Ult ra -Lo w -P o w e r, Op e n -Dra in ,
S in g le /Du a l-S u p p ly Co m p a ra t o rs
__________Ap p lic a t io n s In fo rm a t io n
V+
Hys t e re s is
Hysteresis increases the comparators’ noise margin by
increasing the upper threshold and decreasing the
lower threshold (Figure 2).
R
H
R
PULL-UP
V
IN
Hysteresis (MAX9_1/MAX982/MAX9_3)
To add hysteresis to the MAX9_1, MAX982, or MAX9_3,
c onne c t re s is tor R1 b e twe e n REF a nd HYST, a nd
connect resistor R2 between HYST and V- (Figure 3). If
no hysteresis is required, connect HYST to REF. When
hysteresis is added, the upper threshold increases by
the same amount that the lower threshold decreases.
The hysteresis band (the difference between the upper
V+ OUT
V-
MAX9_4
GND
V
REF
and lower thresholds, V ) is approximately equal to
HB
twice the voltage between REF and HYST. The HYST
input can be adjusted to a maximum voltage of REF
a nd to a minimum volta g e of (REF - 50mV). The
maximum difference between REF and HYST (50mV)
will therefore produce a 100mV max hysteresis band.
Use the following equations to determine R1 and R2:
Figure 4. External Hysteresis
Bo a rd La yo u t a n d Byp a s s in g
Power-supply bypass capacitors are not needed if the
supply impedance is low, but 100nF bypass capacitors
should be used when the supply impedance is high or
when the supply leads are long. Minimize signal lead
lengths to reduce stray capacitance between the input
and output that might cause instability. Do not bypass
the reference output.
V
HB
R1 =
2 × I
(
)
REF
V
HB
1.182 –
2
R2 =
I
REF
Win d o w De t e c t o r
The MAX9_3 is id e a l for ma king wind ow d e te c tors
(undervoltage/overvoltage detectors). The schematic is
shown in Figure 5, with component values selected for a
4.5V undervoltage threshold and a 5.5V overvoltage
threshold. Choose different thresholds by changing the
values of R1, R2, and R3. To prevent chatter at the
output when the supply voltage is close to a threshold,
hysteresis has been added using R4 and R5. Taken
alone, OUTA would provide an active-low undervoltage
ind ic a tion, a nd OUTB would g ive a n a c tive -low
overvoltage indication. Wired-ORing the two outputs
provides an active-high, power-good signal.
whe re I
(the c urre nt s ourc e d b y the re fe re nc e )
REF
s hould not e xc e e d the REF s ourc e c a p a b ility, a nd
s hould b e s ig nific a ntly la rg e r tha n the HYST inp ut
c urre nt.
usually appropriate. If 2.4MΩ is chosen for R2 (I
0.5µA), the e q ua tion for R1 a nd
approximated as:
I
va lue s b e twe e n 0.1µA a nd 4µA a re
REF
=
REF
7/1–MAX984
V
HB
c a n b e
R1 (kΩ) = V (mV)
HB
Whe n hyste re sis is obta ine d in this ma nne r for the
MAX982/MAX9_3, the same hysteresis applies to both
comparators.
Hysteresis (MAX972/MAX9_4)
Hysteresis can be implemented with any comparator
using positive feedback, as shown in Figure 4. This
approach generally draws more current than circuits
using the HYST pin on the MAX9_1/MAX982/MAX9_3,
and the high feedback impedance slows hysteresis. In
addition, because the output does not source current,
any increase in the upper threshold is dependent on
the load or pull-up resistor on the output.
The design procedure is as follows:
1) Choose the required hysteresis level and calculate
values for R4 and R5 according to the formulas in
the Hysteresis (MAX9_1/MAX982/MAX9_3) section.
In this example, ±5mV of hysteresis has been added
at the comparator input (V = V / 2). This means
H
HB
that the hysteresis apparent at V will be larger
because of the input resistor divider.
IN
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