LTC2919
8
2919f
APPLICATIO S I FOR ATIO
WUU
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The LTC2919 is a low power, high accuracy triple/dual
supply monitor with two adjustable inputs and an accurate
UVLO that can monitor a third supply. Reset timeout may
be selected with an external capacitor, set to an internally
generated 200ms, or disabled entirely.
The three-state polarity select pin (SEL) chooses one of
three possible polarity combinations for the adjustable
input thresholds, as described in Table 1. An individual
output is released when its corresponding ADJ input is
valid (above threshold if confi gured for positive polarity,
below threshold if confi gured for negative polarity).
Both input voltages (V
ADJ1
and V
ADJ2
) must be valid and
V
CC
above the UVLO threshold for longer than the reset
timeout period before RST is released. The LTC2919 as-
serts the reset output during power-up, power-down and
brownout conditions on any of the voltage inputs.
Power-Up
The LTC2919 uses proprietary low voltage drive circuitry
for the RST, OUT1 and OUT2 pins which holds them low
with V
CC
as low as 200mV. This helps prevent indeter-
minate voltages from appearing on the outputs during
power-up.
In applications where the low voltage pull-down capabil-
ity is important, the supply to which the external pull-up
resistor connects should be the same supply which pow-
ers the part. Using the same supply for both ensures that
RST, OUT1 and OUT2 never fl oat above 200mV during
power-up, as the pull-down ability of the pin will then
increase as the required pull-down current to maintain a
logic low increases.
Once V
CC
passes the UVLO threshold, polarity selection and
timer initialization will occur. If the monitored ADJ input is
valid, the corresponding OUT will be released. When both
ADJ1 and ADJ2 are valid, the appropriate timeout delay
will begin, after which RST will be released.
Power-Down
On power-down, once V
CC
drops below the UVLO threshold
or either V
ADJ
becomes invalid, RST asserts logic low. V
CC
of at least 0.5V guarantees a logic low of 0.15V at RST.
Shunt Regulator
The LTC2919 contains an internal 6.5V shunt regulator on
the V
CC
pin to allow operation from a high voltage supply.
To operate the part from a supply higher than 6V, the V
CC
pin must have a current-limiting series resistor, R
CC
, to
the supply. This resistor should be sized according to the
following equation:
V
S(MAX)
– 6.2V
10mA
≤ R
CC
≤
V
S(MIN)
– 6.8V
200µA + I
REF
where V
S(MIN)
and V
S(MAX)
are the operating minimum and
maximum of the supply, and I
REF
is the maximum current
the user expects to draw from the reference output.
As an example, consider operation from an automobile bat-
tery which might dip as low as 10V or spike to 60V. Assume
that the user will be drawing 100A from the reference. We
must then pick a resistance between 5.4k and 10.7k.
When the V
CC
pin is connected to a low impedance supply,
it is important that the supply voltage never exceed 6V,
or the shunt regulator may begin to draw large currents.
Some supplies may have a nominal value suffi ciently
close to the shunt regulation voltage to prevent sizing of
the resistor according to the above equation. For such
supplies, a 470 series resistor may be used.
Adjust Polarity Selection
The external connection of the SEL pin selects the polarities
of the LTC2919 adjustable inputs. SEL may be connected
to GND, connected to V
CC
or left unconnected during
normal operation. When left unconnected, the maximum
leakage allowable from the pin is ±5µA. Table 1 shows
the three possible selections of polarity based on SEL
connection.
Table 1. Voltage Threshold Selection
ADJ1 INPUT ADJ2 INPUT SEL
Positive Polarity
(+) UV or (–) OV
Positive Polarity
(+) UV or (–) OV
V
CC
Positive Polarity
(+) UV or (–) OV
Negative Polarity
(–) UV or (+) OV
Open
Negative Polarity
(–) UV or (+) OV
Negative Polarity
(–) UV or (+) OV
Ground
Note: Open = open circuit or driven by a three-state buffer in high impedance
state with leakage current less than 5A.
