ARM28XXT SERIES

(28V Input, Triple Output)

6

2016-07-01

Fig I. Block Diagram

Circuit Operation and Application Information

The ARM28XXT Series of converters have been de-

signed using a single ended forward switched mode con-

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

some advantage in radiation hardened designs in that they elimi-

nate the possibility of simultaneous turn on of both switching ele-

ments during a radiation induced upset; in addition, single

ended topologies are not subject to transformer saturation prob-

lems often associated with double ended implementa-

tions.

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,

under voltage lockout and an external synchronization

port permitting operation at an externally set clock rate.

Operating Guidelines

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 mindful of the results. Characteristic curves

illustrating typical regulation performance are shown in Figures

VII, VIII and IX.

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.