LTC2935
7
2935fa
APPLICATIONS INFORMATION
VOLTAGE MONITORING
Unmanaged power can cause various system problems.
At power-up, voltage fl uctuation 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 fi ltering ensures that
fl uctuations 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 signifi cant 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 V
CC
, 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 V
CC
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 V
CC
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 V
CC
and ground is recommended (0.01μF minimum).
Threshold Confi guration
The LTC2935 monitors and compares the V
CC
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
THRESHOLD
(V)
POWER-FAIL
THRESHOLD
(V)
RESET
THRESHOLD
(V)
POWER-FAIL
THRESHOLD
(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 confi gured reset
threshold. In the LTC2935-3/LTC2935-4 the power-fail
threshold is always 120mV higher than the confi gured
reset threshold. Set the threshold selection inputs (S2,
S1, S0) accordingly.
