lm615amn National Semiconductor Corporation, lm615amn Datasheet - Page 8
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lm615amn
Manufacturer Part Number
lm615amn
Description
Lm615 Quad Comparator And Adjustable Reference
Manufacturer
National Semiconductor Corporation
Datasheet
1.LM615AMN.pdf
(14 pages)
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
cathode may swing from a diode drop below V
erence 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
the 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 refer-
ence voltage Varying that voltage and so varying I
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
FIGURE 1 Voltage Associated with Reference
FIGURE 2 Reference Equivalent Circuit
r
flowing in the reverse direction forces the refer-
(Current Source I
FIGURE 3 1 2V Reference
r
is External)
TL H 11057 – 11
b
TL H 11057 – 10
r
r
TL H 11057 – 9
to the ref-
flowing in
is held at
r
a
has
e
r
8
Capacitors in parallel with the reference are allowed See
the Reference AC Stability Range typical curve for capaci-
tance values from 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
V
hold V
conduct current from Cathode to Anode FEEDBACK cur-
rent always remains low If FEEDBACK is connected to An-
ode then V
BACK is held at a constant voltage above Anode say
3 76V for V
stant V
ode into FEEDBACK node A Thevenin equivalent 3 76V is
generated from FEEDBACK to Anode with R2
Keep I greater than one thousand times larger than FEED-
BACK bias current for
tary grade over the military temperature range (I
for a 1% untrimmed error for an industrial temperature
range part)
R1
R2
ro
e
e
to vary from 1 24V to 6 3V The reference attempts to
V
R1 (V
r
r
r
I
FIGURE 5 Resistors R1 and R2 Program
at 1 24V If V
generates a current I
e
ro
1 24 32m
Reference Output Voltage to be 5V
ro
FIGURE 4 Thevenin Equivalent of
ro
V
r
e
)
e
b
Reference with 5V Output
5V Connecting a resistor across the con-
1
V
e
r
e
e
39k
k
39k (5 1 24)
r
is above 1 24V the reference will
0 1% error I
1 24V For higher voltages FEED-
e
b
R1 V
1
e
t
r
118k
flowing from Cath-
32 mA for the mili-
TL H 11057 – 12
TL H 11057 – 13
e
t
5 5 mA
3 76 I