8

www.irf.com

ARM28XXT Series

Figure III. Synchronization Input Equivalent Circuit

5K

2N2907A

5K

47pf

Sync

Input

Input

Return

+10V

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.

Output Short Circuit Protection

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.

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 Undervoltage Protection

Input Filter

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 Figure IV.

+ Input

Input

Return

3.6 µH

5.4 µfd

10

Ω

Figure IV. Input Filter Circuit

Synchronization

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 Figure 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.