MAX991 Maxim, MAX991 Datasheet - Page 9

• Manufacturer Part Number: MAX991
• Description: The MAX987/MAX988/MAX991/MAX992/MAX995/ MAX996 single/dual/quad micropower comparators feature low-voltage operation and rail-to-rail inputs and outputs
• Manufacturer: Maxim

5) Calculate R2 as shown. For this example, choose an
6) Verify trip voltages and hysteresis as follows:
The MAX988/MAX992/MAX996 have ±2.5mV 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
MAX987/MAX991/MAX995.
Figure 2. Additional Hysteresis (MAX988/MAX992/MAX996)
R2 =
R2 =
Hysteresis
V rising: V
V falling
V
IN
IN
8.2kΩ standard value:
IN
R1
⎛
⎜
⎝
⎛
⎜
⎝
1.2 x 12k
V
REF
:
V
3.0V
V
=
THR
THR
THF
V
x R
R2
THR
V
_______________________________________________________________________________________
Ω
= V
=
REF
1
⎞
⎟ −
⎠
⎞
⎟ −
⎠
1
V
REF
−
High-Speed, Micropower, Low-Voltage,
THR
R3
1
V
12
R
THF
1
x R1 x
V
V
1
V
1
−
k
EE
CC
MAX988/MAX992/MAX996
CC
Ω
SOT23, Rail-to-Rail I/O Comparators
−
⎛
⎜
⎝
MAX988
MAX992
MAX996
R x V
−
1
R
1
⎛
⎜
⎝
3
OUT
R1
R
2 2
1
3
.
1
M
CC
+
Ω
R
⎞
⎟
⎠
1
2
R4
=
+
8 03
R
.
1
3
0.1µF
⎞
⎟
⎠
k
Ω
Use the following procedure to calculate resistor
values:
1) Select R3 according to the formulas R3 = V
2) Choose the hysteresis band required (V
3) Calculate R1 according to the following equation:
4) Choose the trip point for V
5) Calculate R2 as follows:
6) Verify trip voltages and hysteresis as follows:
These comparators’ high-gain bandwidth requires
design precautions to maximize their high-speed capa-
bility. The recommended precautions are:
1) Use a PCB with an unbroken, low-inductance
2) Place a decoupling capacitor (a 0.1µF ceramic
3) On the inputs and outputs, keep lead lengths short
4) Solder the devices directly to the PCB instead of
or R3 = (V
the two resulting resistor values.
example, choose 50mV.
the trip point for V
voltage at which the comparator switches its output
from low to high as V
Hysteresis
V rising: V
V falling
ground plane.
capacitor is a good choice) as close to V
possible.
to avoid unwanted parasitic feedback around the
comparators.
using a socket.
IN
IN
R2 =
R1 = (R3 + R4) x (V
:
REF
⎛
⎜
⎝
⎛
⎜
⎝
V
=
THR
R1
THF
V
1
REF
V
Circuit Layout and Bypassing
- V
THR
V
+
= V
THR
=
CC
R
IN
x R
1
V
2
REF
) / 1µA - R4. Use the smaller of
−
IN
THR
falling). This is the threshold
1
+
V
rises above the trip point.
⎞
⎟ −
⎠
THF
x R1 x
R
−
3
1
IN
⎛
⎜
⎝
HB
R
1
+
R x V
1
R
1
1
rising (V
R
3
/ V
4
−
+
⎞
⎟
⎠
CC
R
R
CC
)
3
4
THR
1
+
⎞
⎟
⎠
HB
R
). For this
REF
4
; V
CC
THF
/ 1µA
as
is
9