lm611cmx National Semiconductor Corporation, lm611cmx Datasheet - Page 14

• Manufacturer Part Number: lm611cmx
• Description: Operational Amplifier And Adjustable Reference
• Manufacturer: National Semiconductor Corporation

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Application Information
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
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
= 5V. Connecting a resistor across the constant 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.)
R1 = Vr/I = 1.24/32µ = 39k
<
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
= V
FIGURE 4. Thevenin Equivalent of
r
= 1.24V. For higher voltages FEEDBACK is
FIGURE 3. 1.2V Reference
Reference with 5V Output
r
flowing from Cathode into FEEDBACK
r
is above 1.24V, the reference will
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(Continued)
r
generates
ro
14
R2 = R1 {(Vro/Vr) − 1} = 39k {(5/1.24) − 1)} = 118k
Understanding that V
resistors, and capacitors may be tied to the FEEDBACK pin,
a range of V
Connecting a resistor across Cathode-to-FEEDBACK cre-
ates a 0 TC current source, but a range of TCs may be
synthesized.
FIGURE 6. Output Voltage has Negative Temperature
Voltage Across R1 and R2 to be Proportional
FIGURE 8. Diode in Series with R1 Causes
FIGURE 7. Output Voltage has Positive TC
FIGURE 5. Resistors R1 and R2 Program
Coefficient (TC) if R2 has Negative TC
Reference Output Voltage to be 5V
r
to Absolute Temperature (PTAT)
temperature coefficients may be synthesized.
if R1 has Negative TC
r
is fixed and that voltage sources,
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