LT4356 Linear Technology Corporation, LT4356 Datasheet - Page 11

• Manufacturer Part Number: LT4356
• Description: Overvoltage Protection Regulator And Inrush Limiter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
ramp of rise time t
exponentially decaying back to V
of t. A common automotive transient specifi cation has
constants of t
condition known as “load dump” has constants of
t
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 heatsink thermal
mass. For short duration transients of less than 100ms,
MOSFET survival is increasingly a matter of safe operating
area (SOA), an intrinsic property of the MOSFET.
SOA quantifi es the time required at any given condition
of V
MOSFET to its rated maximum. MOSFET SOA is expressed
in units of watt-squared-seconds (P
sentially constant for intervals of less than 100ms for any
given device type, and rises to infi nity under DC operating
conditions. Destruction mechanisms other than bulk die
temperature distort the lines of an accurately drawn SOA
graph so that P
I
proaches the maximum rating, rendering some devices
useless for absorbing energy above a certain voltage.
Calculating Transient Stress
To select a MOSFET suitable for any given application, the
SOA stress must be calculated for each input transient
which shall not interrupt operation. It is then a simple mat-
ter to chose a device which has adequate SOA to survive
the maximum calculated stress. P
transient waveform is calculated as follows (Figure 4).
r
D
= 5ms, V
V
and V
IN
DS
and I
DS
Figure 3. Prototypical Transient Waveform
. In particular P
PK
D
r
= 60V and τ = 200ms.
t
r
V
= 10μs, V
to raise the junction temperature of the
2
PK
t is not the same for all combinations of
r
, reaching a peak voltage of V
PK
2
t tends to degrade as V
= 80V and τ = 1ms. A surge
IN
with a time constant
2
2
t for a prototypical
t). This fi gure is es-
PK
DS
43561 F03
and
ap-
V
Let
Then
Typically V
For the transient conditions of V
V
of 3A, P
a D-pak package. The P
is evaluated by integrating the square of MOSFET power
versus time.
Calculating Short Circuit Stress
SOA stress must also be calculated for short circuit condi-
tions. Short circuit P
where, ΔV
overcurrent timer interval.
For V
= 100nF, P
calculated in the previous example. Nevertheless, to ac-
Figure 4. Safe Operating Area Required to Survive Prototypical
Transient Waveform
IN
REG
a = V
b = V
(V
P
P
P
V
REG
2
2
2
IN
IN
t = I
t =
= 16V, t
t = (V
= Nominal Input Voltage)
REG
PK
= 14.7V, V
2
t is 16.7W
LOAD
2
1
SNS
REG
– V
IN
2
– V
I
LOAD
t is 6.6W
• Δ V
r
is the SENSE pin threshold, and t
IN
2
≈ V
IN
= 10μs and τ = 1ms, and a load current
3
2
1
1
2
t
IN
SNS
SNS
V
r
(
t
PK
r
2
V
and τ >> t
2
s—easily handled by a MOSFET in
PK
(
t is given by:
2a
b – a
= 50mV, R
2
/ R
s—less than the transient SOA
2
– V
2
b
t of other transient waveshapes
SNS
ln
)
REG
3
b
a
)2 • t
+
+ 3a
r
)
simplyfying the above to
2
SNS
τ
PK
TMR
2
+ b
= 12mΩ and C
= 80V, V
LT4356-1
2
(W
(W
2
4ab
s)
2
s)
TMR
IN
11
= 12V,
is the
43561fd
TMR
43561 F04