LTC2935CDC-2-TRPBF datasheet(7/12 Pages) LINER

Part #	LTC2935CDC-2-TRPBF

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

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Manufacturer	LINER [Linear Technology]

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LTC2935

7

2935f

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 150mV of voltage mar-

gin 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 to signal preparation for

controlled shutdown. 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,

RSTpullslowuntilVCCrisesabovetheresetthresholdplus

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. The

power-fail threshold is always 150mV higher than the

conﬁgured reset threshold. Set the threshold selection

inputs (S2, S1, S0) accordingly.

Table 1. Falling Threshold Selection

RESET

THRESHOLD

(V)

POWER-FAIL

THRESHOLD

(V)

S2

S1

S0

3.30

3.45

Low

3.15

3.30

Low

High

3.00

3.15

Low

High

2.85

3.00

Low

High

Low

2.70

2.85

High

Low

2.55

2.70

High

Low

2.40

2.55

High

Low

High

2.25

2.40

High

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