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ARM28XXT Series

Figure I. Block Diagram

Circuit Description and Application Information

The ARM28XXT series of converters have been designed

using a single ended forward switched mode converter

topology. (refer to Figure I.) Single ended topologies enjoy

some advantage in radiation hardened designs in that they

eliminate the possibility of simultaneous turn on of both

switching elements during a radiation induced upset; in

addition, single ended topologies are not subject to

transformer saturation problems often associated with

double ended implementations.

The design incorporates an LC input filter to attenuate

input ripple current. A low overhead linear bias regulator

is used to provide bias voltage for the converter primary

control logic and a stable, well regulated reference for the

error amplifier. Output control is realized using a wide band

discrete pulse width modulator control circuit incorporating

a unique non-linear ramp generator circuit. This circuit

helps stabilize loop gain over variations in line voltage for

superior output transient response. Nominal conversion

frequency has been selected as 250 KHz to maximize

efficiency and minimize magnetic element size.

Output voltages are sensed using a coupled inductor and

a patented magnetic feedback circuit. This circuit is

relatively insensitive to variations in temperature, aging,

radiation and manufacturing tolerances making it

particularly well suited to radiation hardened designs. The

control logic has been designed to use only radiation

tolerant components, and all current paths are limited with

series resistance to limit photo currents.

Other key circuit design features include short circuit

protection, undervoltage lockout and an external

synchronization port permitting operation at an externally

set clock rate.

Operating Guidelines

Thermal Considerations

The ARM series of converters is capable of providing

relatively high output power from a package of modest

volume. The power density exhibited by these devices is

obtained by combining high circuit efficiency with effective

methods of heat removal from the die junctions. Good

design practices have

effectively addressed this

requirement inside the device. However when operating

at maximum loads, significant heat generated at the die

junctions must be carried away by conduction from the

base. To maintain case temperature at or below the

specified maximum of 125°C, this heat can be transferred

by attachment to an appropriate heat dissipater held in

intimate contact with the converter base-plate.

The circuit topology used for regulating output voltages in

the ARM28XXT series of converters was selected for a

number of reasons. Significant among these is the ability

to simultaneously provide adequate regulation to three

output voltages while maintaining modest circuit complexity.

These attributes were fundamental in retaining the high

reliability and insensitivity to radiation that characterizes

device performance. Use of this topology dictates that the

user maintain the minimum load specified in the electrical

tables on each output. Attempts to operate the converter

without a load on any output will result in peak charging to

an output voltage well above the specified voltage regulation

limits, potentially in excess of ratings, and should be

avoided. Output loads that are less than specification

minimums will result in regulation performance outside the

limits presented in the tables. In most practical applications,

this lower bound on the load range does not present a

serious constraint; however the user should be mindfull of

the results. Characteristic curves illustrating typical

regulation performance are shown in Figures VII, VIII and IX.

11

10

9

13

14

Sample

Hold

+15 Vout

Dual

Output

Return

-15 Vout

Dual

+5 Vdc

Output

Output

Return

Short

Circuit

Pulse Width

Modulator

Primary Bias

& Reference

EMI

Filter

Under-Voltage

Detector

1

3

4

2

+Input

Enable

Sync In

Input

Return