LM611MN National Semiconductor, LM611MN Datasheet - Page 11

• Manufacturer Part Number: LM611MN
• Description: Operational Amplifier and Adjustable Reference
• Manufacturer: National Semiconductor

Application Information
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 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
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-
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(Continued)
r
.
r
, has small ef-
r
is held at the
r
generates
ro
11
R1 = Vr/I = 1.24/32µ = 39k
R2 = R1 {(Vro/Vr) − 1} = 39k {(5/1.24) − 1)} = 118k
sistors, and capacitors may be tied to the FEEDBACK pin, a
range of V
Understanding that V
FIGURE 6. Output Voltage has Negative Temperature
FIGURE 7. Output Voltage has Positive TC
FIGURE 5. Resistors R1 and R2 Program
Coefficient (TC) if R2 has Negative TC
r
Reference Output Voltage to be 5V
FIGURE 4. Thevenin Equivalent of
temperature coefficients may be synthesized.
Reference with 5V Output
if R1 has Negative TC
r
is fixed and that voltage sources, re-
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