ltc2908its8-c1-trpbf Linear Technology Corporation, ltc2908its8-c1-trpbf Datasheet - Page 11

• Manufacturer Part Number: ltc2908its8-c1-trpbf
• Description: Precision Six Input Supply Monitor
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Adjustable Input
The noninverting input on the V
0.5V. And the high impedance inverting input directly ties
to the V
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:
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 V
R
V
SUPPLY
1
=
7.5
3.3
2.5
1.8
1.5
1.2
1.0
0.9
0.8
0.7
0.6
12
10
V
8
6
5
3
ADJ
⎛
⎝ ⎜
ADJ
V
V
(V)
0 5
TRIP
Figure 3. Setting the Adjustable Trip Point
TRIP
.
R1
1%
R2
1%
pin.
V
–
LTC2908-A1/LTC2908-B1/LTC2908-C1
1
V
⎞
⎠ ⎟
U
TRIP
11.25
4.725
3.055
2.325
1.685
1.410
1.120
0.933
0.840
0.750
0.655
0.561
+
–
2.82
R2
9.4
7.5
5.6
7
0.5V
(V)
–
+
U
ADJ
R1 (kΩ)
2150
1780
1400
1300
1020
86.6
68.1
49.9
30.9
12.1
845
511
464
365
237
182
124
W
comparator is set to
2908 F03
ADJ
Inputs
U
R2 (kΩ)
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
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 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 ±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 over
temperature.
THRESHOLD
Figure 5. Typical Threshold Accuracy vs Temperature
±1.5%
BAND
–0.5
–1.0
–1.5
5.000V
4.750V
4.675V
4.600V
4.500V
1.5
1.0
0.5
0
Figure 4. Threshold Band Diagram
–50
MINIMUM
RELIABLE
REGION OF POTENTIAL MALFUNCTION
VOLTAGE
SYSTEM
–25
TEMPERATURE (°C)
WITH 2.5% MONITOR
0
TOLERANCE
SUPPLY
25
50
SUPERVISOR
THRESHOLD
IDEAL
75
2908 F04
2908 F05
100
LTC2908
–5.0%
–6.5%
–8.0%
–10%
NOMINAL
VOLTAGE
SUPPLY
±2.5%
THRESHOLD
BAND
11
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