LTC4271 datasheet(24/30 Pages) AD

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91013 F17

AGND

VEE

SENSEn

VSSKn

GATEn

OUTn

PORT

S1B

1µF

100V

LTC9103

SMAJ58A

CBULK

TVSBULK

VEE

0.1µF

X7R

100V

RSENSEn

AGND

Figure 17. LTC9103 Surge Protection

Figure 16. Power Supply Configuration with External 3.3V Regulators

LTC9101-3/LTC9103

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Sense Resistors

The LTC9101-3/LTC9103 is designed for a low 0.1Ω

current sense resistance per port, laid out as shown in

the Layout Requirements section, Figure 18. In order to

meet the IHOLD, ICUT, and ILIM accuracy required by the

IEEE specification, the sense resistors should have ±1%

tolerance or better, and no more than ±200ppm/°C tem-

perature coefficient.

Port Output Cap

Each port requires a 0.1µF cap across OUTn to AGND

(see Figure 17) to keep the LTC9103 stable while in cur-

rent 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 capac-

itors rated for at least 100V are recommended and must

be located close to the LTC9103.

91013 F16

1µF

100V

1µF

100V

10k

1/2W

10k

1/2W

1µF

100V

1µF

100V

1µF

47µF

100V

1µF

VEE

3.3V

VEE

AGND

CAP3

EXT3

PWRIN

CAP4

LTC9103

VEE

3.3V

AGND

CAP3

EXT3

PWRIN

CAP4

LTC9103

3.3V

REGULATOR

Surge Protection

Ethernet ports can be subject to significant cable surge

events. To keep PoE voltages below a safe level and pro-

tect the application against damage, protection compo-

nents, as shown in Figure 17, are required at the main

supply, at the 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 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

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.

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