LM611AMJSLASH883 datasheet(11/17 Pages) NSC

11 / 17 page

Application Information (Continued)

The reference equivalent circuit reveals how V

r 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

r, 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

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,

ro, to vary from 1.24V to 6.3V. The reference attempts to

hold V

r at 1.24V. If Vr 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

ro = Vr = 1.24V. For higher voltages FEEDBACK is

held at a constant voltage above Anode — say 3.76V for V

= 5V. Connecting a resistor across the constant V

r generates

a current I=R1/V

r flowing from Cathode into FEEDBACK

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

<0.1% error — I

≥32 µA for 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

r 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

LM611AMJ/883 Datasheet(PDF) 11 Page - National Semiconductor (TI)

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