LM612 National Semiconductor, LM612 Datasheet - Page 7

• Manufacturer Part Number: LM612
• Description: Dual-Channel Comparator and Reference
• Manufacturer: National Semiconductor

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
the ‘‘forward’’ direction there is the familiar diode transfer
function I
ence voltage to be developed from cathode to anode
The reference equivalent circuit reveals how V
the constant 1 2V by feedback for a wide range of reverse
current
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 I
series resistance of less than an ohm at the higher currents
Alternatively an active current source such as the LM134
series may generate I
FIGURE 1 1 24V Reference is Developed between
Cathode and Anode Current Source I
FIGURE 2 Reference Equivalent Circuit
R
flowing in the reverse direction forces the refer-
FIGURE 3 1 2V Reference
R
R
has small effect with the equivalent
R
is External
TL H 11058 – 10
R
R
TL H 11058 – 8
TL H 11058 – 9
flowing in
is held at
7
4
Capacitors in parallel with the reference are allowed See
the Reference AC Stability Range typical curve for capaci-
tance values from 20
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 vary al-
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 V
to V
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
The amount of hysteresis added in Figure 4 is
A good rule of thumb is to add hysteresis of at least the
maximum specified offset voltage More than about 50 mV
FIGURE 4 R
V
H
a
e
V
V
a
a
c
c
R
R
S
(R
S
F
and R
F
R
a
S
R
F
S
Add Hysteresis to Comparator
)
A to 3 mA the reference is stable
b
and the non-inverting tied
for R
TL H 11058– 11
F ll
R
S