LTC2960
APPLICATIONS INFORMATION
VOLTAGE MONITORING
The LTC2960 is a voltage supervisor with a wide operating
voltage range up to 36V with only 850nA quiescent current.
The supervisor has two outputs,
RST
and OUT that pro-
vide voltage monitoring capabilities for system power-up,
power-down and brown-out conditions. Built-in hysteresis
and a reset timeout period ensure that fluctuations due to
load transients or supply noise do not cause chattering
of the status outputs. The LTC2960 can provide reset and
voltage status signals to a microprocessor based system
or can alternatively be used as an Under Voltage Lock Out
(UVLO) for DC/DC switchers or LDOs for control over a
battery operated system.
If the monitored voltage drops below the reset threshold,
RST
pulls low until the ADJ input rises above 0.4V plus
2.5% hysteresis. An internal reset timer delays the return
of the
RST
output to a high state to provide monitored
voltage settling and initialization time. The
RST
output is
typically connected to a processor reset input.
If the monitored supply voltage falls to the IN
+
(LTC2960-1/
LTC2960-3) threshold, the spare comparator pulls OUT low.
OUT remains low until the IN
+
input rises above 0.4V plus
5% hysteresis. OUT is typically used to signal preparation
for controlled shutdown. For example, the OUT output
may be connected to a processor nonmaskable interrupt
(NMI). Upon interrupt, the processor begins shutdown
procedures such as supply sequencing and/or storage/
erasure of system state in nonvolatile memory.
If the monitored supply voltage rises to the IN
–
threshold
(LTC2960-2/LTC2960-4), the spare comparator pulls OUT
low. OUT remains low until the IN
–
falls below 0.4V minus
5% hysteresis. The LTC2960-2/LTC2960-4 operates as an
undervoltage and overvoltage monitor.
Few, if any, external components are necessary for reliable
operation. However, a decoupling capacitor between V
CC
and ground is recommended (0.01μF minimum). Use a
capacitor with a compatible voltage rating.
THRESHOLD CONFIGURATION
The LTC2960 monitors voltage applied to its inputs IN
+
/IN
–
and ADJ. A resistive divider connected between a monitored
voltage and ground is used to bias the inputs. Figure 1
demonstrates how the inputs can be made dependent upon
a single voltage (V1). Only three resistors are required.
To calculate their values, specify desired falling reset (V
R
)
and IN
+
(V
IN+
) thresholds with V
IN+
> V
R
. For example:
V
IN+
= 6.4V, V
R
= 6V
V1
R3
LTC2960-1/
LTC2960-3
ADJ
R2
IN
+
R1
OUT
2960 F01
RST
Figure 1. Configuration for Single Voltage Monitoring
The solution for R1, R2 and R3 provides three equations
and three unknowns. Maximum resistor size is governed
by maximum input leakage current. For the LTC2960,
the maximum input leakage current below 85°C is 1nA.
For a maximum error of 1% due to both input currents,
the resistor divider current should be at least 100 times
the sum of the leakage currents, or 0.2μA. At the reset
threshold, V1 = 6V, R
SUM
= 8MΩ where:
R
SUM
= R1 + R2 + R3
Both the falling reset and IN
+
thresholds are 0.4V, so:
V
•R
0.4V
•
8MΩ
R1=
TH SUM
=
=
500k
V
IN
+
6.4V
The closest 1% value is 499k. R2 can be determined
from:
V
•
R
0.4V
•
8MΩ
R2
=
TH SUM
– R1=
– 499k
V
R
6V
R2 = 34.33k
2960f
8
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
