AFL28XXD Series
Internally, these ports differ slightly in their function. In use,
a low on Enable 1 completely shuts down all circuits in the
converter, while a low on Enable 2 shuts down the secondary
side while altering the controller duty cycle to near zero.
Externally, the use of either port is transparent to the user
save for minor differences in idle current. (See specification
table).
than 100ns, maximum low level of +0.8V and a minimumigh
level of +2.0V. The sync output of another converter which
has been designated as the master oscillator provides a
convenient frequency source for this mode of operation.
When external synchronization is not indicted, the sync in
pin should be left open (unconnected) thereby permitting
the converter to operate at its’ own internally set frequency.
The sync output signal is a continuous pulse train set at
550 ± 50KHz, with a duty cycle of 15 ± 5%. This signal is
referenced to the input return and has been tailored to be
compatible with the AFL sync input port. Transition times
are less than 100ns and the low level output impedance is
less than 50Ω. This signal is active when the DC input
voltage is within the specified operating range and the
converter is not inhibited. This synch output has adequate
drive reserve to synchronize at least five additional
converters. A typical synchronization connection option is
illustrated in Figure III.
Synchronization of Multiple Converters
When operating multiple converters, system requirements
often dictate operation of the converters at a common
frequency. To accommodate this requirement, the AFL
series converters provide both a synchronization input and
output.
The sync input port permits synchronization of an AFL
connverter to any compatible external frequency source
operating between 500KHz and 700KHz. This input signal
should be referenced to the input return and have a 10% to
90% duty cycle. Compatibility requires transition times less
Figure III. Preferred Connection for Parallel Operation
Power
Input
1
Vin
Rtn
Case
Enable 1
Sync Out
Sync In
6
Enable 2
Share
12
AFL
Trim
- Output
Return
+ Output
7
Optional
Synchronization
Connection
Share Bus
1
Vin
Rtn
Case
Enable 1
Sync Out
Sync In
6
Enable 2
Share
12
AFL
Trim
- Output
Return
+ Output
7
to Negative Load
to Positive Load
1
Vin
Rtn
Case
Enable 1
Sync Out
Sync In
6
Enable 2
12
AFL
Share
Trim
- Output
Return
+ Output
7
(Other Converters)
Parallel Operation-Current and Stress Sharing
Figure III. illustrates the preferred connection scheme for
operation of a set of AFL converters with outputs operating
in parallel. Use of this connection permits equal current
sharing among the members of a set whose load current
exceeds the capacity of an individual AFL. An important
feature of the AFL series operating in the parallel mode is
that in addition to sharing the current, the stress induced by
temperature will also be shared. Thus if one member of a
paralleled set is operating at a higher case temperature, the
current it provides to the load will be reduced as
compensation for the temperature induced stress on that
device.
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