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LTC2921 数据表(PDF) 11 Page - Linear Technology

部件名 LTC2921
功能描述  Precision Quad Supply Monitor in 6-Lead SOT-23
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC2921 数据表(HTML) 11 Page - Linear Technology

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11
LTC2903-1
29031f a
Table 1a contains suggested 1% resistor values for the
ADJ inputs to obtain nominal –11.5% thresholds. Connect
unused supervisor inputs to the highest supply voltage
available.
Table 1a. Suggested 1% Resistor Values for the –11.5% ADJ Inputs
VSUPPLY (V)
VTRIP (V)
R1 (kΩ)
R2(kΩ)
12
10.75
2050
100
10
8.95
1690
100
8
7.15
1330
100
7.5
6.7
1240
100
6
5.38
976
100
5
4.435
787
100
3.3
2.935
487
100
3
2.66
432
100
2.5
2.2
340
100
1.8
1.605
221
100
1.5
1.325
165
100
1.2
1.065
113
100
1
0.884
76.8
100
0.9
0.795
59
100
Table 1b contains suggested 1% resistor values for the
ADJ inputs to obtain nominal –6.5% thresholds.
Table 1b. Suggested 1% Resistor Values for the –6.5% ADJ 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
Implications of Threshold Accuracy
Specifying system voltage margin for worst-case opera-
tion requires consideration of three factors: power supply
tolerance, IC supply voltage tolerance and supervisor re-
set 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 ±10% 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 4.5V (5V –
10%), is the exact voltage at which a
perfectly accurate
supervisor generates a reset. Such a perfectly accurate
supervisor does not exist—the actual reset threshold may
vary over a specified band (±1.5% for the LTC2903-1 su-
pervisors). Figure 4 shows the typical relative threshold
accuracy for all four inputs, over temperature.
With this variation of reset threshold in mind, the nominal
reset threshold of the supervisor resides
below the mini-
mum supply voltage, just enough so that the reset thresh-
old band and the power supply tolerance bands do not
overlap. If the two bands overlap, the supervisor could
generate a false or nuisance reset when the power supply
remains within its specified tolerance band (say, at 4.6V).
Adding half of the reset threshold accuracy spread (1.5%)
to the ideal 10% thresholds puts the LTC2903-1 thresh-
olds at 11.5% (typ) below the nominal input voltage. For
example, the 5V typical threshold is 4.425V, or 75mV
below the ideal threshold of 4.500V. The guaranteed
threshold lies in the band between 4.500V and 4.350V
over temperature.
APPLICATIO S I FOR ATIO
Figure 4. LTC2903 Typical Threshold Accuracy vs Temperature
TEMPERATURE (°C)
–50
–1.5
–1.0
–0.5
0
0.5
25
75
100
29031 F04
1.0
1.5
–25
0
50


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