Board Layout and Bypassing
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.
_______________Typical Applications
Auto-Off Power Source
Figure 5 shows the schematic for a 40mA power supply
that has a timed auto power-off function. The
comparator output is the switched power-supply output.
With a 10mA load, it typically provides a voltage of
(V
BATT
- 0.12V), but draws only 3.5µA quiescent current.
This circuit takes advantage of the four key features of
the MAX931: 2.5µA supply current, an internal
reference, hysteresis, and high current output. Using
the component values shown, the three-resistor voltage
divider programs the maximum ±50mV of hysteresis
and sets the IN- voltage at 100mV. This gives an IN+
trip threshold of approximately 50mV for IN+ falling.
The RC time constant determines the maximum power-
on time of the OUT pin before power-down occurs.
This period can be approximated by:
R x C x 4.6sec
For example: 2MΩ x 10µF x 4.6 = 92sec. The actual
time will vary with both the leakage current of the
capacitor and the voltage applied to the circuit.
Window Detector
The MAX933 is ideal for making window detectors
(undervoltage/overvoltage detectors). The schematic
is shown in Figure 6, with component values selected
for an 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. OUTA provides an active-low undervoltage
indication, and OUTB gives an active-low overvoltage
indication. ANDing the two outputs provides an active-
high, power-good signal.
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 (MAX931/MAX932/MAX933)
section.
In this example, ±5mV of hysteresis has been added
at the comparator input
(V
H
= V
HB
/2). This means
that the hysteresis apparent at V
IN
will be larger
because of the input resistor divider.
2. Select R1. The leakage current into INB- is normally
under 1nA, so the current through R1 should exceed
MAX931-MAX934
Ultra Low-Power, Low-Cost
Comparators with 2% Reference
______________________________________________________________________________________ 11
IN+
MAX931
OUT
IN-
HYST
REF
V-
V+
VBATT -0.15V
10mA
21
4
5
6
8
3
100k
1.1M
47k
4.5V TO 6.0V
MOMENTARY SWITCH
7
R
C
GND
Figure 5. Auto-off power switch operates on 2.5µA quiescent
current.
MAX933
INB-
REF
HYST
INA+
V-
V+
OUTA
OUTB
R5
10k
R1
R2
R3
UNDERVOLTAGE
V
IN
V
OTH
= 5.5V
V
UTH
= 4.5V
+5V
POWER GOOD
OVERVOLTAGE
R4
2.4M
Figure 6. Window Detector
