerance or better, and no more than

±200ppm/°C temperature coefficient.

Port Output Cap

Each port requires a 0.1μF cap across OUTn to AGND (see

Figure 30) to keep the LTC9102/LTC9103 stable while in

current limit during startup or overload. Common ceramic

capacitors often have significant voltage coefficients; this

means the capacitance is reduced as the applied voltage

increases. To minimize this problem, X7R ceramic capaci-

tors rated for at least 100V are recommended and must

be located close to the LTC9102/LTC9103.

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