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

+

= 3V is

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.

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

ro

= 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

≥32 µA for the military grade over the military

temperature range (I

≥5.5 µA for a 1% untrimmed error for a

commercial part.)

Understanding that V

sistors, and capacitors may be tied to the FEEDBACK pin, a

range of V

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FIGURE 1. Voltages Associated with Reference

(Current Source I

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FIGURE 2. Reference Equivalent Circuit

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FIGURE 3. 1.2V Reference

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FIGURE 4. Thevenin Equivalent

of Reference with 5V Output

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R1 = Vr/I = 1.24/32µ = 39k

R2 = R1 {(Vro/Vr) − 1} = 39k {(5/1.24) − 1)} = 118k

FIGURE 5. Resistors R1 and R2 Program

Reference Output Voltage to be 5V

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