Linear Technology LT4363 Datasheet page 13

applicaTions inForMaTion
matically initiated once the OV pin falls below 1.268V. OV
has no effect on initial start-up when power is first applied
and upon exiting shutdown. The cool down phase may
be interrupted in the LT4363-2 by pulling SHDN low for
at least 1s/µF of C .
TMR
For both the LT4363-1 and LT4363-2 the FLT pin goes
high in shutdown and is cleared high when power is first
applied to V . If FLT is set low, it can be reset during the
CC
cool down phase by pulling SHDN low for at least 1s/µF
of C .
TMR
Intermittent Fault Conditions
Brief overvoltage or overcurrent conditions interrupt the
operation of the timer. If the TMR pin has not yet reached
1.275V when the input falls below the regulation value
or drops out of current limit, the timer capacitor is dis-
charged back to 0.5V with a 2µA current sink. If the TMR
voltage crosses 1.275V FLT is set low. If the overvoltage
or overcurrent abates before reaching 1.375V, the timer
capacitor discharges with 2µA back to 0.5V, whereupon
FLT resets high. If several short overvoltage or overcurrent
events occur in rapid succession, the timer capacitor will
integrate the charging and discharging currents.
MOSFET Selection
The LT4363 drives an N-channel MOSFET to conduct the
load current. The important features of the MOSFET are
on-resistance R , the maximum drain-source voltage
DS(ON)
V , the threshold voltage, and the SOA.
(BR)DSS
The maximum allowable drain-source voltage must be
higher than the supply voltage. If the output is shorted
to ground or during an overvoltage event, the full supply
voltage will appear across the MOSFET.
The gate drive for the MOSFET is guaranteed to be more
than 10V and less than 16V for those applications with V
higher than 9V. This allows the use of standard threshold
voltage N-channel MOSFETs. For systems with V
than 9V, a logic level MOSFET is required since the gate
drive can be as low as 4.5V.
The SOA of the MOSFET must encompass all fault condi-
tions. In normal operation the pass transistor is fully on,
dissipating very little power. But during either overvolt-
age or overcurrent faults, the GATE pin is controlled to
regulate either the output voltage or the current through
the MOSFET. Large current and high voltage drop across
the MOSFET can coexist in these cases. The SOA curves
of the MOSFET must be considered carefully along with
the selection of the fault timer capacitor.
Transient Stress in the MOSFET
During an overvoltage event, the LT4363 drives a series
pass MOSFET to regulate the output voltage at an acceptable
level. The load circuitry may continue operating throughout
this interval, but only at the expense of dissipation in the
MOSFET pass device. MOSFET dissipation or stress is a
function of the input voltage waveform, regulation voltage
and load current. The MOSFET must be sized to survive
this stress.
Most transient event specifications use the prototypi-
cal waveshape shown in Figure 3, comprising a linear
ramp of rise time t
exponentially decaying back to V
of τ. A common automotive transient specification has
constants of t
condition known as load dump commonly has constants
of t = 5ms, V
r
MOSFET stress is the result of power dissipated within
the device. For long duration surges of 100ms or more,
stress is increasingly dominated by heat transfer; this is
a matter of device packaging and mounting, and heat sink
thermal mass. This is best analyzed by simulation, using
the MOSFET thermal model.
For short duration transients of less than 100ms, MOSFET
survival is increasingly a matter of safe operating area
CC
less
CC
V
IN
For more information www.linear.com/LT4363
, reaching a peak voltage of V
r
IN
= 10µs, V = 80V and τ = 1ms. A surge
r PK
= 60V and τ = 200ms.
PK
V
PK
τ
t
r
Figure 3. Prototypical Transient Waveform
LT4363
and
PK
with a time constant
4363 F03
4363fb
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