lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 8

• Manufacturer Part Number: lt4351
• Description: Mosfet Diode-or Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
LT4351
Increasingly, system designers have to deal with multiple
supply sources. The multiplicity may provide parallel, re-
dundant supplies for increased reliability or provide a means
of connecting disparate supplies. In all cases the desire is
for behavior like a diode but with no loss or voltage drop.
ORing diodes have been the conventional means of
connecting these supplies. The disadvantage of this ap-
proach is that diodes introduce efficiency loss because of
their forward voltage drop. This variable voltage drop also
degenerates supply tolerance. Additionally, diodes pro-
vide no information concerning the status of the sourcing
supply. Separate control must also be added to ensure
that a supply that is out of range is not allowed to affect
the common supply.
The LT4351 eliminates these problems by using N-channel
MOSFETs as the pass elements. The MOSFET is turned on
when power is being passed, allowing for a low voltage
drop from the supply to the load. When the input source
voltage drops below the output common supply voltage it
turns off the MOSFET, thereby matching the function and
performance of an ideal diode.
The LT4351 drives either a single MOSFET or dual back-
to-back MOSFETs. Dual MOSFETs are chosen to eliminate
current flow from the input supply to the output supply
when the V
A driver amplifier monitors the input (V
(OUT) and controls the MOSFETs. If V
15mV, GATE goes high and turns on the MOSFET(s)
allowing for power passage.
Undervoltage and overvoltage comparators C
C
conjunction with the UV and OV pins sets appropriate
thresholds such that the MOSFET(s) is off when the UV pin
is below 300mV or OV pin is above 300mV.
To help deal with the transients on the supply lines, the UV
input has current hysteresis. When the UV voltage drops
below the 300mV threshold, a 10 A current is pulled from
the pin. Thus the user can set the hysteresis level through
appropriate values in the divider.
Overvoltage shutdown occurs in two stages. The first
occurs when the OV pin exceeds the 300mV reference.
8
OVF
also control power passage. A resistive divider in
IN
voltage is greater than OUT.
U
IN
exceeds OUT by
IN
) and output
UV
, C
OV
and
When OV just exceeds the reference, an internal capacitor
starts charging, delaying the signal to turn off the
MOSFET(s).
The second occurs when the OV pin exceeds 330mV. The
OVF comparator will immediately trip pulling GATE to
GND. This affords a delay inversely proportional to the
amount of overdrive. This also provides for glitch immu-
nity without compromising response time in the event of
a serious overvoltage condition.
The FAULT output indicates the status of the C
C
also pulls low when GATE is at compliance and V
by more than 0.21V indicating a probable nonfunctioning
MOSFET. Compliance occurs when GATE is at the lesser
of OUT + 7.4V or V
the greater of V
than 0.9V. If V
is not guaranteed.
The gate drive consists of a high current, wide bandwidth
amplifier (Driver). When the amplifier is enabled, it at-
tempts to regulate the GATE voltage such that the voltage
across the MOSFET(s) is approximately 15mV. If the
MOSFET(s) on resistance is so high as to prevent regula-
tion, then GATE goes to compliance and the MOSFET(s)
fully turns on. The inputs to the amplifier are V
The GATE pin sources current from V
to GND. The maximum GATE to V
V
voltage).
The STATUS comparator, ST, pulls low when GATE ex-
ceeds V
The STATUS pin pulls low as an indication that power is
passing through the MOSFET(s).
If V
or at compliance (GATE = V
high as an indication of a likely open MOSFET. FAULT will
pull low in this state indicating the probable fault.
The gate drive amplifier and STATUS function derive
power from V
present, the gate drive amplifier and STATUS are active
independent of the state of V
UV
DD
IN
comparators. It pulls low during a fault condition. It
– 2.3V or 7.4V above V
is greater than OUT by 0.21V and GATE > V
IN
by 0.7V. This occurs when V
DD
IN
IN
or OUT is below this level, the output state
. The circuit requires V
or OUT. It is active if V
DD
– 2.3V. FAULT derives its drive from
DD
OUT
IN
. If in a fault, GATE pulls
– 2.3V), STATUS will go
IN
or V
voltage is the lesser of
DD
IN
DD
IN
IN
and sinks current
or OUT is greater
(internal clamp
> 2.5V. If V
> OUT + 15mV.
OV
IN
, C
and OUT.
sn4351 4351fs
IN
IN
OVF
+ 7.4V
> OUT
DD
and
is