Applications Information - Linear Technology LT4363 Datasheet

applicaTions inForMaTion

The LT4363 limits the voltage and current delivered to the
load during supply transient or output overload events. The
total fault timer period is set to ride through short-duration
faults, while longer events cause the output to shut off
and protect the MOSFET pass device from damage. The
MOSFET provides a low resistance path from the input to
the load during normal operation, while in fault conditions
it operates as a series regulator.
Overvoltage Fault
The LT4363 limits the voltage at the output during an
overvoltage at the input. An internal amplifier regulates
the GATE pin to maintain 1.275V at the FB pin. During
this interval the MOSFET is on and supplies current to
the load. This allows uninterrupted operation during short
overvoltage events. If the overvoltage condition persists,
the timer causes the MOSFET to turn off.
Overcurrent Fault
The LT4363 features and adjustable current limit that pro-
tects against output short circuits or excessive load current.
During an overcurrent event, the GATE pin is regulated to
limit the current sense voltage across the SNS and OUT
pins to 50mV. In the case of a severe short at the output,
where OUT is less than 2V, the current sense voltage is
reduced to 25mV to further reduce power dissipation in
the MOSFET. If the overcurrent condition persists, the
timer causes the MOSFET to turn off.
Fault Timer Overview
Overvoltage and overcurrent conditions are limited in
duration by an adjustable timer. A capacitor at the TMR pin
sets the delay time before a fault condition is reported at
the FLT pin as well as the overall delay before the MOSFET
is turned off. The same capacitor also sets the cool down
time before the MOSFET is allowed to turn back on.
When either an overvoltage or overcurrent fault condition
occurs, a current source charges the TMR pin capacitor.
The exact current level varies as a function of the type of
fault and the V voltage drop across the MOSFET. This
DS
scheme takes better advantage of the MOSFET's available
Safe Operating Area (SOA) than would a fixed timer current.
The TMR pin is biased to 0.5V under normal operating
conditions. In the presence of a fault the timer first charges
to 1.275V, and then enters the early warning phase of
operation. At this point the FLT pin pulls low and after
charging to 1.375V, the timer shuts off the MOSFET. The
warning phase is indicated by FLT low and gives time for
the load to perform house-keeping chores such as data
storage in anticipation of impending power loss. After
faulting off, the timer enters the cool down phase. At the
end of the cool down period the LT4363-1 remains off until
reset, while the LT4363-2 automatically restarts. For the
LT4363-2 retry is inhibited if the OV pin is greater than
1.275V. This prevents motorboating in the event there is
a sustained input overvoltage condition.
Fault Timer Operation in Overvoltage
In the presence of an overvoltage condition when the
LT4363 regulates the output voltage, the timer charges
from 0.5V to 1.275V with a current that varies as a func-
tion of V
across V
from around 4µA with V
Because V
at the V
of Overvoltage TMR Current vs (V
the Typical Performance Characteristics.
When TMR reaches 1.275V, the FLT pin is latched low as
an early warning of impending shutdown. The timer cur-
rent is cut to a fixed value of 6µA and continues to run
until TMR reaches 1.375V, producing a fixed early warning
period given by:
C
TMR
When TMR reaches 1.375V, the MOSFET is turned off and
allowed to cool for an extended period. The total elapsed
time between the onset of output regulation and turn-off
is given by:
t
REG
Because I
regulation depends upon the input waveform and the time
required for the output voltage to come into regulation.
For more information www.linear.com/LT4363
(see Figure 1). V is inferred from the drop
DS DS
and OUT. The timer current increases linearly
CC
≤ 0.5V, to 50µA with V
DS
is measured indirectly, clamping or filtering
DS
pin affects the timer current response. A graph
CC
6µA
= t •
WARNING
100mV
 0.775V 100mV
= C • +
TMR

I 6µA
TMR
is a function of V
TMR CC
LT4363
= 75V.
DS
– V ) is shown in
CC OUT


– V , the exact time in
OUT
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