MAX9647 Maxim, MAX9647 Datasheet - Page 9

• Manufacturer Part Number: MAX9647
• Description: The MAX9647/MAX9648 comparators are drop-in, pin-for-pin compatible replacements for the LMX331/LMX331H
• Manufacturer: Maxim

Additional hysteresis can be generated with two resis-
tors using positive feedback
procedure to calculate resistor values:
1) Find output voltage when output is high:
2) Find the trip points of the comparator using these
where V
tor switches its output from high to low as V
the trip point, and V
the comparator switches its output from low to high as
V
3) The hysteresis band is:
In this example, let V
50nA, and R
Figure 1. Threshold Hysteresis Band (Not to Scale)
IN
V
formulas:
V
V
OUT(HIGH)
OUT
drops below the trip point.
IN+
IN-
TH
TL
= V
TH
= V
V
V
HYST
HYST
is the threshold voltage at which the compara-
REF
REF
V
L
V
OUT(HIGH)
V
= 5.1kI.
TH
= 5.0V - (50 x 10
(1 - (R2/(R1 + R2)))
= V
TL
+ ((V
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= 2.5 + 2.5(R2/(R1 + R2))
= 2.5(1 - (R2/(R1 + R2)))
TH
TL
OUT(HIGH)
- V
DD
is the threshold voltage at which
TL
= V
THRESHOLDS
= 5V, V
HYSTERESIS
= V
DD
BAND
(Figure
DD
-9
- I
- V
LOAD
(R2/(R1 + R2))
x 5.1 x 10
REF
REF
2). Use the following
)R2)/(R1 + R2)
= 2.5V, I
x R
IN
L
3
I) ≈ 5.0V
rises above
General-Purpose, Low-Voltage,
V
V
LOAD
TH
TL
=
Select R2. In this example, choose 1kI.
Select V
Solve for R1.
where R1 ≈ 100kI, V
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
The sink current is (V
Choose R
current, yet small enough to supply the necessary cur-
rent to drive the load. R
10kI. Choose R1 to be much larger than R
lowering V
Use 0.1FF bypass capacitors from V
mize performance, minimize stray inductance by putting
this capacitor close to the V
lengths. For slow moving input signals (rise time > 1ms),
use a 1nF capacitor between IN+ and IN- to reduce high
frequency noise.
Figure 2. Adding Hysteresis with External Resistors
Tiny Pack Comparators
V
REF
V
IN
HYST
V
L
OUT(HIGH)
R2
HYST
MAX9647/MAX9648
to be large enough to avoid drawing excess
0.050V = 5(1000/(R1 + 1000))V
. In this example, choose 50mV.
= V
IN+
IN-
Board Layout and Bypassing
OUT(HIGH)
OUT(HIGH)
TH
ir raising V
R1
= 2.525V, and V
L
should be between 1kI and
DD
(R2/(R1 + R2))V
V
V
V
DD
OUT(LOW)
DD
- V
SS
REF
pin and reducing trace
REF
MAX9647
/(R1 + R2).
DD
) x (R1 + R2).
R
to V
L
TL
.
OUT
= 2.475V
SS
L
. To maxi-
to avoid
V
OUT