LM613AIN National Semiconductor, LM613AIN Datasheet - Page 15

• Manufacturer Part Number: LM613AIN
• Description: Dual Operational Amplifiers/ Dual Comparators/ and Adjustable Reference
• Manufacturer: National Semiconductor

Typical Performance Distributions
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 6.3V reference with V
allowed.
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
fect 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
(Continued)
FIGURE 1. Voltage Associated with Reference
r
flowing in the reverse direction forces the reference
Voltage Reference Broad-Band
(current source I
Noise Distribution
r
is external)
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−
r
.
to the reference
r
r
, has small ef-
r
flowing in the
is held at the
+
= 3V is
15
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.
Adjustable Reference
The FEEDBACK pin allows the reference output voltage,
V
hold V
duct current from Cathode to Anode; FEEDBACK current al-
ways 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 = R1/V
node. A Thevenin equivalent 3.76V is generated from FEED-
BACK to Anode with R2 = 3.76/I. Keep I greater than one
thousand times larger than FEEDBACK bias current for
temperature range (I 5.5 µA for a 1% untrimmed error for a
commercial part).
= 5V. Connecting a resistor across the constant V
<
ro
0.1% error — I 32 µA for the military grade over the military
, to vary from 1.24V to 6.3V. The reference attempts to
FIGURE 4. Thevenin Equivalent of Reference
r
ro
at 1.24V. If V
FIGURE 2. Reference Equivalent Circuit
= V
r
= 1.24V. For higher voltages FEEDBACK is
FIGURE 3. 1.2V Reference
r
flowing from Cathode into FEEDBACK
r
is above 1.24V, the reference will con-
with 5V Output
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r
generates
ro