LTC2935 データシートの表示（PDF） - Linear Technology

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LTC2935

Ultra-Low Power Supervisor with Power-Fail Output, Selectable Thresholds

Linear Technology 

LTC2935

APPLICATIONS INFORMATION

VOLTAGE MONITORING

Unmanaged power can cause various system problems.

At power-up, voltage fluctuation around critical thresholds

can cause improper system or processor initialization.

The LTC2935 provides power management capabilities

for the system power-up phase. The supervisory device

issues a system reset after the monitored voltage has

stabilized. Built-in hysteresis and filtering ensures that

fluctuations due to load transients or supply noise do

not cause chattering of the status outputs. Comparator

undervoltage glitch immunity is shown in the Typical

Performance Characteristics section. The curve dem-

onstrates the transient amplitude and width required to

switch the comparators.

Because many batteries exhibit large series resistance,

load currents can cause significant voltage drops. The

low DC current draw of the LTC2935 (at any input volt-

age) does not add to the loading problem. When voltage

is initially applied to VCC, RST and PFO pull low once there

is enough voltage to turn on the pull-down devices (1V

maximum).

If the monitored supply voltage falls to the power-fail

threshold, the built-in power-fail comparator pulls PFO

low. At this point, there remains a voltage margin before

the RST output pulls low. PFO remains low until VCC rises

above the power-fail threshold plus 2.5% hysteresis. PFO

is typically used in signal preparation for controlled shut-

down. For example, the PFO output may be connected to

a processor nonmaskable interrupt. Upon interrupt, the

processor may begin shutdown procedures. Shutdown

events may include supply sequencing and/or storage/

erasure of system state in nonvolatile memory.

If the monitored voltage drops below the reset threshold,

RST pulls low until VCC rises above the reset threshold plus

5% hysteresis. This may occur through battery charging

or replacement. An internal reset timer delays the return

of the RST output to a high state to provide settling and

initialization time. The RST output is typically connected

to a processor reset input.

Few, if any external components are necessary for reliable

operation. However, a decoupling capacitor between VCC

and ground is recommended (0.01μF minimum).

Threshold Configuration

The LTC2935 monitors and compares the VCC voltage

against two internal thresholds during operation. The

higher threshold (power-fail) is the level used for early

warning of a low voltage condition, reported on the PFO

output. The lower threshold (reset) is the level at which the

RST output pulls low. Use system requirements to choose

the appropriate reset threshold from Table 1.

Table 1. Falling Threshold Selection

LTC2935-1/LTC2935-2 LTC2935-3/LTC2935-4

RESET POWER-FAIL RESET POWER-FAIL

THRESHOLD THRESHOLD THRESHOLD THRESHOLD

(V)

(V)

(V)

(V)

S2 S1 S0

3.30

3.45

2.44

2.56 Low Low Low

3.15

3.30

2.32

2.44 Low Low High

3.00

3.15

2.20

2.32 Low High High

2.85

3.00

2.08

2.20 Low High Low

2.70

2.85

1.96

2.08 High High Low

2.55

2.70

1.84

1.96 High Low Low

2.40

2.55

1.72

1.84 High Low High

2.25

2.40

1.60

1.72 High High High

In the LTC2935-1/LTC2935-2 the power-fail threshold

is always 150mV higher than the configured reset

threshold. In the LTC2935-3/LTC2935-4 the power-fail

threshold is always 120mV higher than the configured

reset threshold. Set the threshold selection inputs (S2,

S1, S0) accordingly.

2935fa

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