LTC2912 データシートの表示（PDF） - Linear Technology

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LTC2912

Single UV/OV Voltage Monitor

Linear Technology 

LTC2912

APPLICATIONS INFORMATION

Power-Up/Power-Down

As soon as VCC reaches 1V during power up, the UV output

asserts low and the OV output weakly pulls to VCC.

The LTC2912 is guaranteed to assert UV low, OV high

(LTC2912-1, LTC2912-2) and OV low (LTC2912-3) under

conditions of low VCC, down to VCC = 1V. Above VCC = 2V

(2.1V maximum), the VH and VL inputs take control.

Once the VH input and VCC become valid an internal timer

is started. After an adjustable delay time, UV weakly pulls

high.

Threshold Accuracy

Reset threshold accuracy is important in a supply-sensitive

system. Ideally, such a system resets only if supply voltages

fall outside the exact thresholds for a specified margin.

Both LTC2912 inputs have a relative threshold accuracy

of ±1.5% over the full operating temperature range.

For example, when the LTC2912 is programmed to moni-

tor a 5V input with a 10% tolerance, the desired UV trip

point is 4.5V. Because of the ±1.5% relative accuracy of

the LTC2912, the UV trip point can be anywhere between

4.433V and 4.567V which is 4.5V ±1.5%.

Likewise, the accuracy of the resistances chosen for RA,

RB and RC can affect the UV and OV trip points as well.

Using the example just given, if the resistances used to

set the UV trip point have 1% accuracy, the UV trip range

is between 4.354V and 4.650V. This is illustrated in the

following calculations.

The UV trip point is given as:

VUV

=

0.5V

1+

RC

RA + RB





The two extreme conditions, with a relative accuracy of

1.5% and resistance accuracy of 1%, result in:

VUV(MIN)

=

0.5V

•

0.985

•

1+

RC

(RA +

• 0.99 

RB) • 1.01

and

VUV(MAX)

=

0.5V

•

1.015

•

1+

RC • 1.01 

(RA + RB) • 0.99

For a desired trip point of 4.5V, RC = 8

RA + RB

Therefore,

VUV(MIN)

=

0.5V

• 0.985

•

1+

8

0.99

1.01

=

4.354V

and

VUV(MAX)

=

0.5V

•

1.015

•

1+

8

1.01

0.99

=

4.650V

Glitch Immunity

In any supervisory application, noise riding on the moni-

tored DC voltage causes spurious resets. To solve this

problem without adding hysteresis, which causes a new

error term in the trip voltage, the LTC2912 lowpass filters

the output of the first stage comparator at each input. This

filter integrates the output of the comparator before as-

serting the UV or OV logic. A transient at the input of the

comparator of sufficient magnitude and duration triggers

the output logic. The Typical Performance Characteristics

show a graph of the Transient Duration vs Comparator

Overdrive.

UV/OV Timing

The LTC2912 has an adjustable timeout period (tUOTO) that

holds OV, OV or UV asserted after each fault has cleared.

This delay assures a minimum reset pulse width allowing

settling time for the monitored voltage after it has entered

the “valid” region of operation.

2912fa

8

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