LTC2909

11

2909fa

APPLICATIO S I FOR ATIO

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.

A common solution to the problem of spurious reset is

to introduce hysteresis around the nominal threshold.

However, this hysteresis degrades the effective accuracy

of the monitor and increases the range over which the

system must operate. The LTC2909 therefore does not

have hysteresis, except in comparator mode (see Setting

the Reset Timeout). If hysteresis is desired in other modes,

it may be added externally. See Typical Applications for

an example.

The LTC2909 uses two techniques to combat spurious

reset without sacrificing threshold accuracy. First, the

timeout period helps prevent high frequency variation

⎯R⎯S⎯T output.

When either ADJ1 or ADJ2 becomes invalid, 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 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 pre-

venting nuisance resets due to short glitches (from load

transients or other effects) on a valid supply. To reduce

sensitivity to these short glitches, the comparator outputs

go through a lowpass filter before triggering the output

logic. Any transient at the input of a comparator needs to

be of sufficient magnitude and duration to pass the filter

before it can change the monitor state.

The combination of the reset timeout and comparator

filtering prevents spurious changes in the output state

without sacrificing threshold accuracy. If further supply

glitch immunity is needed, the user may place an external

capacitor from the ADJ input to ground. The resultant RC

lowpass filter with the resistor divider will further reject

high frequency components of the supply, at the cost of

slowing the monitor’s response to fault conditions.

Selecting External Resistors

In a typical positive supply monitoring application, the

ADJx pin connects to a tap point on an external resistive

divider between a positive voltage being monitored and

ground, as shown in Figure 1.

When monitoring a negative supply, the ADJx pin connects

to a tap point on a resistive divider between the negative

voltage being monitored and the buffered reference (REF),

as shown in Figure 2.

–

+

+

–

0.5V

2909 F01

ADJx

–

+

+

–

0.5V

2909 F02

ADJx

REF

Figure 1. Setting Positive Supply Trip Point

Figure 2. Setting Negative Supply Trip Point

Normally the user will select a desired trip voltage based

on their supply and acceptable tolerances, and a value of

resistor divider will be approximately:

I

V

=

05

1

.

Recommended range is 1k to 1M.