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參數資料
| 型號： | LM611 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 2.54mm CGrid Hdr Shrd /Slt .76AuLF 20Ckt RoHS Compliant: Yes |
| 中文描述： | 運算放大器和可調參考 |
| 文件頁數： | 11/17頁 |
| 文件大小： | 735K |
| 代理商： | LM611 |
Application Information
(Continued)
The reference equivalent circuit reveals how V
is held at the
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
, has small ef-
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
r
.
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
, to vary from 1.24V to 6.3V. The reference attempts to
hold V
at 1.24V. If V
is above 1.24V, the reference will con-
duct current from Cathode to Anode; FEEDBACK current al-
ways remains low. If FEEDBACK is connected to Anode,
then V
= V
= 1.24V. For higher voltages FEEDBACK is
held at a constant voltage above Anode— say 3.76V for V
ro
=5V. Connecting a resistor across the constant V
generates
a current I=R1/V
flowing from Cathode into FEEDBACK
node.AThevenin 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
<
0.1% error— I
≥
32 μAfor the military grade over the military
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
is fixed and that voltage sources, re-
sistors, and capacitors may be tied to the FEEDBACK pin, a
range of V
r
temperature coefficients may be synthesized.
DS009221-15
FIGURE 2. Reference Equivalent Circuit
DS009221-16
FIGURE 3. 1.2V Reference
DS009221-17
FIGURE 4. Thevenin Equivalent of
Reference with 5V Output
DS009221-18
FIGURE 5. Resistors R1 and R2 Program
Reference Output Voltage to be 5V
DS009221-19
FIGURE 6. Output Voltage has Negative Temperature
Coefficient (TC) if R2 has Negative TC
DS009221-20
FIGURE 7. Output Voltage has Positive TC
if R1 has Negative TC
www.national.com
11
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