LTC2919HDDB-3.3-TRPBF データシートの表示（PDF） - Linear Technology

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LTC2919HDDB-3.3-TRPBF

Precision Triple/Dual Input UV, OV and Negative Voltage Monitor

Linear Technology 

LTC2919

APPLICATIO S I FOR ATIO

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 configured for positive polarity,

below threshold if configured for negative polarity).

Both input voltages (VADJ1 and VADJ2) must be valid and

VCC 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 VCC 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 float 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 VCC 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 VCC drops below the UVLO threshold

or either VADJ becomes invalid, RST asserts logic low. VCC

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

8

Shunt Regulator

The LTC2919 contains an internal 6.5V shunt regulator on

the VCC pin to allow operation from a high voltage supply.

To operate the part from a supply higher than 6V, the VCC

pin must have a current-limiting series resistor, RCC, to

the supply. This resistor should be sized according to the

following equation:

VS(MAX) – 6.2V

10mA

≤

RCC

≤

VS(MIN) – 6.8V

200µA + IREF

where VS(MIN) and VS(MAX) are the operating minimum and

maximum of the supply, and IREF 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 100μA from the reference. We

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

When the VCC 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 sufficiently

close to the shunt regulation voltage to prevent sizing of

the resistor according to the above equation. For such

supplies, a 470Ω 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 VCC 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

Positive Polarity

(+) UV or (–) OV

Positive Polarity

(+) UV or (–) OV

Positive Polarity

(+) UV or (–) OV

Negative Polarity

(–) UV or (+) OV

Negative Polarity

(–) UV or (+) OV

Negative Polarity

(–) UV or (+) OV

SEL

VCC

Open

Ground

Note: Open = open circuit or driven by a three-state buffer in high impedance

state with leakage current less than 5μA.

2919f

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