Datasheet

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
V
SUPPLY
(V) V
TRIP
(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
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
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
WUUU
Figure 4. LTC2903 Typical Threshold Accuracy vs Temperature
TEMPERATURE (°C)
–50
–1.5
TYPICAL THRESHOLD ACCURACY (%)
–1.0
–0.5
0
0.5
25
75
100
29031 F04
1.0
1.5
–25 0
50