Rainbow Electronics MAX993 User Manual
Page 8
MAX985/MAX986/MAX989/MAX990/MAX993/MAX994
Micropower, Low-Voltage, UCSP/SC70,
Rail-to-Rail I/O Comparators
8
_______________________________________________________________________________________
5) Calculate R2 as follows. For this example, choose an
8.2k
Ω standard value:
6) Verify trip voltages and hysteresis as follows:
MAX986/MAX990/MAX994
The MAX986/MAX990/MAX994 have ±3mV internal
hysteresis. They have open-drain outputs and require
an external pullup resistor (Figure 2). Additional hys-
teresis can be generated using positive feedback, but
the formulas differ slightly from those of the
MAX985/MAX989/MAX993.
Use the following procedure to calculate resistor
values:
1) Select R3 according to the formulas R3 = V
REF
/
500µA or R3 = (V
REF
- V
CC
) / 500µA - R4. Use the
smaller of the two resulting resistor values.
2) Choose the hysteresis band required (V
HB
). For this
example, choose 50mV.
3) Calculate R1 according to the following equation:
R1 = (R3 + R4) x (V
HB
/ V
CC
)
4) Choose the trip point for V
IN
rising (V
THR
; V
THF
is
the trip point for V
IN
falling). This is the threshold
voltage 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 trip voltages and hysteresis as follows:
Board Layout and Bypassing
Power-supply bypass capacitors are not typically need-
ed, but use 100nF bypass capacitors when supply
impedance is high, when supply leads are long,
or when excessive noise is expected on the supply
lines. Minimize signal trace lengths to reduce stray
capacitance.
V rising: V
= V
x R1 x
1
R1
V falling
IN
THR
REF
IN
:
+
+
+
=
−
+
=
−
1
2
1
3
4
1
3
4
R
R
R
V
V
R x V
R
R
Hysteresis
V
V
THF
THR
CC
THR
THF
R2 =
1
V
V
THR
REF
x R
R
R
R
1
1
1
1
3
4
−
−
+
V rising: V
= V
x R1 x
1
R1
V falling
IN
THR
REF
IN
:
+
+
=
−
=
−
1
2
1
3
1
3
R
R
V
V
R x V
R
Hysteresis
V
V
THF
THR
CC
THR
THF
R2 =
1
V
V
R2 =
1
3.0V
1.2 x 12k
THR
REF
.
.
x R
R
R
k
M
k
1
1
1
1
3
1
12
1
2 2
8 03
−
−
−
−
=
Ω
Ω
Ω
Ω
V
EE
V
CC
OUT
R3
R2
R1
R4
V
REF
V
IN
V
CC
MAX986
MAX990
MAX994
Figure 2. Additional Hysteresis (MAX986/MAX990/MAX994)