LTC2909

8

2909fa

APPLICATIO S I FOR ATIO

The LTC2909 is a low power, high accuracy dual/triple

supply monitor with two adjustable inputs and an ac-

curate UVLO. 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. Both input voltages

figured for positive polarity, below threshold if configured

for the reset timeout before

⎯R⎯S⎯T is released. The LTC2909

asserts the reset output during power-up, power-down and

brownout conditions on any of the voltage inputs.

Power-Up

The LTC2909 uses proprietary low voltage drive circuitry

for the

⎯R⎯S⎯T pin which holds ⎯R⎯S⎯T low with as little as

on the

⎯R⎯S⎯T line 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 powers

the part. Using the same supply for both ensures that

⎯R⎯S⎯T

never floats 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.

and timer initialization will occur. If the monitored supplies

(ADJ1 and ADJ2) are valid, the appropriate timeout delay

will begin, after which

⎯R⎯S⎯T will be released. Otherwise, the

the UVLO threshold) before beginning the timeout.

Power-Down

of at least 0.5V guarantees a logic low of 0.15V at

⎯R⎯S⎯T.

Shunt Regulator

The LTC2909 contains an internal 6.5V shunt regulator on

should be sized according to the following equation:

VV

mA

R

VV

µA I

SMAX

CC

SMIN

VRE

()

(

)

–.

–.

62

10

68

200

≤≤

+

FF

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 100μA from the reference. We

must then pick a resistance between 5.4k and 10.7k.

it is important that the supply voltage never exceed 6V,

or the shunt regulator may begin to draw large currents.

Some supplies may have nominal value sufficiently close

to the shunt regulation voltage to prevent sizing of the

resistor according to the above equation. For such sup-

plies, a 470Ω series resistor may be used.

Polarity Selection

The external connection of the SEL pin selects the polarities

of the LTC2909 adjustable inputs. SEL may be connected to

operation. When left unconnected, the maximum leakage

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

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 10μA.

If the user’s application requires, the SEL pin may be driven

leakage of the three-state pin.