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ADDC02812DA/ADDC02815DA
REV. A
–17–
What users should be interested in is the probability of a power
supply not failing prior to some time t. Given the assumption of
a constant failure rate, this probability is defined as
R
(
t
)
=
e
±
λ
t
where
R
(
t
) is the probability of a device not failing prior to some
time, t.
If we substitute
l
= 1/MT BF in the above formula, then the
expression becomes
R
(
t
)
=
e
±
t
MTBF
T his formula is the correct way to interpret the meaning of
MT BF.
If we assume t = MT BF = 1,000,000 hours, then the probability
that a power supply will not fail prior to 1,000,000 hours of use
is e
–1
, or 36.8%. T his is quite different from saying the power
supply will last 1,000,000 hours before it fails. T he probability
that the power supply will not fail prior to 50,000 hours of use is
e
–.05
, or 95%. For t = 10,000 hours, the probability of no failure
is e
–.01
, or 99%.
T emperature and E nvironmental Factors
: Although the
calculation of MT BF per MIL-HDBK -217 is a detailed process,
there are two key variables that give the manufacturer significant
leeway in predicting an MT BF rating. T hese two variables are
temperature and environmental factor. T herefore, for users to
properly compare MT BF numbers from two different manufac-
turers, the environmental factor and the temperature must be
identical. Contact the factory for MT BF calculations for specific
environmental factors and temperatures.
ME CHANICAL CONSIDE RAT IONS
When mounting the converter into the next higher level assem-
bly, it is important to insure good thermal contact is made
between the converter and the external heat sink. Poor thermal
connection can result in the converter shutting off, due to the
temperature shutdown feature (Pin 9), or reduced reliability for
the converter due to higher than anticipated junction and case
temperatures. For these reasons the mounting tab locations
were selected to insure good thermal contact is made near the
hot spots of the converter which are shown in the shaded areas
of Figure 26.
Figure 26. Hot Spots (Shaded Areas) of DC/DC Converter
T he pins of the converter are typically connected to the next
higher level assembly by bending them at right angles, either
down or up, and cutting them shorter for insertion in printed
circuit board through holes. In order to maintain the hermetic
integrity of the seals around the pins, a fixture should be used
for bending the pins without stressing the pin-to-sidewall seals.
It is recommended that the minimum distance between the
package edge and the inside of the pin be 100 mils (2.54 mm)
for the 40 mil (1.02 mm) diameter pins; 120 mils (3.05 mm)
from the package edge to the center of the pin as shown in
Figure 27.
0.100"
(2.54mm)
0.120"
(3.05mm)
Figure 27. Minimum Bend Radius of 40 Mil (1.02 mm) Pins