LTC2917/LTC2918
10
29178fb
APPLICATIONS INFORMATION
Threshold Accuracy
The trip threshold for the supplies monitored is selected
by confi guring the three-state input pins. When using the
adjustable input, a external resistive divider sets the trip
threshold, allowing the user complete control over the
trip point. Selection of this trip voltage is crucial to the
reliability of the system.
Any power supply has some tolerance band within which
it is expected to operate (e.g. 5V±10%). It is generally
undesirable that a supervisor issue a reset when the power
supply is inside this tolerance band. Such a “nuisance”
reset reduces reliability by preventing the system from
functioning under normal conditions.
To prevent nuisance resets, the supervisor threshold must
be guaranteed to lie outside the power supply tolerance
band. To ensure that the threshold lies outside the power
supply tolerance range, the nominal threshold must lie out-
side that range by the monitor’s accuracy specifi cation.
All 27 of the selectable thresholds have the same relative
threshold accuracy of ±1.5% of the programmed nominal
input voltage (over the full operating temperature range).
Consider the example of monitoring a 5V supply with a 10%
tolerance. The nominal threshold internal to the LTC2917
is 11.5% below the 5V input at 4.425V. With ±1.5% ac-
curacy, the trip threshold range is 4.425V±75mV over
temperature (i.e. 10% to 13% below 5V). The monitored
system must thus operate reliably down to 4.35V or 13%
below 5V over temperature.
Glitch Immunity
The above discussion is concerned only with the DC
value of the monitored supply. Real supplies also have
relatively high-frequency variation, from sources such as
load transients, noise, and pickup. These variations should
not be considered by the monitor in determining whether
a supply voltage is valid or not. The variations may cause
spurious outputs at RST, particularly if the supply voltage
is near its trip threshold.
Two techniques are used to combat spurious reset without
sacrifi cing threshold accuracy. First, the timeout period
helps prevent high-frequency variation whose frequency
is above 1/ t
RST
from appearing at the RST output.
When the voltage at VM goes below the threshold, the
RST pin asserts low. When the supply recovers past
the threshold, the reset timer starts (assuming it is not
disabled), and RST does not go high until it fi nishes. If
the supply becomes invalid any time during the timeout
period, the timer resets and starts a fresh when the supply
next becomes valid.
While the reset timeout is useful at preventing toggling
of the reset output in most cases, it is not effective at
preventing nuisance resets due to short glitches (due to
load transients or other effects) on a valid supply.
To reduce sensitivity to these short glitches, the comparator
has additional anti-glitch circuitry. Any transient at the input
of the comparator needs to be of suffi cient magnitude and
duration t
UV
before it can change the monitor state.
The combination of the reset timeout and anti-glitch cir-
cuitry prevents spurious changes in output state without
sacrifi cing threshold accuracy.
Adjustable Input
When the monitor threshold is confi gured as ADJ, the
internal comparator input is connected to the pin without
a resistive divider, and the pin is high-impedance. Thus,
any desired threshold may be chosen by attaching VM to
a tap point on an external resistive divider between the
monitored supply and ground, as shown in Figure 1.
Figure 1. Setting the Trip Point Using the
Adjustable Threshold.
–
+
+
–
0.5V
VM
R2
R1
29178 F01
V
MON
