lm614amn National Semiconductor Corporation, lm614amn Datasheet - Page 13

• Manufacturer Part Number: lm614amn
• Description: Quad Operational Amplifier And Adjustable Reference
• Manufacturer: National Semiconductor Corporation

Application Information
VOLTAGE REFERENCE
Reference Biasing
The voltage reference is of a shunt regulator topology that
models as a simple zener diode. With current I
“forward” direction there is the familiar diode transfer func-
tion. I
voltage to be developed from cathode to anode. The cath-
ode may swing from a diode drop below V
voltage or to the avalanche voltage of the parallel protection
diode, nominally 7V. A 5.0V reference with V
lowed.
The reference equivalent circuit reveals how V
constant 1.2V by feedback, and how the FEEDBACK pin
passes little current.
To generate the required reverse current, typically a resistor
is connected from a supply voltage higher than the reference
voltage. Varying that voltage, and so varying I
effect with the equivalent series resistance of less than an
ohm at the higher currents. Alternatively, an active current
source, such as the LM134 series, may generate I
Adjustable Reference
The FEEDBACK pin allows the reference output voltage,
V
hold V
conduct current from Cathode to Anode; FEEDBACK current
always remains low. If FEEDBACK is connected to Anode,
then V
held at a constant voltage above Anode — say 3.76V for V
a current I=V
node. A Thevenin equivalent 3.76V is generated from FEED-
BACK to Anode with R2=3.76/I. For a 1% error, use R1 such
that I is greater than one hundred times the FEEDBACK bias
current. For example, keep I
= 5V. Connecting a resistor across the constant V
ro
, to vary from 1.24V to 5.0V. The reference attempts to
FIGURE 1. Voltages Associated with Reference
r
r
ro
flowing in the reverse direction forces the reference
at 1.24V. If V
= V
r
(Current Source I
r
= 1.24V. For higher voltages FEEDBACK is
/R1 flowing from Cathode into FEEDBACK
r
is above 1.24V, the reference will
5.5µA.
r
is External)
00932609
−
to the reference
r
r
+
flowing in the
is held at the
r
, has small
= 3V is al-
r
generates
r
.
ro
13
Capacitors in parallel with the reference are allowed. See the
Reference AC Stability Range typical curve for capacitance
values — from 20 µA to 3 mA any capacitor value is stable.
With the reference’s wide stability range with resistive and
capacitive loads, a wide range of RC filter values will perform
noise filtering.
FIGURE 2. Reference Equivalent Circuit
FIGURE 4. Thevenin Equivalent
of Reference with 5V Output
FIGURE 3. 1.2V Reference
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