MAX44268 Maxim, MAX44268 Datasheet - Page 12

• Manufacturer Part Number: MAX44268
• Description: The MAX44268 is an ultra-small and low-power dual comparator ideal for battery-powered applications such as cell phones, notebooks, and portable medical devices that have extremely aggressive board space and power constraints
• Manufacturer: Maxim

=
(R
then be computed using the basic time domain equa-
tions for the charging RC circuit as:
The t
component values in such a way that R3 >> R1 and R5
>> R1. This ensures that the output of the comparator
goes close to V
~ V
can be approximated as:
The frequency of the relaxation oscillator is:
Figure 7. Relaxation Oscillator Waveforms
f
THEVENIN
C1 WAVEFORM
CC
t
t
WAVEFORM
HIGH
HIGH
=
HIGH
V
V
and R
THEVENIN
V
T_FALL
T_RISE
OUT
R
=
1
calculation can be simplified by selecting the
+
THEVENIN
) in series with the capacitor C1. t
R
t
t
HIGH
LOW
THEVENIN
THEVENIN
CC
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=
x
when at logic-high (that is, V
V
R5C1 ln
R5C11n
(R2 R4) R3 R1
(R2 R4) R3 R1
CC
=
[
C1 ln
(R2 R4) R3 R1 R5

~ R5). With this selection, t

[
(R2 R4) R3





R1








V

V
+
V
V
+
V
CC
V
CC
T_FALL
T_RISE
THEVENIN
THEVENIN
+
- V
- V
+
+
+
T_FALL
T_RISE
1
]
(
(
]

V
V
+
CC
CC
- V
V
- V
R2 R4
Dual Comparator with Reference
+
CC




-
- V
T_FALL
T_RISE
V
+
x R4
T_FALL
T_RISE
HIGH
THEVENIN




HIGH
1.3mm x 1.3mm, Low-Power
can
)
)




The IC is ideal for window detectors (undervoltage/over-
voltage detectors).
simple window detector circuit. By changing the values
of R1, R2, and R3 different voltage threshold values can
be chosen. For this example, assume a single-cell Li+
battery with a 2.9V end-of-life charge, a peak charge of
4.2V, and a nominal value of 3.6V. OUTA provides an
active-low undervoltage indication, and OUTB provides
an active-low overvoltage indication. The open-drain out-
puts of both the comparators are wired ORed to give an
active-high power-good signal.
The design procedure is as follows:
1) Select R1. The input bias current into INB- is less than
2) Calculate R2 + R3. The overvoltage threshold should
3) Calculate R2. The undervoltage threshold should be
4) Calculate R3:
For this example, choose a 1.58MI standard value 1%
resistor.
15nA, so the current through R1 should exceed 1.5µA
for the thresholds to be accurate. In this example,
choose R1 = 825kI (1.236V/1.5µA).
be 4.2V when V
follows:
2.9V when V
follows:
For this example, choose a 374kI standard value 1%
resistor.
=
=
R2 (R1 R2 R3)x
=
(
370k
R2 R3 R1 x
(
825
IN
+
+
R3 (R2 R3) - R2
=
Ω
IN
is falling. The design equation is as
Typical Application Circuit
+
=1 .595M
=
is rising. The design equation is as
+
= 
1969k - 374k
=
= 1969k
1969 x 1 .236/2.9 - 825
825 x
Window Detector Circuit
+
) (






Ω
Ω



V
V



V
Ω
V



UTH
REF
MAX44268
OTH
1.236
REF
4.2






- R1
Ω
- 1



- 1




)



)
shows a