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參數(shù)資料
| 型號(hào)： | LM612IN |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 比較器 |
| 英文描述： | Dual-Channel Comparator and Reference |
| 中文描述： | DUAL COMPARATOR, 5000 uV OFFSET-MAX, 1500 ns RESPONSE TIME, PDIP8 |
| 封裝： | PLASTIC, DIP-8 |
| 文件頁(yè)數(shù)： | 7/10頁(yè) |
| 文件大小： | 244K |
| 代理商： | LM612IN |
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
R
flowing in
the ‘‘forward’’ direction there is the familiar diode transfer
function. I
R
flowing in the reverse direction forces the refer-
ence voltage to be developed from cathode to anode.
TL/H/11058–8
FIGURE 1. 1.24V Reference is Developed between
Cathode and Anode; Current Source I
R
is External
The reference equivalent circuit reveals how V
R
is held at
the constant 1.2V by feedback for a wide range of reverse
current.
TL/H/11058–9
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 I
R
, 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 I
R
.
TL/H/11058–10
FIGURE 3. 1.2V Reference
Capacitors in parallel with the reference are allowed. See
the Reference AC Stability Range typical curve for capaci-
tance valuesDfrom 20
m
A 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 varyDal-
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
b
and the non-inverting tied
to V
a
.
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
4.
TL/H/11058–11
FIGURE 4. R
S
and R
F
Add Hysteresis to Comparator
The amount of hysteresis added in Figure 4 is
V
H
e
V
a
c
R
S
(R
F
a
R
S
)
&
V
a
c
R
S
R
F
for R
F
ll
R
S
A good rule of thumb is to add hysteresis of at least the
maximum specified offset voltage. More than about 50 mV
7
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