LTC4354 Linear Technology, LTC4354 Datasheet - Page 7

• Manufacturer Part Number: LTC4354
• Description: Negative Voltage Diode-OR Controller and Monitor
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Input Power Supply
The power supply for the device is derived from –48_RTN
through an external current limiting resistor (R
internal shunt regulator clamps the voltage at V
11V. A 1µF decoupling capacitor to V
It also provides a soft-start to the part.
R
supply current requirement of 2mA at the expected input
operating voltage.
The power dissipation of the resistor is calculated at the
maximum DC input voltage:
If the power dissipation is too high for a single resistor, use
multiple low power resistors in series instead of a single
high power component.
MOSFET SELECTION
The LTC4354 drives N-channel MOSFETs to conduct the
load current. The important features of the MOSFETs are
on-resistance R
age V
IN
R
P
should be chosen to accommodate the maximum
IN
=
DSS
≤
(
V
(
, and the threshold voltage.
IN MAX
V
(
IN MIN
(
I
CC MAX
)
DS(ON)
R
)
−
(
IN
−
U
V
CC MIN
V
Z MAX
, the maximum drain-source volt-
)
(
(
U
)
)
)
)
2
W
SS
is recommended.
U
CC
IN
pin to
). An
The gate drive for the MOSFET is guaranteed to be more
than 10V and less than 12V. This allows the use of
standard threshold voltage N-channel MOSFETs. An ex-
ternal zener diode can be used to clamp the potential at the
V
breakdown voltage is less than 12V.
The maximum allowable drain-source voltage, V
must be higher than the supply voltages. If the inputs are
shorted, the full supply voltage will appear across the
MOSFETs.
The LTC4354 tries to servo the voltage drop across the
MOSFET to 30mV in the forward direction by controlling
the gate voltage and sends out a fault signal when the
voltage drop exceeds the 250mV fault threshold. The
R
load current while not triggering a fault, and to stay within
the MOSFET’s power rating at the maximum load current
(I
Fault Conditions
LTC4354 monitors fault conditions and turns on an LED or
optocoupler to indicate a fault. When the voltage drop
across the pass transistor is higher than the 250mV fault
threshold, the internal pull-down at the FAULT pin turns off
and allows the current to flow through the LED or
optocoupler. Conditions that cause high voltage across
the pass transistor include: short in the load circuitry,
excessive load current, FET open while conducting cur-
rent, and FET short on the channel with the higher supply
voltage. The fault threshold is internally set to 250mV.
CC
DS(ON)
2
• R
pin to as low as 4.5V if the gate to source rated
DS(ON)
should be small enough to conduct the maximum
).
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LTC4354
(BR)DSS,
7
4354f