Datasheet

MAX987/MAX988/MAX991/MAX992/MAX995/MAX996
High-Speed, Micropower, Low-Voltage,
SOT23, Rail-to-Rail I/O Comparators
_______________________________________________________________________________________ 9
5) Calculate R2 as shown. For this example, choose an
8.2kΩ standard value:
6) Verify trip voltages and hysteresis as follows:
MAX988/MAX992/MAX996
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.
Use the following procedure to calculate resistor
values:
1) Select R3 according to the formulas R3 = V
REF
/ 1µA
or R3 = (V
REF
- V
CC
) / 1µA - 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:
Circuit Layout and Bypassing
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
ground plane.
2) Place a decoupling capacitor (a 0.1µF ceramic
capacitor is a good choice) as close to V
CC
as
possible.
3) On the inputs and outputs, keep lead lengths short
to avoid unwanted parasitic feedback around the
comparators.
4) Solder the devices directly to the PCB instead of
using a socket.
V rising: V = V x R1 x
1
R1
V falling
IN THR REF
IN
:
++
+
⎛
⎝
⎜
⎞
⎠
⎟
= −
+
⎛
⎝
⎜
⎞
⎠
⎟
= −
1
2
1
34
1
34
RRR
VV
RxV
RR
Hysteresis V V
THF THR
CC
THR THF
R2 =
1
V
V
THR
REF
xR R R R1
1
1
1
34
⎛
⎝
⎜
⎞
⎠
⎟
−−
+
V rising: V = V x R1 x
1
R1
V falling
IN THR REF
IN
:
++
⎛
⎝
⎜
⎞
⎠
⎟
= −
⎛
⎝
⎜
⎞
⎠
⎟
= −
1
2
1
3
1
3
RR
VV
RxV
R
Hysteresis V V
THF THR
CC
THR THF
R2 =
1
V
V
R2 =
1
3.0V
1.2 x 12k
THR
REF
.
.
xR R R
kM
k
1
1
1
1
3
1
12
1
22
803
⎛
⎝
⎜
⎞
⎠
⎟
−−
⎛
⎝
⎜
⎞
⎠
⎟
−−
=
ΩΩ Ω
Ω
V
EE
V
CC
OUT
R3
R2
R1
R4
0.1µF
V
REF
V
IN
V
CC
MAX988
MAX992
MAX996
Figure 2. Additional Hysteresis (MAX988/MAX992/MAX996)