ARM2815T/EM 查看數據表（PDF） - International Rectifier
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ARM2815T/EM Datasheet PDF : 13 Pages
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ARM28XXT Series
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.
Figure III. Synchronization Input Equivalent Circuit
+10V
5K
Sync
Input
47pf
2N2907A
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.
Input Undervoltage 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
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.
Figure IV. Input Filter Circuit
10 Ω
5K
Input
Return
Output Short Circuit Protection
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.
8
+ Input
Input
Return
3.6 µH
5.4 µfd
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