Ult ra Lo w -P o w e r, Lo w -Co s t
Co m p a ra t o rs w it h 2 % Re fe re n c e
1-MAX934
For example: 2MΩ x 10µF x 4.6 = 92sec. The actual
time will va ry with b oth the le a ka g e c urre nt of the
capacitor and the voltage applied to the circuit.
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.
Win d o w De t e c t o r
The MAX933 is id e a l for ma king wind ow d e te c tors
(undervoltage/overvoltage detectors). The schematic
is shown in Figure 6, with component values selected
for a n 4.5V und e rvolta g e thre s hold , a nd 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.
_______________Typ ic a l Ap p lic a t io n s
Au t o -Off P o w e r S o u rc e
Figure 5 shows the schematic for a 40mA power supply
tha t ha s a time d a uto p owe r-off func tion. The
comparator output is the switched power-supply output.
With a 10mA load, it typically provides a voltage of
(VBATT - 0.12V), but draws only 3.5µA quiescent current.
This circuit takes advantage of the four key features of
the MAX931: 2.5µA s up p ly c urre nt, a n inte rna l
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 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 (VH = VHB/2). This means
that the hysteresis apparent at VIN will be larger
because of the input resistor divider.
The RC time constant determines the maximum power-
on time of the OUT pin before power-down occurs.
This period can be approximated by:
2. Select R1. The leakage current into INB- is normally
under 1nA, so the current through R1 should exceed
R x C x 4.6sec
MOMENTARY SWITCH
V
IN
+5V
V+
V
V
UTH
= 5.5V
= 4.5V
OTH
4.5V TO 6.0V
7
V+
R3
R2
INA+
UNDERVOLTAGE
POWER GOOD
OVERVOLTAGE
OUTA
OUTB
MAX931
IN+
3
8
6
REF
HYST
47k
R
C
5
4
R5
10k
HYST
IN-
REF
INB-
OUT
1.1M
100k
VBATT -0.15V
10mA
V-
GND
1
R1
R4
2.4M
2
MAX933
V-
Figure 6. Window Detector
Figure 5. Auto-off power switch operates on 2.5µA quiescent
current.
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
