LTC2909 datasheet(13/16 Pages) LINER

13 / 16 page

LTC2909

2909fa

APPLICATIO S I FOR ATIO

If the user wishes to avoid having an external capacitor,

the TMR pin should be tied to ground, switching the part

to an internal 200ms timer.

If the user requires a shorter timeout than 400μs, or

wishes to perform application-speciﬁc processing of the

reset output, the part may be put in comparator mode by

tying the TMR pin to VCC. In comparator mode, the timer

is bypassed and comparator outputs go straight to the

reset output.

The current required to hold TMR at ground or VCC is

about 2μA. To force the pin from the ﬂoating state to

ground or VCC may require as much as 100μA during the

transition.

When the part is in comparator mode, one of the two

means of preventing false reset has been removed, so

a small amount of one-sided hysteresis is added to the

inputs to prevent oscillation as the monitored voltage

passes through the threshold. This hysteresis is such

that the valid-to-invalid transition threshold is unchanged,

but the invalid-to-valid threshold is moved by about

0.7%. Thus, when the ADJ input polarity is positive,

the threshold voltage is 500mV nominal when the in-

put is above 500mV. As soon as the input drops below

500mV, the threshold moves up to 503.5mV nominal.

Conversely, when conﬁgured as a negative-polarity input,

the threshold is 500mV when the input is below 500mV,

and switches to 496.5mV when the input goes above

500mV.

The comparator mode feature should be enabled by directly

shorting the TMR pin to the VCC pin. Connecting the pin to

any other voltage may have unpredictable results.

Selecting the Reset Timing Capacitor

Connecting a capacitor, CTMR, between the TMR pin and

ground sets the reset timeout, tRST. The following formula

approximates the value of capacitor needed for a particular

timeout:

CTMR = t⎯R⎯S⎯T • 110 [pF/ms]

Leaving the TMR pin open with no external capacitor

generates a reset timeout of approximately 400μs.

Maximum length of the reset timeout is limited by the

ability of the part to charge a large capacitor on start-up.

Initially, with a large (discharged) capacitor on the TMR

pin, the part will assume it is in internal timer mode (since

the pin voltage will be at ground). If the 2μA ﬂowing out of

the TMR pin does not charge the capacitor to the ground-

sense threshold within the ﬁrst 200ms after supplies

become good, the internal timer cycle will complete and

⎯R⎯S⎯T will go high too soon.

APPLICATIO S I FOR ATIO

Table 3. Suggested Resistor Values for 5% Monitoring

NOMINAL

VOLTAGE

5% UV

5% OV

5% UV and OV

RX1

RX2

RX1

RX2

RX4

RX5

RX6

232k

10.2M

102k

5.11M

82.5k

11.5k

4.12M

115k

3.09M

200k

6.19M

76.8k

10.7k

2.37M

49.9k

1.07M

102k

2.49M

76.8k

10.7k

1.87M

115k

1.82M

78.7k

1.43M

162k

22.6k

2.94M

137k

1.15M

137k

1.33M

76.8k

10.7k

732k

3.3

221k

1.15M

340k

2.05M

76.8k

10.7k

453k

2.5

115k

422k

51.1k

221k

137k

19.1k

576k

1.8

63.4k

150k

115k

324k

82.5k

11.5k

221k

1.5

59.0k

107k

137k

301k

76.8k

10.7k

158k

1.2

127k

158k

102k

158k

187k

26.1k

267k

200k

174k

100k

113k

107k

15.0k

105k

–5

133k

1.37M

118k

1.37M

174k

20.0k

2.00M

–9

97.6k

1.74M

115k

2.32M

182k

22.6k

3.65M

–12

107k

2.49M

40.2k

1.07M

40.2k

5.11k

1.07M

–15

107k

3.09M

309k

10.2M

309k

40.2k

10.2M

Trip points are nominal voltage ±6.5%.

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