LT4356CDE-1-PBF LINER [Linear Technology], LT4356CDE-1-PBF Datasheet - Page 9

• Manufacturer Part Number: LT4356CDE-1-PBF
• Description: Overvoltage Protection Regulator and Inrush Limiter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The LT4356-1 can limit the voltage and current to the load
circuitry during supply transients or overcurrent events.
The total fault timer period should be set to ride through
short overvoltage transients while not causing damage
to the pass transistor. The selection of this N-channel
MOSFET pass transistor is critical for this application.
It must stay on and provide a low impedance path from
the input supply to the load during normal operation and
then dissipate power during overvoltage or overcurrent
conditions.
The following sections describe the overcurrent and the
overvoltage faults, and the selection of the timer capacitor
value based on the required warning time. The selection
of the N-channel MOSFET pass transistor is discussed
next. Auxiliary amplifi er, reverse input, and the shutdown
functions are covered after the MOSFET selection. External
component selection is discussed in detail in the Design
Example section.
Overvoltage Fault
The LTC4356-1 limits the voltage at the OUT pin during an
overvoltage situation. An internal voltage amplifi er regu-
lates the GATE pin voltage to maintain a 1.25V threshold at
the FB pin. During this period of time, the power MOSFET
is still on and continues to supply current to the load. This
allows uninterrupted operation during short overvoltage
transient events.
When the voltage regulation loop is engaged for longer
than the time-out period, set by the timer capacitor con-
nected from the TMR pin to ground, an overvoltage fault is
detected. The GATE pin is pulled down to the OUT pin by a
150mA current. After the fault condition has disappeared
and a cool down period has transpired, the GATE pin starts
to pull high again. This prevents the power MOSFET from
being damaged during a long period of overvoltage, such
as during load dump in automobiles.
Overcurrent Fault
The LT4356-1 features an adjustable current limit that
protects against short circuits or excessive load current.
During an overcurrent event, the GATE pin is regulated to
limit the current sense voltage across the V
pins to 50mV.
CC
and SNS
An overcurrent fault occurs when the current limit circuitry
has been engaged for longer than the time-out delay set
by the timer capacitor. The GATE pin is then immediately
pulled low by a 10mA current to GND turning off the
MOSFET. After the fault condition has disappeared and a
cool down period has transpired, the GATE pin is allowed
to pull back up and turn on the pass transistor.
Fault Timer
The LT4356-1 includes an adjustable fault timer pin. Con-
necting a capacitor from the TMR pin to ground sets the
delay timer period before the MOSFET is turned off. The
same capacitor also sets the cool down period before the
MOSFET is allowed to turn back on after the fault condition
has disappeared.
Once a fault condition, either overvoltage or overcurrent,
is detected, a current source charges up the TMR pin.
The current level varies depending on the voltage drop
across the drain and source terminals of the power
MOSFET(V
OUT pin. This scheme takes better advantage of the avail-
able Safe Operating Area (SOA) of the MOSFET than would
a fi xed timer current. The timer function operates down to
V
Fault Timer Current
The timer current starts at around 2μA with 0.5V or less
of V
ing an overvoltage fault (Figure 1). During an overcurrent
fault, it starts at 4μA with 0.5V or less of V
to 260μA with 80V across the MOSFET (Figure 2). This
arrangement allows the pass transistor to turn off faster
during an overcurrent event, since more power is dissipated
during this condition. Refer to the Typical Performance
Characteristics section for the timer current at different
V
When the voltage at the TMR pin, V
threshold, the FLT pin pulls low to indicate the detection
of a fault condition and provide warning to the load of
the impending power loss. In the case of an overvoltage
fault, the timer current then switches to a fi xed 5μA.
CC
DS
DS
= 5V across the whole temperature range.
in both overvoltage and overcurrent events.
, increasing linearly to 50μA with 75V of V
DS
), which is typically from the V
TMR
, reaches the 1.25V
LT4356-1
DS
but increases
CC
pin to the
DS
43561fd
dur-
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