LM614MN National Semiconductor, LM614MN Datasheet - Page 12
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LM614MN
Manufacturer Part Number
LM614MN
Description
Quad Operational Amplifier and Adjustable Reference
Manufacturer
National Semiconductor
Datasheet
1.LM614MN.pdf
(17 pages)
www.national.com
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
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 1. Voltages Associated with Reference
r
flowing in the reverse direction forces the reference
FIGURE 2. Reference Equivalent Circuit
(Current Source I
r
is External)
DS009326-10
DS009326-9
−
r
.
to the reference
r
r
, has small ef-
r
flowing in the
is held at the
+
= 3V is
12
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
= 5V. Connecting a resistor across the constant V
a current I = 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.)
R1 = Vr/I = 1.24/32µ = 39k
R2 = R1 {(Vro/Vr) − 1} = 39k {(5/1.24) − 1)} = 118k
Understanding that V
sistors, and capacitors may be tied to the FEEDBACK pin, a
range of 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
r
ro
at 1.24V. If V
FIGURE 5. Resistors R1 and R2 Program
= V
r
Reference Output Voltage to be 5V
temperature coefficients may be synthesized.
FIGURE 4. Thevenin Equivalent
r
r
of Reference with 5V Output
= 1.24V. For higher voltages FEEDBACK is
/R1 flowing from Cathode into FEEDBACK
FIGURE 3. 1.2V Reference
r
r
is above 1.24V, the reference will con-
is fixed and that voltage sources, re-
DS009326-11
DS009326-12
DS009326-13
r
generates
ro