LT4321

8

4321f

For more information www.linear.com/LT4321

applicaTions inForMaTion

Transient Voltage Suppressor

The LT4321 specifies an absolute maximum voltage of

100V and is designed to tolerate brief overvoltage events.

However, pins that interface to Ethernet cables or remote

telecomsuppliescanroutinelyseeexcessivepeakvoltages.

To protect the LT4321, install a unidirectional transient

voltage suppressor (TVS) such as an SMAJ60A between

OUTP and OUTN. This TVS must be mounted as close as

possible to the LT4321.

For extremely high cable discharge and surge protection

contact Linear Technology Applications.

MOSFET Selection

Select external MOSFETs that have a drain-source break-

down voltage higher than the maximum input voltage.

For PoE systems the drain-source breakdown should be

at least 100V. For all applications the gate threshold must

be a minimum of 2V.

The amount of power saved by the LT4321 depends on the

For example, a PoE+ class 4 PD’s maximum average cur-

drop of 40mV reduces power consumption to 1/15th that

of a B2100 Schottky diode bridge.

Enable Pins

and EN will control whether the LT4321 is in shutdown

mode or ideal bridge mode (Table 1 and Table 2). EN and

EN may be driven by a 3.3V or 5V logic signal, or with an

open drain or collector.

The EN pin is pulled up to the internally generated voltage

down to OUTN by an internal 250kΩ resistor. When OUTP

is less than 12V the enable pins are high impedance to

prevent these resistors from corrupting PoE detection.

The enable pins tolerate 100V (absolute maximum) and

may be tied directly to the OUTP or OUTN pins as needed.

Figure 3 and Figure 4 show how to interface the enable

pins to a PD interface controller. In these configurations,

the LT4321 PoE ideal bridge will be enabled after detec-

tion and classification are complete and before the PD is

consuming a significant amount of current.