LTC2908
APPLICATIO S I FOR ATIO
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. The following formula de-
rives the value of the R1 resistor in the divider from a
particular value of R2 and the desired trip voltage:
⎛
V
⎞
R1
=
⎜
TRIP
– 1
⎟
R2
⎝
0.5V
⎠
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
ADJ
Inputs
V
SUPPLY
(V)
12
10
8
7.5
6
5
3.3
3
2.5
1.8
1.5
1.2
1.0
0.9
0.8
0.7
0.6
V
TRIP
(V)
11.25
9.4
7.5
7
5.6
4.725
3.055
2.82
2.325
1.685
1.410
1.120
0.933
0.840
0.750
0.655
0.561
R1 (kΩ)
2150
1780
1400
1300
1020
845
511
464
365
237
182
124
86.6
68.1
49.9
30.9
12.1
R2 (kΩ)
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
4.750V
±1.5%
THRESHOLD 4.675V
BAND
4.600V
4.500V
If an application has less than six supply voltages, the
unused supervisor inputs should be tied to the closest
higher supply voltage available.
U
V
TRIP
R1
1%
V
ADJ
R2
1%
LTC2908-A1/LTC2908-B1
W
U U
–
+
+
–
0.5V
2908 F03
Figure 3. Setting the Adjustable Trip Point
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.
5.000V
MINIMUM
IDEAL
SUPPLY
RELIABLE TOLERANCE SUPERVISOR
SYSTEM
THRESHOLD
VOLTAGE
NOMINAL
SUPPLY
VOLTAGE
–5.0%
–6.5%
REGION OF POTENTIAL MALFUNCTION
WITH 2.5% MONITOR
2908 F04
–8.0%
–10%
±2.5%
THRESHOLD
BAND
Figure 4. Threshold Band Diagram
sn2908 2908fs
9
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
