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
100nA for the thresholds to be accurate. R1 values
up to about 10MΩ can be used, but values in the
100kΩ to 1MΩ range are usually easier to deal with.
In this example, choose R1 = 294kΩ.
The full-s c a le thre s hold (a ll LEDs on) is g ive n b y
= (R1 + R2)/R1 volts. The other thresholds are at
3/4 full scale, 1/2 full scale, and 1/4 full scale. The
output resistors limit the current into the LEDs.
V
IN
3. Ca lc ula te R2 + R3. The ove rvolta g e thre s hold
s hould b e 5.5V whe n VIN is ris ing . The d e s ig n
equation is as follows:
Le ve l S h ift e r
Figure 8 shows a circuit to shift from bipolar ±5V inputs
to TTL s ig na ls . The 10kΩ re s is tors p rote c t the
comparator inputs, and do not materially affect the
operation of the circuit.
V
OTH
R2 + R3 = R1 ×
− 1
V
+ V
H
REF
Tw o -S t a g e Lo w -Vo lt a g e De t e c t o r
Fig ure 9 s hows the MAX932 monitoring a n inp ut
5.5
(1.182 + 0.005)
= 294k ×
− 1
volta g e in two s te p s . Whe n V is hig he r tha n the
IN
LOW and FAIL thresholds, outputs are high. Threshold
c a lc ula tions a re s imila r to thos e for the wind ow-
detector application.
= 1.068MΩ
4. Calculate R2. The undervoltage threshold should
be 4.5V when V is falling. The design equation is
1-MAX934
IN
as follows:
R2
R1
(V
− V )
H
REF
V
R2 = (R1 + R2 + R3) ×
− R1
V
IN
UTH
+5V
3
(1.182 − 0.005)
= (294k + 1.068M) ×
= 62.2kΩ
− 294k
V+
4.5
MAX934
1.182V
Choose R2 = 61.9kΩ (1% standard value).
8
REF
5. Calculate R3.
V-
9
2
R3 = (R2 + R3) − R2
= 1.068M − 61.9k
182k
250k
250k
5
4
INA+
INA-
OUTA
1V
= 1.006MΩ
330Ω
330Ω
330Ω
330Ω
Choose R3 = 1MΩ (1% standard value).
7
6
INB+
INB-
6. Verify the resistor values. The equations are as
follows, evaluated for the above example.
1
OUTB
750mV
Overvoltage threshold :
(R1 + R2 + R3)
V
= (V
+ V ) ×
OTH
REF H
R1
INC+
11
16
= 5.474V.
Undervoltage threshold :
OUTC
OUTD
500mV
250mV
10 INC-
(R1 + R2 + R3)
V
= (V
− V ) ×
UTH
REF H
(R1 + R2)
13
IND+
250k
250k
15
= 4.484V,
12 IND-
R5
where the hysteresis voltage V = V
×
REF
.
H
R4
GND
14
Ba r-Gra p h Le ve l Ga u g e
The high output source capability of the MAX931 series
is useful for driving LEDs. An example of this is the
simple four-stage level detector shown in Figure 7.
Figure 7. Bar-Graph Level Gauge
12 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
