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AFL270XXS Series

The AFL270XXS series converters incorporate a single

stage LC input filter whose elements dominate the input

load impedance characteristic during the turn-on sequence.

The input circuit is as shown in Figure IV.

Figure IV. Input Filter Circuit

8.4µH

Pin 1

Pin 2

0.54µfd

A minimum voltage is required at the input of the converter

to initiate operation. This voltage is set to 150

± 5 volts. To

preclude the possibility of noise or other variations at the

input falsely initiating and halting converter operation, a hys-

teresis of approximately 10 volts is incorporated in this cir-

cuit. Thus if the input voltage droops to 140

± 5 volts, the

converter will shut down and remain inoperative until the

input voltage returns to

≈ 150 volts.

In addition to permitting close voltage regulation of remotely

located loads, it is possible to utilize the converter sense

pins to incrementally increase the output voltage over a

limited range.The adjustments made possible by this method

are intended as a means to “trim” the output to a voltage

setting for some particular application, but are not intended

to create an adjustable output converter. These output

voltage setting variations are obtained by connecting an

appropriate resistor value between the +sense and -sense

pins while connecting the -sense pin to the output return pin

as shown in Figure V. below. The range of adjustment and

corresponding range of resistance values can be deter-

mined by use of the equation presented below.

R

= 100

-

-.025

adj

NOM

OUT

NOM

•













V

VV

Where

V

V

Finding a resistor value for a particular output voltage, is

simply a matter of substituting the desired output voltage

and the nominal device voltage into the equation and solv-

ing for the corresponding resistor value.

Figure V. Connection for V

Enable 2

Share

+ Sense

- Sense

Return

+ V

out

To Load

R

ADJ

AFL270xxS

Caution: Do not set R

adj

< 500

Ω

Attempts to adjust the output voltage to a value greater than

120% of nominal should be avoided because of the poten-

tial of exceeding internal component stress ratings and sub-

sequent operation to failure. Under no circumstance should

the external setting resistor be made less than 500

Ω. By

remaining within this specified range of values, completely

safe operation fully within normal component derating is

assured.

Examination of the equation relating output voltage and re-

sistor value reveals a special benefit of the circuit topology

utilized for remote sensing of output voltage in the

AFL270XXS series of converters. It is apparent that as the

resistance increases, the output voltage approaches the

nominal set value of the device. In fact the calculated limit-

ing value of output voltage as the adjusting resistor be-

comes very large is

≅ 25mV above nominal device voltage.

The consequence is that if the +sense connection is un-

intentionally broken, an AFL270XXS has a fail-safe output

voltage of Vout + 25mV, where the 25mV is independent of

the nominal output voltage. It can be further demonstrated

that in the event of both the + and - sense connections

being broken, the output will be limited to Vout + 440mV.

This 440 mV is also essentially constant independent of the

nominal output voltage. While operation in this condition is

not damaging to the device, not all performance parameters

will be met.

Typical performance data is graphically presented on the follow-

ing pages for selected parameters on a variety of AFL270XXS

type converters. The data presented was selected as repre-

sentative of more critical parameters and for general interest in

typical converter applications.

Input Filter

Undervoltage Lockout

Output Voltage Adjust

Performance Data