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AFL270XXS Series

level of +2.0 volts. The sync output of another converter

which has been designated as the master oscillator pro-

vides a convenient frequency source for this mode of op-

eration. When external synchronization is not required, the

sync in pin should be left 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

±50 KHz, with a duty cycle of 15 ±5%. This signal is refer-

enced to the input return and has been tailored to be com-

patible with the AFL sync input port. Transition times are

less than 100 ns and the low level output impedance is less

than 50 ohms. This signal is active when the DC input

voltage is within the specified operating range and the con-

verter is not inhibited. This output has adequate drive re-

serve to synchronize at least five additional converters. A

typical synchronization connection option is illustrated in

Figure III.

Figure III. Preferred Connection for Parallel Operation

Optional

Synchronization

Connection

Power

Input

(Other Converters)

Share Bus

1

6

AFL

7

12

- Sense

Enable 2

+ Vout

Return

+ Sense

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

1

6

AFL

7

12

- Sense

Enable 2

+ Vout

Return

+ Sense

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

1

6

AFL

7

12

- Sense

Enable 2

+ Vout

Return

+ Sense

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

to Load

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 temperture, the current it pro-

vides to the load will be reduced as compensation for the

temperature induced stress on that device.

When operating multiple converters, system requirements

often dictate operation of the converters at a common fre-

quency. To accommodate this requirement, the AFL series

converters provide both a synchronization input and out-

put.

The sync input port permits synchronization of an AFL co-

nverter to any compatible external frequency source oper-

ating between 500 and 700 KHz. This input signal should

be referenced to the input return and have a 10% to 90%

duty cycle. Compatibility requires transition times less th an

100 ns, maximum low level of +0.8 volts and a minimum

high

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 sharing of

a load current exceeding the capacity of an individual AFL

among the members of the set. An important feature of the

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 second-

ary side while altering the controller duty cycle to near zero.

Externally, the use of either port is transparent save for

minor differences in idle current. (See specification table).

Synchronization of Multiple Converters

Parallel Operation-Current and Stress Sharing