LTC2908 データシートの表示（PDF） - Linear Technology
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LTC2908 Datasheet PDF : 18 Pages
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LTC2908
APPLICATIONS INFORMATION
Adjustable Input
The noninverting input on the VADJ comparator is set to
0.5V. And the high impedance inverting input directly ties
to the VADJ pin.
In a typical application, this pin connects to a tap point on an
external resistive divider between the positive voltage being
monitored and ground (see Figure 3). The following formula
derives the value of the R1 resistor in the divider from a
particular value of R2 and the desired trip voltage:
R1=
VTRIP
0.5V
– 1R2
R2 = 100k is recommended. Table 1 shows suggested
1% resistor values for various adjustable applications and
their corresponding trip thresholds.
Table 1. Suggested 1% Resistor Values for the VADJ Inputs
VSUPPLY (V)
VTRIP (V)
R1 (kΩ)
R2 (kΩ)
12
11.25
2150
100
10
9.4
1780
100
8
7.5
1400
100
7.5
7
1300
100
6
5.6
1020
100
5
4.725
845
100
3.3
3.055
511
100
3
2.82
464
100
2.5
2.325
365
100
1.8
1.685
237
100
1.5
1.410
182
100
1.2
1.120
124
100
1.0
0.933
86.6
100
0.9
0.840
68.1
100
0.8
0.750
49.9
100
0.7
0.655
30.9
100
0.6
0.561
12.1
100
VTRIP
R1
1%
VADJ
R2
1%
LTC2908-A1/LTC2908-B1/LTC2908-C1
–
+
+– 0.5V
2908 F03
Figure 3. Setting the Adjustable Trip Point
In an application with less than six supply voltages, the
unused supervisor inputs should be tied to the closest
higher supply voltage available.
Threshold Accuracy
Specifying system voltage margin for worst-case operation
requires the consideration of three factors: power supply
tolerance, IC supply voltage tolerance and supervisor reset
threshold accuracy. Highly accurate supervisors ease the
design challenge by decreasing the overall voltage margin
required for reliable system operation.
Consider a 5V system with a ±5% power supply tolerance
band. System ICs powered by this supply must operate
reliably within this band (and a little more, as explained
below). The bottom of the supply tolerance band, at 4.75V
(5% below 5V), is the exact voltage at which a perfectly
accurate supervisor generates a reset (see Figure 4).
Such a perfectly accurate supervisor does not exist—the
actual reset threshold may vary over a specified band
(±1.5% for the LTC2908 supervisors). Figure 5 shows
the typical relative threshold accuracy for all six inputs
overtemperature.
5.000V
MINIMUM
RELIABLE
SYSTEM
VOLTAGE
SUPPLY
TOLERANCE
IDEAL
SUPERVISOR
THRESHOLD
NOMINAL
SUPPLY
VOLTAGE
±1.5%
THRESHOLD
BAND
4.750V
4.675V
4.600V
4.500V
–5.0%
–6.5%
–8.0%
REGION OF POTENTIAL MALFUNCTION
WITH 2.5% MONITOR
2908 F04 –10%
±2.5%
THRESHOLD
BAND
Figure 4. Threshold Band Diagram
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–50 –25 0
25 50
TEMPERATURE (°C)
75 100
2908 F05
Figure 5. Typical Threshold Accuracy vs Temperature
2908fd
11
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