Datasheet

LTC2909
10
2909fb
APPLICATIO S I FOR ATIO
WUU
U
Adjust Input Trip Point
The trip threshold for the supplies monitored by the adjust-
able inputs is set with an external resistor divider, allowing
the user complete control over the trip point. Selection of
this trip voltage is crucial to the reliability of the system.
Any power supply has some tolerance band within which
it is expected to operate (e.g., 5V ±10%). It is generally
undesirable that a supervisor issue a reset when the power
supply is inside this tolerance band. Such a “nuisance”
reset reduces reliability by preventing the system from
functioning under normal conditions.
To prevent nuisance resets, the supervisor threshold must
be guaranteed to lie outside the power supply tolerance
Table 2b. Possible Combinations of Supply Monitoring. For Example Purposes, All Supplies are Monitored at 5% Tolerance and
Connections are Shown Only for ADJ1, ADJ2, REF, SEL
ADJ1
ADJ2 SEL
15V (UV/OV)
R
P6
2.37M
R
P5
10.7k
R
P4
76.8k
REF
SEL OPEN
1 Positive UV and OV
ADJ1
ADJ2 SEL
5V (OV)15V (UV)
R
P2B
1.33M
R
P1B
137k
R
P1A
115k
R
P2A
3.09M
REF
1 Positive UV, 1 Positive OV
1 Negative OV, 1 Positive OV
ADJ1
ADJ2 SEL
–15V (UV/OV)
R
N6
10.2M
R
N4
309k
R
N5
40.2k
REF
1 Negative UV and OV
ADJ1
ADJ2 SEL
15V (OV)–15V (OV)
R
P2
6.19M
R
P1
200k
R
N1
309k
R
N2
10.2M
REF
ADJ1
ADJ2 SEL
–5V (UV)–15V (OV)
R
N2B
1.37M
R
N1B
133k
R
N1A
309k
R
N2A
10.2M
REF
ADJ1
ADJ2 SEL
–15V (UV)15V (UV)
R
N2
3.09M
R
N1
107k
R
P1
115k
R
P2
3.09M
REF
1 Positive UV, 1 Negative UV
1 Negative UV, 1 Negative OV
band. To ensure that the threshold lies outside the power
supply tolerance range, the nominal threshold must lie out-
side that range by the monitor’s accuracy specifi cation.
All three of the LTC2909 inputs (ADJ1, ADJ2, V
CC
UVLO)
have the same relative threshold accuracy of ±1.5% of the
programmed nominal input voltage (over the full operating
temperature range). Therefore, using the LTC2909, the
typical 10% UV threshold is at 11.5% below the nominal
input voltage level. For a 5V input, the threshold is nominally
4.425V. With ±1.5% accuracy, the trip threshold range is
4.425V ±75mV over temperature (i.e., 10% to 13% below
5V). The monitored system must thus operate reliably
down to 4.35V or 13% below 5V over temperature.