LM615 datasheet(8/14 Pages) NSC

8 / 14 page

Application Information

VOLTAGE REFERENCE

Reference Biasing

The voltage reference is of a shunt regulator topology that

models as a simple zener diode With current Ir flowing in

the ‘‘forward’’ direction there is the familiar diode transfer

function Ir flowing in the reverse direction forces the refer-

ence voltage to be developed from cathode to anode The

cathode may swing from a diode drop below Vb to the ref-

erence voltage or to the avalanche voltage of the parallel

protection diode nominally 7V A 63V reference with Va e

3V is allowed

TLH11057 – 9

FIGURE 1 Voltage Associated with Reference

(Current Source Ir is External)

The reference equivalent circuit reveals how Vr is held at

the constant 12V 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 refer-

ence voltage Varying that voltage and so varying Ir has

small effect 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 Ir

TLH11057 – 10

FIGURE 2 Reference Equivalent Circuit

TLH11057 – 11

FIGURE 3 12V Reference

Capacitors in parallel with the reference are allowed See

the Reference AC Stability Range typical curve for capaci-

tance valuesfrom 20 mA to 3 mA any capacitor value is

stable With the reference’s wide stability range with resis-

tive and capacitive loads a wide range of RC filter values

will perform noise filtering

Adjustable Reference

The FEEDBACK pin allows the reference output voltage

Vro to vary from 124V to 63V The reference attempts to

hold Vr at 124V If Vr is above 124V the reference will

conduct current from Cathode to Anode FEEDBACK cur-

rent always remains low If FEEDBACK is connected to An-

ode then Vro e Vr e 124V For higher voltages FEED-

BACK is held at a constant voltage above Anodesay

376V for Vro e 5V Connecting a resistor across the con-

stant Vr generates a current I e R1Vr flowing from Cath-

ode into FEEDBACK node A Thevenin equivalent 376V is

generated from FEEDBACK to Anode with R2 e 376I

Keep I greater than one thousand times larger than FEED-

BACK bias current for k01% errorI t 32 mA for the mili-

tary grade over the military temperature range (I t 55 mA

for a 1% untrimmed error for an industrial temperature

range part)

TLH11057 – 12

FIGURE 4 Thevenin Equivalent of

Reference with 5V Output

TLH11057 – 13

R1 e Vr I e 12432m e 39k

R2 e R1 (Vro Vr) b 1 e 39k (5124) b 1 e 118k

FIGURE 5 Resistors R1 and R2 Program

Reference Output Voltage to be 5V

LM615 Scheda tecnica(PDF) 8 Page - National Semiconductor (TI)

LM615 Scheda tecnica(HTML) 8 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Scheda tecnica Scarica

Link URL