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 )

where R(t) is the probability of a device not failing prior to some

time, t.

If we substitute

= 1/MTBF in the above formula, then the

expression becomes

R(t )

= e

–t

MTBF

This formula is the correct way to interpret the meaning of

MTBF.

If we assume t = MTBF = 1,000,000 hours, then the probability

that a power supply will not fail prior to 1,000,000 hours of use

is e

supply will last 1,000,000 hours before it fails. The probability

that the power supply will not fail prior to 50,000 hours of use is

e

is e

Temperature and Environmental Factors: Although the

calculation of MTBF per MIL-HDBK-217 is a detailed process,

there are two key variables that give the manufacturer significant

leeway in predicting an MTBF rating. These two variables are

temperature and environmental factor. Therefore, for users to

properly compare MTBF numbers from two different manufac-

turers, the environmental factor and the temperature must be

identical. Contact the factory for MTBF calculations for specific

environmental factors and temperatures.

MECHANICAL CONSIDERATIONS

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

The 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