AFL5005S Datasheet, PDF (8/11 Pages) International Rectifier – ADVANCED ANALOG HIGH RELIABILITY HYBRID DC/DC CONVERTERS

AFL50XXS Series
Input Filter
The AFL50XXS 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.
Figure IV. Input Filter Circuit
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 Adjustment
OUT
Pin 1
0.75µH
Pin 2
2.7µfd
AFL50xxS
Enable 2
Share
+ Sense
- Sense
Return
+ V_out
R _ADJ
To Load
Undervoltage Lockout
A minimum voltage is required at the input of the converter
to initiate operation. This voltage is set to 26.5 ± 1.5 volts. To
preclude the possibility of noise or other variations at the
input falsely initiating and halting converter operation, a hys-
teresis of approximately 2 volts is incorporated in this cir-
cuit. Thus if the input voltage droops to 24.5 ± 1.5 volts, the
converter will shut down and remain inoperative until the
input voltage returns to ≈ 25 volts.
Output Voltage 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
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 following equation.
Radj
=
100
•
_VOUT
-
VNOM
VNOM
-.025

Note: R_adj 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
subsequent operation to failure. Under no circumstance
should the external setting resistor be made less than 500W.
By remaining within this specified range of values, com-
pletely safe operation fully within normal component derat-
ing limits 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 AFL50XXS
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 limiting value of
output voltage as the adjusting resistor becomes very large
is ≈ 25mV above nominal device voltage.
The consequence is that if the +sense connection is unin-
tentionally broken, an AFL50XXS has a fail-safe output volt-
age 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.
Where
V_NOM = device nominal output voltage, and
V_OUT = desired output voltage
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