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9

ARH28XXS Series

When an external frequency source is not available, an

internal clock signal is provided through appropriate

buffering at the sync out port. This port can drive a

minimum of 3 ARH sync in ports thereby allowing all to

operate at the same clock frequency.

Output Short Circuit Protection

Protection against accidental short circuits on the output

is provided in the ARH28XXS converter. This protection is

implemented by sensing primary switching current and

reducing the switching pulse widths when a short occurs.

The output current is therefore limited to a maximum value,

which protects the converter. Under this condition the

internal power dissipation is nearly the same as for

maximum loading.

Input Undervoltage Protection

Input Filter

Figure III. Input Filter

EMI Filtering

Output Noise

Although the internal filtering provided at both input and

output terminals of the ARH series converters is sufficient

for most applications, some critical applications may

require additional filtering in order to accommodate particular

system requirements.

While the internal input filter maintains input ripple current

below 50 mA

p-p,

an external filter can be applied to further

attenuate this ripple to a level below the CE03 limits imposed

by MIL-STD-461. International Rectifier currently supplies

such a filter housed in a complementary package. The

catalog number of this part is ARF461.

When attempting noise measurement at the output of

switching converters, measurement techniques employed

can have a significant influence on results during these

tests. Any noise measurements should be undertaken

only with test leads dressed as close to the device output

pins as is physically possible. Probe ground leads should

be kept to a minimum (<< 1") length to minimize the influence

of parasitic impedances on results.

To attenuate input ripple current, the ARH28XXS series

converters incorporate a two-stage LC input filter illustrated

in Figure III following. The elements of this filter comprise

the dominant input load impedance characteristic, and

therefore determine the nature of the current inrush at

turn-on.

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

voltage variations at the input falsely initiating and halting

converter operation, a hysteresis of approximately 1.0

volts is incorporated into this circuit. The converter is

guaranteed to operate at 18 Volts input under all specified

conditions.