AFL120XXS Series
Input Filter
The AFL120XXS series converters incorporate a LC input
filter whose elements dominate the input load impedance
characteristic at turn-on. The input circuit is as shown in
Figure IV.
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 VOUT Adjustment
Figure IV. Input Filter Circuit
Enable
2
16.8uH
Share
Sense
Sense
Return
R ADJ
Pin 1
+
-
AFL120xxS
0.78uF
To Load
+
Vout
Pin 2
Note: Radj must be set ≥ 500Ω
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 500Ω. By
remaining within this specified range of values, completely
safe operation fully within normal component derating limits
is assured.
Undervoltage Lockout
A minimum voltage is required at the input of the converter
to initiate operation. This voltage is set to 74 ± 4 volts. To
preclude the possibility of noise or other variations at the
input falsely initiating and halting converter operation, a hys-
teresis of approximately 7 volts is incorporated in this cir-
cuit. Thus if the input voltage droops to 67 ± 4 volts, the
converter will shut down and remain inoperative until the
input voltage returns to ≈ 74 volts.
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
AFL120XXS 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-
OutputVoltage Adjust
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 volt-
age setting variations are obtained by connecting an appro-
priate 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-
comes very large is ≈ 25mV above nominal device voltage.
The consequence is that if the +sense connection is unin-
tentionally broken, an AFL120XXS 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 toVout + 440mV. This
440 mV is also essentially constant independent of the nomi-
nal output voltage.
mined by use of the following equation.
NOM
V
adj
R
= 100•
OUT
NOM
V
- V
-.025
Where VNOM = device nominal output voltage, and
VOUT = desired output voltage
8
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