ltc2903-1 Linear Technology Corporation, ltc2903-1 Datasheet - Page 11

• Manufacturer Part Number: ltc2903-1
• Description: Precision Quad Supply Monitor In 6-lead Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Calculate the trip voltage from:
Table 1 contains suggested 1% resistor values for the ADJ
input to obtain nominal –11.5% thresholds.
Table 1. Suggested 1% Resistor Values for the ADJ Input
Connect unused supervisor inputs to the highest supply
voltage available (typically V1). On the LTC2903-C1, the
negative V4 input must always be applied.
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
V
V
SUPPLY
TRIP
7.5
3.3
2.5
1.8
1.5
1.2
0.9
12
10
8
6
5
3
1
(V)
0 5 1
.
V
V
TRIP
U
10.75
4.435
2.935
1.605
1.325
1.065
0.884
0.795
8.95
7.15
5.38
2.66
6.7
2.2
R
R
(V)
2
1
U
R1 (k )
2050
1690
1330
1240
76.8
976
787
487
432
340
221
165
113
59
W
U
R2(k )
100
100
100
100
100
100
100
100
100
100
100
100
100
100
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.
The powered system must work reliably down to the
lowest voltage in the threshold band or risk malfunction
before the reset line falls. In the 5V example, using the
1.5% accurate supervisor, the system ICs must work
down to 4.35V. System ICs working with a 2.5% accurate
supervisor must operate down to 4.25V, increasing the
required system voltage margin and the probability of
system malfunction.
Figure 4. LTC2903 Typical Threshold Accuracy vs Temperature
–0.5
–1.0
–1.5
1.5
0.5
1.0
0
–50
–25
TEMPERATURE ( C)
0
25
50
LTC2903-1
75
29031 F04
100
11
29031f