LTC2907IDDB datasheet(10/16 Pages) LINER

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LTC2906/LTC2907

29067f

Threshold Accuracy

Reset threshold accuracy is of the utmost importance in a

supply sensitive system. Ideally such a system should not

reset while supply voltages are within a specified margin

below the rated nominal level. Both of the LTC2906/

LTC2907 inputs have the same relative threshold accu-

racy. The specification for LTC2906/LTC2907 is

±1.5% of

the programmed nominal input voltage (over the full

operating temperature range).

For example, when the LTC2906/LTC2907 are programmed

to handle a 5V input with 10% tolerance (S1 = V1 and TOL

= GND, refer to Table 1 and Table 3), it does not issue a

reset command when V1 is above 4.5V. The typical 10%

trip threshold is at 11.5% below the nominal input voltage

level. Therefore, the typical trip threshold for the 5V input

is 4.425V. With

±1.5% accuracy, the trip threshold range

is 4.425V

±75mV over temperature (i.e. 10% to 13%

below 5V). This implies that the monitored system must

operate reliably down to 4.35V or 13% below 5V over

temperature.

The same system using a supervisor with only

±2.5%

accuracy needs to work reliably down to 4.25V (4.375V

±125mV) or 15% below 5V, requiring the monitored

system to work over a much wider operating voltage

range.

In any supervisory application, supply noise riding on the

monitored DC voltage can cause spurious resets, particu-

larly when the monitored voltage is near the reset thresh-

old. A less desirable but common solution to this problem

is to introduce hysteresis around the nominal threshold.

Notice however, this hysteresis introduces an error term

in the threshold accuracy. Therefore, a

±2.5% accurate

monitor with

±1% hysteresis is equivalent to a ±3.5%

monitor with no hysteresis.

The LTC2906/LTC2907 take a different approach to solve

this problem of supply noise causing spurious reset. The

first line of defense against this spurious reset is a first

order low pass filter at the output of the comparator. Thus,

the comparator output goes through a form of integration

before triggering the output logic. Therefore, any kind of

transient at the input of the comparator needs to be of

sufficient magnitude and duration before it can trigger a

change in the output logic.

The second line of defense is the programmed delay time

tRST (200ms for LTC2906 and adjustable using an external

capacitor for LTC2907). This delay will eliminate the effect

of any supply noise, whose frequency is above 1/ tRST, on

the RST and RST output.

When either V1 or VADJ drops below its programmed

threshold, the RST pin asserts low (RST weakly pulls

high). When the supply recovers above the programmed

threshold, the reset-pulse-generator timer starts

counting.

If the supply remains above the programmed threshold

when the timer finishes counting, the RST pin weakly pulls

high (RST asserts low). However, if the supply falls below

the programmed threshold any time during the period

when the timer is still counting, the timer resets and starts

fresh when the supply next rises above the programmed

threshold.

Note that this second line of defense is only effective for a

rising supply and does not affect the sensitivity of the

system to a falling supply. Therefore, the first line of

defense that works for both cases of rising and falling is

necessary. These two approaches prevent spurious reset

caused by supply noise without sacrificing the threshold

accuracy.

Selecting the Reset Timing Capacitor

The reset time-out period for LTC2907 is adjustable in

order to accommodate a variety of microprocessor appli-

cations. Connecting a capacitor, CTMR, between the TMR

pin and ground sets the reset time-out period, tRST. The

following formula determines the value of capacitor needed

for a particular reset time-out period:

CTMR = tRST • 110 • 10–9 [F/s]

For example, using a standard capacitor value of 22nF

gives a 200ms delay.

The graph in Figure 2 shows the desired delay time as a

function of the value of the timer capacitor that should be

used:

APPLICATIO S I FOR ATIO

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