LT4321
6
4321f
For more information www.linear.com/LT4321
OVERVIEW
The LT4321 is a dual ideal diode bridge controller designed
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
voltage, ∆V
SD
.
A very common application is an IEEE 802.3 powered device
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
the forward drop generated across two conducting diodes.
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.
Some
designs use ideal diode bridge circuits implemented
with discrete components. These bridges often suffer from
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
enters ideal bridge mode when OUTP is greater than V
UVLO
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 when OUTP < V
UVLO
. It can be held 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
1.4
600
0.6
0.4
0.2
1.6
200
400
800 1000
IN12 = 55V
IN36 = 0V
IN45 = FLOAT
IN78 = FLOAT
LT4321 (50mΩ FETs)
DIODES (S2B)
POWER
SAVED
