Datasheet

LTC4364-1/LTC4364-2
15
436412f
gate and source of each MOSFET for extra protection
(Figures 8 to 10).
Transient Stress in the MOSFET
The SOA of the MOSFET must encompass all fault condi-
tions. In normal operation the pass devices are fully on,
dissipating very little power. But during either overvoltage or
overcurrent faults, the HGATE pin is controlled to regulate
either the output voltage or the current through MOSFET
M1. Large current and high voltage drop across M1 can
coexist in these cases. The SOA curves of the MOSFET
must be considered carefully along with the selection of
the fault timer capacitor.
During an overvoltage event, the LTC4364 drives the pass
MOSFET M1 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 model shown
in Figure 5. The idealized waveform comprises a linear
ramp of rise time t
r
, reaching a peak voltage of V
PK
and
exponentially decaying back to V
IN
with a time constant
of τ. A typical automotive transient specification has
constants of t
r
= 10μs, V
PK
= 80V and τ = 1ms. A surge
condition known as “load dump” has constants of t
r
=
5ms, V
PK
= 60V and τ = 200ms.
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 analyzed by simulation, using the
MOSFET’s thermal model.
For short duration transients of less than 100ms, MOS-
FET survival is increasingly a matter of SOA, an intrinsic
property of the MOSFET. SOA quantifies the time required
at any given condition of V
DS
and I
D
to raise the junction
temperature of the MOSFET to its rated maximum. MOSFET
SOA is expressed in units of watt-squared-seconds (P
2
t),
which is an integral of P(t)
2
dt over the duration of the
transient. This figure is essentially constant for intervals
of less than 100ms for any given device type, and rises
to infinity under DC operating conditions. Destruction
mechanisms other than bulk die temperature distort the
lines of an accurately drawn SOA graph so that P
2
t is not
the same for all combinations of I
D
and V
DS
. In particular
P
2
t tends to degrade as V
DS
approaches 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 of M1 must be calculated for each input
transient which shall not interrupt operation. It is then
a simple matter to choose a device which has adequate
SOA to survive the maximum calculated stress. P
2
t for a
prototypical transient waveform is calculated as follows
(Figure 6).
Let:
a = V
REG
– V
IN
b = V
PK
– V
IN
where V
IN
= Nominal Input Voltage.
V
PK
τ
V
IN
436412 F05
t
r
Figure 5. Prototypical Transient Waveform
Figure 6. Safe Operating Area Required to Survive Prototypical
Transient Waveform
V
PK
= 80V
τ
= 1ms
V
IN
= 12V
436412 F06
V
REG
= 16V
t
r
= 10µs
APPLICATIONS INFORMATION