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MAX917–MAX920
SOT23, 1.8V, Nanopower, Beyond-the-Rails
Comparators With/Without Reference
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11
THRESHOLDS
OUT
IN+
IN-
VHB
HYSTERESIS
BAND
VTHF
VTHR
Figure 2. Threshold Hysteresis Band
VCC
MAX917
MAX919
OUT
R3
R1
R2
VREF
VEE
VIN
VCC
Figure 3. MAX917/MAX919 Additional Hysteresis
Additional Hysteresis (MAX917/MAX919)
The MAX917/MAX919 have a 4mV internal hysteresis
band (VHB). Additional hysteresis can be generated
with three resistors using positive feedback (Figure 3).
Unfortunately, this method also slows hysteresis re-
sponse time. Use the following procedure to calculate
resistor values.
1) Select R3. Leakage current at IN is under 2nA, so
the current through R3 should be at least 0.2A to
minimize errors caused by leakage current. The cur-
rent through R3 at the trip point is (VREF - VOUT)/R3.
Considering the two possible output states in solving
for R3 yields two formulas: R3 = VREF/IR3 or R3 =
(VCC - VREF)/IR3. Use the smaller of the two resulting
resistor values. For example, when using the
MAX917 (VREF = 1.245V) and VCC = 5V, and if we
choose IR3 = 1A, then the two resistor values are
1.2M
Ω and 3.8MΩ. Choose a 1.2MΩ standard value
for R3.
2) Choose the hysteresis band required (VHB). For this
example, choose 50mV.
3) Calculate R1 according to the following equation:
R1 = R3 (VHB / VCC)
For this example, insert the values
R1 = 1.2M
Ω (50mV/5V) = 12kΩ
4) Choose the trip point for VIN rising (VTHR) such that
VTHR > VREF (R1 + R3)/R3 (VTHF is the trip point for
VIN falling). This is the threshold voltage at which the
comparator switches its output from low to high as
VIN rises above the trip point. For this example,
choose 3V.
5) Calculate R2 as follows:
R2 = 1/[VTHR/(VREF R1) - (1 / R1) - (1 / R3)]
R2 = 1/[3.0V/(1.2V 12k
Ω) - (1 / 12kΩ) -
(1/1.2M
Ω)] = 8.05kΩ
For this example, choose an 8.2k
Ω standard value.
6) Verify the trip voltages and hysteresis as follows:
VIN rising: VTHR = VREF R1 [(1 / R1) + (1 / R2)
+ (1 / R3)]
VIN falling: VTHF = VTHR - (R1 VCC / R3)
Hysteresis = VTHR - VTHF
Additional Hysteresis (MAX918/MAX920)
The MAX918/MAX920 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 MAX917/
MAX919. Use the following procedure to calculate
resistor values.
1) Select R3 according to the formulas R3 = VREF / 1A
or R3 = (VCC - VREF)/1A - R4. Use the smaller of
the two resulting resistor values.
2) Choose the hysteresis band required (VHB).
3) Calculate R1 according to the following equation:
R1 = (R3 + R4) (VHB/VCC)
4) Choose the trip point for VIN rising (VTHR) (VTHF is
the trip point for VIN falling). This is the threshold
voltage at which the comparator switches its output
from low to high as VIN rises above the trip point.
5) Calculate R2 as follows:
R2
1/ V
/ V
R1
1
R1
1
R3
THR
REF
=
()