MIC2593 Micrel Semiconductor, MIC2593 Datasheet - Page 23

• Manufacturer Part Number: MIC2593
• Description: Dual-Slot PCI Hot Plug Controller
• Manufacturer: Micrel Semiconductor

50ms before the MIC2593 circuit breaker trips. During that
time, the dissipation in the MOSFET is given by:
At first glance, it would appear that a really hefty MOSFET is
required to withstand this sort of fault condition. This is where
the transient thermal impedance curves become very useful.
Figure 13 shows the curve for the Vishay (Siliconix) Si4430DY,
a commonly used SO-8 power MOSFET.
Taking the simplest case first, we’ll assume that once a fault
event such as the one in question occurs, it will be a long time,
several seconds, before the fault is isolated and the channel
is reset. In such a case, we can approximate this as a “single
pulse” event, that is to say, there’s no significant duty cycle.
Then, reading up from the X-axis at the point where “Square
Wave Pulse Duration” is equal to 0.1sec (=100msec), we see
that the Z
this duration is only 7% of its continuous R
This particular part is specified as having an R
35 C/W for intervals of 10 seconds or less. Thus:
Assume T
drain leads, no airflow.
Recalling from our previous approximation hint, the part has
an R
Assume it has been carrying just about 5A for some time.
When performing this calculation, be sure to use the highest
anticipated ambient temperature (T
April 2004
MIC2593
ON
P = E
P
MOSFET
of (0.014/2) = 7m at 25 C.
(J-A)
A
= 55 C maximum, 1 square inch of copper at the
I; E
of this MOSFET to a highly infrequent event of
= (2V
MOSFET
0.01
5A) = 10W for 50ms
0.1
Figure 13. Si4430DY MOSFET Transient Thermal Impedance Curve
2
1
= [5V–5A(0.6 )] = 2V
10
-4
Duty Cycle = 0.5
0.2
0.1
0.05
0.02
10
A(MAX)
-3
Normalized Thermal Transient Imperance, Juction-to-Ambient
Single Pulse
(J-A)
) in which the
.
10
-2
(J-A)
Square Wave Pulse Duration (sec)
of
10
23
-1
MOSFET will be operating as the starting temperature, and
find the operating junction temperature increase ( T
that point. Then, as shown next, the final junction temperature
is found by adding T
form equation, getting a close approximation may take one or
two iterations, but it’s not a hard calculation to perform and
tends to converge quickly.
Then the starting (steady-state)T
Iterate the calculation once to see if this value is within a few
percent of the expected final value. For this iteration we will
start with T
So our original approximation of 62.0 C was very close to the
correct value. We will use T
Finally, add (10W)(35 C/W)(0.07) = 24.5 C to the steady-state
T
maximum junction temperature for this part.
J
to get T
T
T
T
T
T
J
J
J
J
J
1
( 55 C + (0.21008W)(35 C/W)
J(TRANSIENT MAX.)
T
T
55 C + [7m
(55 C + (0.20125W)(35 C/W)
62.0 C
T
J
A(MAX)
A(MAX)
A
equal to the already calculated value of 62.0 C:
(5A)
+ [7m + (62.0 C-25 C)(0.005)(7m )]
2
(5A)
+ T
+ [R
A(MAX)
10
I
1. Duty Cycle, D =
2. Per Unit Base = R
3. T
4. Surface Mounted
2
Notes:
P
(35 C/W)
DM
JM
2
ON
J
R
– T
+ (55 C–25 C)(0.005)(7m )]
t
1
A
and T
(J-A)
(35 C/W)
+ (T
t
= P
J
2
= 86.5 C. This is an acceptable
= 62 C.
DM
Z
A(MAX)
100
JA
t
t
1
2
JA
(t)
J
J
= 67°C/W
. Since this is not a closed-
is:
– T
600
A
)(0.005/ C)(R
62.35 C
M9999-042204
J
) from
Micrel
ON
)]