Datasheet

LTC2909
9
2909fb
If the state of the SEL pin confi gures a given input as
“negative polarity,” the voltage at the ADJx pin must be
below the trip point (0.5V nominal), or the
⎯
R
⎯
S
⎯
T output will
be pulled low. Conversely, if a given input is confi gured
as “positive polarity,” the pin voltage must be above the
trip point or
⎯
R
⎯
S
⎯
T will assert low.
Thus, a “negative polarity” input may be used to determine
whether a monitored negative voltage is smaller in absolute
value than it should be (–UV), or a monitored positive
voltage is larger than it should be (+OV). The opposite is
true for a “positive polarity” input (–OV or +UV). These
usages are also shown in Table 1. For purposes of this
APPLICATIO S I FOR ATIO
WUU
U
data sheet, a negative voltage is considered “undervoltage”
if it is closer to ground than it should be (e.g., –4.3V for
a –5V supply).
Proper confi guration of the SEL pin and setting of the
trip-points via external resistors allows for any two fault
conditions to be detected. For example, the LTC2909 may
monitor two supplies (positive, negative or one of each)
for UV or for OV (or one UV and one OV). It may also
monitor a single supply (positive or negative) for both UV
and OV. Tables 2a and 2b show example confi gurations
for monitoring possible combinations of fault condition
and supply polarity.
ADJ1
ADJ2 SEL
5V (UV)15V (UV)
R
P2B
1.15M
R
P1B
137k
R
P1A
115k
R
P2A
3.09M
REF
SEL = V
CC
2 Positive UV
ADJ1
ADJ2 SEL
5V (OV)15V (OV)
R
P2B
1.33M
R
P1B
137k
R
P1A
200k
R
P2A
6.19M
REF
2 Positive OV
ADJ1
ADJ2 SEL
–5V (UV)–15V (UV)
R
N2B
1.37M
R
N1B
133k
R
N1A
107k
R
N2A
3.09M
REF
2 Negative UV
SEL = GND
ADJ1
ADJ2 SEL
–5V (OV)–15V (OV)
R
N2B
1.37M
R
N1B
118k
R
N1A
309k
R
N2A
10.2M
REF
2 Negative OV
ADJ1
ADJ2 SEL
–15V (OV)15V (UV)
R
N2
10.2M
R
N1
309k
R
P1
115k
R
P2
3.09M
REF
1 Positive UV, 1 Negative OV
ADJ1
ADJ2 SEL
–15V (UV)15V (OV)
R
N2
3.09M
R
N1
107k
R
P1
200k
R
P2
6.19M
REF
1 Positive OV, 1 Negative UV
Table 2a. 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