LTC2909

9

2909fa

If the state of the SEL pin configures 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 configured

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

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 configuration 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 configurations

for monitoring possible combinations of fault condition

and supply polarity.

ADJ1

ADJ2

SEL

5V (UV)

15V (UV)

1.15M

137k

115k

3.09M

REF

2 Positive UV

ADJ1

ADJ2

SEL

5V (OV)

15V (OV)

1.33M

137k

200k

6.19M

REF

2 Positive OV

ADJ1

ADJ2

SEL

–5V (UV)

–15V (UV)

1.37M

133k

107k

3.09M

REF

2 Negative UV

SEL = GND

ADJ1

ADJ2

SEL

–5V (OV)

–15V (OV)

1.37M

118k

309k

10.2M

REF

2 Negative OV

ADJ1

ADJ2

SEL

–15V (OV)

15V (UV)

10.2M

309k

115k

3.09M

REF

1 Positive UV, 1 Negative OV

ADJ1

ADJ2

SEL

–15V (UV)

15V (OV)

3.09M

107k

200k

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