ARM28XXT SERIES

(28V Input, Triple Output)

7

2016-07-01

Effectiveness of this heat transfer is dependent on the

intimacy of the baseplate-heatsink interface. It is therefore

suggested that a heat transferring medium possessing

good thermal conductivity is inserted between the baseplate and

heat sink. A material utilized at the factory during testing

and burn-in processes is sold under the trade name of

electrically conductive versions are also available. Use of

these materials assures optimum surface contact with the

heat dissipater by compensating for minor surface variations.

While other available types of heat conducting materials

and thermal compounds provide similar effectiveness,

these alternatives are often less convenient and are frequently

messy to use.

A conservative aid to estimating the total heat sink surface area

(

T) above ambient temperature is given by the following ex-

pression:

Where

As an example, assume that it is desired to maintain the

case temperature of an ARM2815T at +65°C or less while

operating in an open area whose ambient temperature

does not exceed +35°C; then

T = 65 - 35 = 35°C

From the Specification Table, the worst case full load efficiency for

this device is 80%; therefore the maximum power dissipation at

full load is given by

and the required heat sink area is

Thus, a total heat sink surface area (including fins, if any)

rise to 35°C above ambient. A flat aluminum plate, 0.25"

would suffice for this application in a still air environment. Note

that to meet the criteria, both sides of the plate require

unrestricted exposure to the ambient air.

Inhibiting Converter Output

Fig. II. Enable Input Equivalent Circuit

Synchronization

As an alternative to application and removal of the DC

voltage to the input, the user can control the converter

output by providing an input referenced, TTL compatible,

logic signal to the enable pin 3. This port is internally

pulled “high” so that when not used, an open connection

on the pin permits normal converter operation. When inhibited

outputs are desired, a logical “low” on this port will shut

the converter down. An open collector device capable of

sinking at least 100 µA connected to enable pin 3 will

work well in this application.

A benefit of utilization of the enable input is that following

initial charge of the input capacitor, subsequent turn-on

commands will induce no uncontrolled current inrush.

 













T

P

80

594

085

143

.

.

.

T

PP

Eff

OUT



















Case temperature rise above ambient

Device dissipation in Watts

1

1



P





















30

1

80

130 0 25

7 5

.

.. W

A

=

35

80 7.5

in

HEAT SINK

0.85

















143

2

594 318

.

..

5K

2N2907A

150K

Enable

Input

Input

Return

150K

2N2222A

2N2222A

V

in

64K

186K

150K

5.6 V

Converter inhibit is initiated when

this transistor is turned off

When using multiple converters, system requirements

may dictate operating several converters at a common

system frequency. To accommodate this requirement, the

ARM28XXT type converter provides a synchronization

input port (pin 4). Circuit topology is as illustrated in Fig. III.

The sync input port permits synchronization of an ARM

converter to any compatible external frequency source

operating in the band of 225 to 310 kHz. The synchronization

input is edge triggered with synchronization initiated on

the negative transition. This input signal should be a negative

going pulse referenced to the input return and have a 20%

to 80% duty cycle. Compatibility requires the negative transition

time to be less than 100 ns with a minimum pulse amplitude of

+4.25 volts referred to the input return. In the event of

failure of an external synchronization source, the converter will

revert to its own internally set frequency. When external

synchronization is not desired, the sync in port may be left

open (unconnected) permitting the converter to operate at its’

own internally set frequency.