AFL28XXS Series
Input Filter
The AFL28XXS series converters incorporate a two stage
LC input filter whose elements dominate the input load
impedance characteristic during the turn-on. The input
circuit is as shown in Figure IV.
Figure V. Connection for V
OUT
Adjustment
Enable 2
Share
Figure IV. Input Filter Circuit
900nH
130nH
AFL28xxS
+ Sense
- Sense
Return
R
ADJ
To Load
+ V
out
Pin 1
Note: R
adj
must be set
≥
500Ω
6 µfd
11.2 µfd
Pin 2
Undervoltage Lockout
A minimum voltage is required at the input of the converter
to initiate operation. This voltage is set to 14V
±
0.5V. To
preclude the possibility of noise or other variations at the
input falsely initiating and halting converter operation, a
hysteresis of approximately 1.0V is incorporated in this
circuit. Thus if the input voltage droops to 13V
±
0.5V, the
converter will shut down and remain inoperative until the
input voltage returns to
≈14V.
Attempts to adjust the output voltage to a value greater than
120% of nominal should be avoided because of the potential
of exceeding internal component stress ratings and
subsequent 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.
Examination of the equation relating output voltage and
resistor value reveals a special benefit of the circuit topology
utilized for remote sensing of output voltage in the AFL28XXS
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
unintentionally broken, an AFL28XXS 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 440mV is also essentially constant independent of the
nominal output voltage.
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 determined
by use of the following equation.
General Application Information
The AFL28XXS series of converters are capable of
providing large transient currents to user loads on demand.
Because the nominal input voltage range in this series is
relatively low, the resulting input current demands will be
correspondingly large. It is important therefore, that the line
impedance be kept very low to prevent steady state and
transient input currents from degrading the supply voltage
between the voltage source and the converter input. In
applications requiring high static currents and large
transients, it is recommended that the input leads be made
of adequate size to minimize resistive losses, and that a
good quality capacitor of approximately 100µfd be
connected directly across the input terminals to assure an
adequately low impedance at the input terminals. Table I
relates nominal resistance values and selected wire sizes.
⎫
⎧
V
NOM
⎬
R
adj
= 100
• ⎨
⎩
V
OUT
-
V
NOM
-.025
⎭
Where
V
NOM
= device nominal output voltage, and
V
OUT
= desired output voltage
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 solving
for the corresponding resistor value.
8
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