LM615 National Semiconductor, LM615 Datasheet - Page 8

• Manufacturer Part Number: LM615
• Description: LM615 Quad Comparator and Adjustable 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
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
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 32
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-
A to 3 mA any capacitor value is
e
b
R1 V
1
e
t
r
118k
flowing from Cath-
32 A for the mili-
TL H 11057 – 12
TL H 11057 – 13
e
t
5 5
3 76 I
A