Datasheet

MAX9117–MAX9120
SC70, 1.6V, Nanopower, Beyond-the-Rails
Comparators With/Without Reference
______________________________________________________________________________________ 11
THRESHOLDS
OUT
IN+
IN-
V
HB
HYSTERESIS
BAND
V
THF
V
THR
Figure 2. Threshold Hysteresis Band
V
CC
MAX9117
MAX9119
OUT
R3
R1
R2
V
REF
V
EE
V
IN
V
CC
Figure 3. MAX9117/MAX9119 Additional Hysteresis
MAX9117 (V
REF
= 1.252V) and V
CC
= +5V, and if
we choose I
R3
= 1µA, then the two resistor values
are 1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard
value for R3.
2) Choose the hysteresis band required (V
HB
). For this
example, choose 50mV.
3) Calculate R1 according to the following equation:
R1 = R3 (V
HB
/ V
CC
)
For this example, insert the values:
R1 = 1.2MΩ (50mV / 5V) = 12kΩ
4) Choose the trip point for V
IN
rising (V
THR
) such that
V
THR
> V
REF
✕
(R1 + R3) / R3, (V
THR
is the trip point
for V
IN
rising). This is the threshold voltage at which
the comparator switches its output from low to high
as V
IN
rises above the trip point. For this example,
choose 3V.
5) Calculate R2 as follows:
R2 = 1 / [V
THR
/ (V
REF
✕
R1) - (1 / R1) - (1 / R3)]
R2 = 1 / [3.0V / (1.252V
✕
12kΩ) - (1 / 12kΩ) -
(1 / 1.2MΩ)] = 8.655kΩ
For this example, choose an 8.66kΩ standard 1% value.
6) Verify the trip voltages and hysteresis as follows:
V
IN
rising: V
THR
= V
REF
✕
R1 [(1 / R1) + (1 / R2)
+ (1 / R3)] = 3V
V
IN
falling: V
THF
= V
THR
- (R1
✕
V
CC
/ R3) = 2.95V
Hysteresis = V
THR
- V
THF
= 50mV
Additional Hysteresis (MAX9118/MAX9120)
The MAX9118/MAX9120 have a 4mV internal hysteresis
band. They have open-drain outputs and require an
external pullup resistor (Figure 4). Additional hysteresis
can be generated using positive feedback, but the for-
mulas differ slightly from those of the MAX9117/
MAX9119. Use the following procedure to calculate
resistor values.
1) Select R3 according to the formulas R3 = V
REF
/ 1µA
or R3 = (V
CC
- V
REF
) / 1µA - R4. Use the smaller of
the two resulting resistor values.
2) Choose the hysteresis band required (V
HB
).
3) Calculate R1 according to the following equation:
R1 = (R3 + R4) (V
HB
/ V
CC
)
4) Choose the trip point for V
IN
rising (V
THR
) (V
THR
is
the trip point for V
IN
rising). This is the threshold volt-
age at which the comparator switches its output
from low to high as V
IN
rises above the trip point.
5) Calculate R2 as follows:
6) Verify the trip voltages and hysteresis as follows:
Hysteresis = V
THR
- V
THF
V falling
VVR
RR RR
R
RR
V
IN
THF REF CC
:
=× ++
+
⎛
⎝
⎜
⎞
⎠
⎟
+
×
−1
1
1
1
2
1
34
1
34
Vri g V V R
RR R
IN THR REF
sin : =× ++
⎛
⎝
⎜
⎞
⎠
⎟
1
1
1
1
2
1
3
R
V
VRRR
THR
REF
21
1
1
1
1
3
=
×
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
−− /