LTC2906/LTC2907

11

29067f

Leaving the TMR pin open with no external capacitor

generates a reset time-out of approximately 200

µs. For

long reset time-out, the only limitation is the availability of

a large value capacitor with low leakage. The TMR capaci-

tor will never charge if the leakage current exceeds the

TMR charging current of 2.1

µA (typical).

RST and RST Output Characteristics

The DC characteristics of the RST and RST pull-up and

pull-down strength are shown in the Typical Performance

Characteristics section. Both RST and RST have a weak

The weak pull-up and strong pull-down arrangement

allows these two pins to have open-drain behavior while

possessing several other beneficial characteristics.

The weak pull-ups eliminate the need for external pull-up

resistors when the rise time on these pins is not critical. On

the other hand, the open-drain RST configuration allows

for wired-OR connections and can be useful when more

than one signal needs to pull-down on the RST line.

As noted in the Power-Up and Power-Down sections, the

on. Therefore, an external pull-up resistor of no more than

100k is recommended on the RST pin if the state and pull-

Note however, by adding an external pull-up resistor, the

pull-up strength on the RST pin is increased. Therefore, if

it is connected in a wired-OR connection, the pull-down

strength of any single device needs to accommodate this

additional pull-up strength.

Output Rise and Fall Time Estimation

The RST and RST output have strong pull-down capability.

The following formula estimates the output fall time (90%

transistor estimated to be typically 40

room temperature (25

capacitance on the pin. Assuming a 150pF load capaci-

tance, the fall time is about 13ns.

The rise time on the RST and RST pins is limited by weak

formula estimates the output rise time (10% to 90%) at the

RST and RST pins:

Notice that this pull-up transistor is modeled as a

6

µA current source in the Block Diagram as a typical

representation.

estimated as follows:

R

MAX V V

V

PU

CC

=Ω

610

11

5

•

(,

) –

capacitance, the rise time is 86

µs. A smaller external pull-

up resistor maybe used if the output needs to pull up faster

and/or to a higher voltage. For example, the rise time

reduces to 3.3

µsfora150pFloadcapacitance,whenusing

a 10k pull-up resistor.

APPLICATIO S I FOR ATIO

10p

100p

1n

10n

100n

1

µ

29067 F02

10000

1000

100

10

1

0.1

Figure 2. Reset Time-Out Period vs Capacitance