LM612MN datasheet(7/10 Pages) NSC

7 / 10 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

TLH11058 – 8

FIGURE 1 124V Reference is Developed between

Cathode and Anode Current Source IR is External

The reference equivalent circuit reveals how VR is held at

the constant 12V by feedback for a wide range of reverse

current

TLH11058 – 9

FIGURE 2 Reference Equivalent Circuit

To generate the required reverse current typically a resistor

is connected from a supply voltage higher than the refer-

ence voltage to the Reference Output pin Varying that volt-

age 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

TLH11058 – 10

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 the reference is stable

for any value of capacitance With the reference’s wide sta-

bility range with resistive and capacitive loads a wide range

of RC filter values will perform noise filtering when neces-

sary

Reference Hysteresis

The reference voltage depends slightly on the thermal his-

tory of the die Competitive micro-power products varyal-

ways check the datasheet for any given device Do not as-

sume that no specification means no hysteresis

COMPARATORS

Either comparator or the reference may be biased in any

way with no effect on the other sections of the LM612 ex-

cept when a substrate diode conducts (see Electrical Char-

acteristics Note 3) For example one or both inputs of one

comparator may be outside the input voltage range limits

the reference may be unpowered and the other comparator

will still operate correctly The inverting input of an unused

comparator should be tied to Vb and the non-inverting tied

to Va

Hysteresis

Any comparator may oscillate or produce a noisy output if

the applied differential input voltage is near the compara-

tor’s offset voltage This usually happens when the input

signal is moving very slowly across the comparator’s switch-

ing threshold This problem can be prevented by the addi-

tion of hysteresis or positive feedback as shown in

Figure

TLH11058 – 11

FIGURE 4 RS and RF Add Hysteresis to Comparator

The amount of hysteresis added in

Figure 4 is

VH e Va c

(RF a RS)

Va c

for RF ll RS

A good rule of thumb is to add hysteresis of at least the

maximum specified offset voltage More than about 50 mV

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