LTC2908 Linear Technology, LTC2908 Datasheet - Page 11

• Manufacturer Part Number: LTC2908
• Description: Precision Six Input Supply Monitor
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
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
V
R1=
SUPPLY
7.5
3.3
2.5
1.8
1.5
1.2
1.0
0.9
0.8
0.7
0.6
12
10
8
6
5
3
ADJ
V
V
(V)
0.5V
ADJ
Figure 3. Setting the Adjustable Trip Point
TRIP
V
pin.
TRIP
R1
1%
R2
1%
– 1 R2
V
TRIP
LTC2908-A1/LTC2908-B1/LTC2908-C1
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
9.4
7.5
5.6
7
+
–
(V)
0.5V
–
+
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
comparator is set to
2908 F03
ADJ
Inputs
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 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 ac-
curate supervisor generates a reset (see Figure 4). Such
a perfectly accurate supervisor does not exist—the
actual reset threshold may vary over a specifi ed 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
5.000V
4.750V
4.675V
4.600V
4.500V
–0.5
–1.0
–1.5
Figure 4. Threshold Band Diagram
1.5
1.0
0.5
0
–50
MINIMUM
RELIABLE
REGION OF POTENTIAL MALFUNCTION
VOLTAGE
SYSTEM
–25
WITH 2.5% MONITOR
TEMPERATURE (°C)
TOLERANCE
0
SUPPLY
25
SUPERVISOR
THRESHOLD
50
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IDEAL
2908 F04
75
2908 F05
LTC2908
–5.0%
–6.5%
–8.0%
–10%
NOMINAL
VOLTAGE
SUPPLY
100
±2.5%
THRESHOLD
BAND
11
2908fc