LTC2917/LTC2918

10

29178f

APPLICATIONS INFORMATION

Threshold Accuracy

The trip threshold for the supplies monitored is selected

by configuring 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 specification.

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 ⎯R⎯S⎯T, particularly if the supply voltage

is near its trip threshold.

Two techniques are used to combat spurious reset without

sacrificing threshold accuracy. First, the timeout period

helps prevent high-frequency variation whose frequency

When the voltage at VM goes below the threshold, the

⎯R⎯S⎯T pin asserts low. When the supply recovers past

the threshold, the reset timer starts (assuming it is not

disabled), and ⎯R⎯S⎯T does not go high until it finishes. 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 sufficient magnitude and

The combination of the reset timeout and anti-glitch cir-

cuitry prevents spurious changes in output state without

sacrificing threshold accuracy.

Adjustable Input

When the monitor threshold is configured 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.

+

VM

R2

R1

29178 F01