Datasheet

LTC2908
11
2908fd
APPLICATIONS INFORMATION
Adjustable Input
The noninverting input on the V
ADJ
comparator is set to
0.5V. And the high impedance inverting input directly ties
to the V
ADJ
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=
V
TR IP
0.5V
–1






R2
R2 = 100k is recommended. Table 1 shows suggested
1% resistor values for various adjust able applications and
their corresponding trip thresholds.
Table 1. Suggested 1% Resistor Values for the V
ADJ
Inputs
V
SUPPLY
(V) V
TRIP
(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
Figure 3. Setting the Adjustable Trip Point
–
+
+
–
R1
1%
R2
1%
0.5V
LTC2908-A1/LTC2908-B1/LTC2908-C1
V
ADJ
2908 F03
V
TRIP
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 specifi ed band
(±1.5% for the LTC2908 supervisors). Figure 5 shows
the typical relative threshold accuracy for all six inputs
overtemperature.
REGION OF POTENTIAL MALFUNCTION
WITH 2.5% MONITOR
MINIMUM
RELIABLE
SYSTEM
VOLTAGE
SUPPLY
TOLERANCE
IDEAL
SUPERVISOR
THRESHOLD
5.000V
±1.5%
THRESHOLD
BAND
±2.5%
THRESHOLD
BAND
NOMINAL
SUPPLY
VOLTAGE
4.750V
4.675V
4.600V
4.500V
–5.0%
–6.5%
–8.0%
–10%
2908 F04
Figure 4. Threshold Band Diagram
Figure 5. Typical Threshold Accuracy vs Temperature
TEMPERATURE (°C)
–50
–1.5
TYPICAL THRESHOLD ACCURACY (%)
–1.0
–0.5
0
0.5
1.5
–25
02550
2908 F05
75 100
1.0