LT4321

6

4321f

For more information www.linear.com/LT4321

OVERVIEW

TheLT4321isadualidealdiodebridgecontrollerdesigned

to rectify two independent DC channels into a single

output. The LT4321 senses the greater of the two input

channels, |IN12-IN36| or |IN45-IN78|, and connects them

to the output with the correct polarity. Smooth crossover

between channels is guaranteed by the enforced dropout

AverycommonapplicationisanIEEE802.3powereddevice

which is required to accept voltage in either polarity at its

RJ-45 input. Polarity correction devices allow the PD to

work equally well with standard or cross-over cables and

endspan or midspan PSEs. They also prevent the PD from

back feeding current into the Ethernet cable.

PD polarity correction is commonly done with a traditional

diode bridge, but this results in an efficiency loss due to

theforwarddropgeneratedacrosstwoconductingdiodes.

This voltage drop reduces the available supply voltage and

dissipates significant power.

The LT4321 uses actively driven MOSFETs to nearly elimi-

nate the forward voltage drop. By maximizing available

voltage and reducing power dissipation (Figure 1), the

LT4321 simplifies PD design and reduces power supply

cost. It can also eliminate thermal design problems, costly

heat sinks, and reduce PC board area.

Somedesignsuseidealdiodebridgecircuitsimplemented

withdiscretecomponents.Thesebridgesoftensufferfrom

a trade-off between quiescent current and tolerance to

transients and leakage. With quiescent current properly

tuned for PoE, stray PCB leakage between bridge compo-

nents can be enough to cause accidental turn-on, latchup,

and destruction of the circuit.

The LT4321 offers significant improvements over discrete

solutions. The integrated bridge controller allows for

sophisticated sensing and control of the PowerPath™

MOSFETs, ensuring that MOSFETs that are supposed to

be off, stay off. An ideal bridge controlled by the LT4321 is

tolerant to hot-plugs, input short-circuits, common mode

shift, and PCB leakage in the application circuit.

applicaTions inForMaTion

OPERATING MODES

Ideal Diode Bridge Mode

In ideal bridge mode the LT4321 saves power by activat-

ing MOSFETs in place of power path diodes. The LT4321

and either EN or EN is asserted.

When the LT4321 is enabled, it senses the inputs with

respect to the output to decide which external MOSFETs

to turn on. Inputs are grouped into pairs, IN12/IN36 and

IN45/IN78. Within each pair, one input voltage must be

greater than OUTP and one must be less than OUTN before

the external MOSFETs related to that pair are enabled. For

example, if IN36 is greater than OUTP and IN12 is less than

OUTN, TG36 and BG12 will turn on. Table 1 and Table 2

outline the conditions that activate the ideal diode bridge.

Shutdown Mode

Shutdown mode is intended to keep the LT4321 quiescent

current from interfering with detection and classification

in a PoE system (Figure 2). The LT4321 is always in

shutdown mode over the full operating voltage range by

deasserting both the EN and EN pins.

Figure 1. Power Dissipation vs Load Current

CURRENT (mA)

0

0

0.8

1.0

1.2

1.8

1.4

600

4321 F01

0.6

0.4

0.2

1.6

200

400

800

1000

IN12 = 55V

IN36 = 0V

IN45 = FLOAT

IN78 = FLOAT

LT4321 (50m FETs)

DIODES (S2B)

POWER

SAVED