LTC6995-1/LTC6995-2

Rev. B

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16

APPLICATIONS INFORMATION

For shorter power-on reset times (1ms to 73ms) the timer

startup delay becomes a significant part of the total POR

time. To take this delay into account the value for RSET can

be modified from the values shown in Table 2. For a POR

time in the range from 1ms to 16ms (DIVCODE = 0), RSET

should be tPOR(ms) • 49.5. For a POR time in the range

from 4.5ms to 73ms (DIVCODE = 1), RSET is tPOR(ms) •

10.9. For longer POR times (DIVCODE 2 through 7) the

startup time is insignificant. After power on, the delay fol-

lowingaresetconditionwillbeinthesamerangeasshown

for tDELAY in Table 2 for these two DIVCODE selections.

For short POR times, a more precise estimation of the

startup time can be found from the following:

tSTART(µs)= 256+16 •(12 –DIVCODE)

(

)

RSET(kΩ)

50

+ 80

Supply bounce resets the internal timer so the POR circuit

automatically debounces supply noise. POR timing starts

fromthetimethattheV+supplyhasreachedapproximately

1.4 volts.

Long Timer One Shots and Delay Generators

The POR circuit of Figure 12 is also useful when the reset

inputsaredriven.Thiscreatesedgetriggeredtimingevents

that are active low and can either be re-triggered or can

stop after one programmed interval. The programmed

time interval can range from only 500µs to over 4 hours

with just resistor value changes.

The circuits in Figure 13 show how a POR or active low

intervalcanbere-startedtoprovideafullsystemresettime.

The Figure 14 circuit requires an indication from the

system being reset that it is ready before timing out. The

LTC6995-2 can accommodate an active high OK signal.

By forcing a reset condition at power on the LTC6995 can

be used to create a long time delayed rising edge triggered

by either a falling edge signal (LTC6995-1) or a rising edge

signal (LTC6995-2) as show in Figure 15.

Figure 13. System Resets On Command with Full POR Time Interval. Reset Pulse Is Debounced Automatically

LTC6995-1

ACTIVE HIGH RESET

RST

GND

SET

OUT

V+

DIV

RSET

100k

R2

R1

0.1µF

V+

V+

POR

OUT

POL = 0

RST

tSTART +

1/2 tOUT

1/2 tOUT

TIMER

STOPPED

TIMER

STOPPED

RESET

POR

POR

V+

LTC6995-2

ACTIVE LOW RESET

RST

GND

SET

OUT

V+

DIV

RSET

100k

R2

R1

0.1µF

V+

V+

POR

OUT

POL = 0

RST

699512 F13

tSTART +

1/2 tOUT

1/2 tOUT

TIMER

STOPPED

TIMER

STOPPED

RESET

POR

POR

V+