Chip information – Rainbow Electronics LMX339 User Manual
Page 8
LMX331/LMX393/LMX339
General-Purpose, Low-Voltage,
Single/Dual/Quad, Tiny-Pack Comparators
8
_______________________________________________________________________________________
1) Find output voltage when output is high:
V
OUT(HIGH)
= V
DD
- I
LOAD
✕
R
L
2) Find the trip points of the comparator using these
formulas:
V
TH
= V
REF
+ ((V
OUT(HIGH)
- V
REF
)R2) / (R1 + R2)
V
TL
= V
REF
(1 - (R2 / (R1 + R2)))
where V
TH
is the threshold voltage at which the com-
parator switches its output from high to low as V
IN
rises
above the trip point, and V
TL
is the threshold voltage at
which the comparator switches its output from low to
high as V
IN
drops below the trip point.
3) The hysteresis band will be:
V
HYST
= V
TH
- V
TL
= V
DD
(R2 / (R1 + R2))
In this example, let V
DD
= 5V, V
REF
= 2.5V, I
LOAD
=
50nA, R
L
= 5.1k
Ω:
V
OUT(HIGH)
= 5.0V - (50
✕
10
-9 ✕
5.1
✕
10
3
Ω) ≈ 5.0V
V
TH
= 2.5V + 2.5V(R2 / (R1 + R2))
V
TL
= 2.5V(1 - (R2 / (R1 + R2)))
Select R2. In this example, we will choose 1k
Ω.
Select V
HYST
. In this example, we will choose 50mV.
Solve for R1:
V
HYST
= V
OUT(HIGH)
(R2 / (R1 + R2)) V
0.050V = 5(1000 / (R1 + 1000)) V
where R1
≈ 100kΩ, V
TH
= 2.525V, and V
TL
= 2.475V.
Choose R1 and R2 to be large enough as not to exceed
the amount of current the reference can supply.
The source current required is V
REF
/ (R1 + R2).
The sink current is (V
OUT(HIGH)
- V
REF
)
✕
(R1 + R2).
Choose R
L
to be large enough to avoid drawing excess
current, yet small enough to supply the necessary cur-
rent to drive the load. R
L
should be between 1k
Ω and
10k
Ω.
Board Layout and Bypassing
Use 0.1µF bypass capacitors from V
DD
to V
SS
. To max-
imize performance, minimize stray inductance by
putting this capacitor close to the V
DD
pin and reduc-
ing trace lengths. For slow-moving input signals (rise
time > 1ms), use a 1nF capacitor between IN+ and IN-
to reduce high-frequency noise.
Chip Information
LMX331/LMX331H TRANSISTOR COUNT: 112
LMX393/LMX393H TRANSISTOR COUNT: 211
LMX339/LMX339H TRANSISTOR COUNT: 411
LMX331
R1
R2
IN+
IN-
V
DD
V
OUT
V
SS
V
REF
V
IN
V
DD
OUT
R
L
Figure 2. Adding Hysteresis with External Resistors