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15

AHF28XX Series

Application Information

Connecting the enable input (Pin 1) to input common (Pin 7)

will cause the converter to shut down. It is recommended

that the enable pin be driven by an open collector device

capable of sinking at least 400 µA of current. The open

circuit voltage of the enable input is 15 ±1 VDC. If the inhibit

function is not used, this input can be left unconnected

because it is internally pulled-up.

Thermal Management

Assuming that there is no forced air flow, the package tem-

perature rise above ambient (

∆T) may be calculated using

the following expression:

where A = the effective surface area in square inches (in-

cluding heat sink if used), P = power dissipation in watts.

The total surface area of the AHF package is 4.9 square

inches. If a worse case full load efficiency of 78% is as-

sumed, then the case temperature rise can be calculated

as follows:

Hence, if T

AMBIENT

= +25

°C, the DC/DC converter case

temperature will be approximately 100

°C if no heat sink

or air flow is provided.

Inhibit Function

W

Eff

P

P

OUT

4

.

3

1

78

.

0

1

12

1

1

=









−

=













−

=

To calculate the heat sink area required to maintain a

specific case temperature rise, the above equation may

be manipulated as follows:

PKG

Α

−









Ρ

∆Τ

− 43

.

1

85

.

0

80

A

As an example, if a maximum case temperature rise of

50°C rise above ambient is desired, then the required

effective heat sink area is:

2

43

.

1

85

.

0

.

75

.

3

9

.

4

)

4

.

3

(

80

50

in

HEATSINK

=

−













=

Α

−