LM614CWM National Semiconductor, LM614CWM Datasheet - Page 12

• Manufacturer Part Number: LM614CWM
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LM614CWM

• No. Of Amplifiers: 4
• Bandwidth: 800MHz
• Slew Rate: 0.7V/µs
• No. Of Pins: 16
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Package / Case: 16-WSOIC
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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