Microprocessor Voltage Monitors
with Programmable Voltage Detection
MAX8211/MAX8212
V
IN
1.230
1.210
1.190
V
TH
(V)
1.170
1.150
1.130
1.110
1.090
1.070
1.050
-55
-25
25
T
A
(°C)
75
125
R1
750k
1%
V+ = 2V
V+ = 16.5V
R2
20M
1%
THRESH
GND
MAX8211,8212-FIG 4
1.250
R3
48.7k
1%
HYST
V+
MAX8211
OUT
V
OUT
(LOW FOR
V
IN
< 4.5V)
Figure 4. MAX8211/MAX8212 Threshold Trip Voltage vs.
Ambient Temperature
Figure 5. MAX8211 Logic-Supply Low-Voltage Detector
Calculate resistor values for Figure 3 as follows:
1) Choose a value for R1. Typical values are in
the 10kΩ to 10MΩ range.
2) Calculate R2 for the desired upper trip point
V
U
using the formula:
R2 = R1
×
(VU
−
1.15V)
(VU
−
VTH )
=
R1
×
1.15V
VTH
Calculate resistor values for Figure 5 as follows:
1) Choose a value for R1. Typical values are in
the 10kΩ to 10MΩ range.
2) Calculate R2:
R2
=
R1
×
(VL
−
1.15V)
(VL
−
VTH )
=
R1
×
1.15V
VTH
(VU
−
VL )
1.15V
3) Calculate R3:
R3
=
R1
×
3) Calculate R3 for the desired amount of
hysteresis, where V
L
is the lower trip point:
R3
=
R2
×
(V
+ −
VTH )
(V +
−
1.15V)
=
R2
×
(VU
−
VL )
(VU
−
VL )
Low-Voltage Detector for Logic Supply
The circuit of Figure 5 will detect when a 5.0V (nominal)
supply goes below 4.5V, which is the V
MIN
normally
specified in logic systems. The selected resistor values
ensure that false undervoltage alarms will not be gener-
ated, even with worst-case threshold trip values and
resistor tolerances. R3 provides approximately 75mV of
hysteresis.
or, if V
+
= V
IN
:
R3
=
R2
×
(VL
−
1.15V)
(VL
−
VTH )
=
R2
×
(VU
−
VL )
(VU
−
VL )
Figure 5 shows an alternate circuit, suitable only when the
voltage being detected is also the power-supply voltage
for the MAX8211 or MAX8212.
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5
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