ARM28XXT SERIES

(28V Input, Triple Output)

8

2016-07-01

Output Short Circuit Protection

Input Under voltage Protection

A minimum voltage is required at the input of the converter to

initiate operation. This voltage is set to a nominal value of

16.8 volts. To preclude the possibility of noise or other

variations at the input falsely initiating and halting converter

operation, a hysteresis of approximately 1.0 volts is incorporated in

this circuit. The converter is guaranteed to operate at 19 Volts

input under all specified conditions.

Input Filter

Protection against accidental short circuits on any output

is provided in the ARM28XXT converters. This protection

is implemented by sensing primary switching current and,

when an over-current condition is detected, switching action is

terminated and a restart cycle is initiated. If the short circuit

condition has not been cleared by the time the restart

cycle has completed, another restart cycle is initiated.

The sequence will repeat until the short circuit condition is

cleared at which time the converter will resume normal

operation. The effect is that during a shorted condition, a

series of narrow pulses are generated at approximately

5% duty cycle which periodically sample the state of the

load. Thus device power dissipation is greatly reduced

during this mode of operation.

Fig. III. Synchronization Input Equivalent Circuit

Parallel Operation

Although no special provision for forced current sharing

has been incorporated in the ARM28XXT Series, multiple

units may be operated in parallel for increased output

power applications. The 5 volt outputs will typically share

to within approximately 10% of their full load capability

and the dual (±15 volt) outputs will typically share to within

50% of their full load. Load sharing is a function of the

individual impedance of each output and the converter

with the highest nominal set voltage will furnish the predominant

load current.

Additional Filtering

To attenuate input ripple current, the ARM28XXT Series

converters incorporate a single stage LC input filter. The

elements of this filter comprise the dominant input load

impedance characteristic, and therefore determine the

nature of the current inrush at turn-on.The input filter circuit

elements are as shown in Fig. IV.

Fig. IV. Input Filter Circuit

Although internal filtering is provided at both the input and

output terminals of the ARM2800 series, additional filtering may

be desirable in some applications to accommodate more

stringent system requirements.

While the internal input filter of Fig. IV keeps input ripple

current below 100 mAp-p, an external filter is available that

will further attenuate this ripple content to a level

below the CE03 limits imposed by MIL-STD-461B. Fig. V

is a general diagram of the Advanced Analog ARF461

filter module designed to operate in conjunction with the

ARM2800 Series converters to provide that attenuation.

5K

2N2907A

5K

47pf

Sync

Input

Input

Return

+10V

+ Input

Input

Return

3.6 µH

5.4 µfd

10



Fig. V. ARF461 Input EMI Filter

This circuit as shown in Fig. V is constructed using the

same quality materials and processes as those employed

in the ARM2800 Series converters and is intended for use

in the same environments. This filter is fabricated in a

complementary package style whose output pin configuration

allows pin to pin connection between the filter and the

converter. More complete information on this filter can be

obtained from the ARF461 data sheet.