14

LTC690/LTC691

LTC694/LTC695

Low

External Clock Input

1024 clks

4096 clks

512 clks

2048 clks

Low

External Capacitor*

Floating or High

Low

100ms

1.6 sec

50ms

200ms

Floating or High

Floating or High

1.6 sec

1.6 sec

50ms

200ms

S

APPLICATI

I FOR ATIO

IMMEDIATELY

AFTER RESET

(Long Period)

1.6 sec

• C

70pF

200ms

• C

70pF

800ms

• C

70pF

NORMAL

(Short Period)

WATCHDOG TIME-OUT PERIOD

RESET ACTIVE TIME

LTC691

LTC695

400ms

• C

70pF

OSC IN

OSC SEL

Table 2. LTC691 and LTC695 Reset Active Time and Watchdog Time-Out Selections

Figure 13. The External Push-Button Reset

Figure 14. The External Push-Button Reset with the LTC1235

Push-Button Reset

The LTC690 family does not provide a logic input for direct

connection to a pushbutton. However, a push-button in

series with a 100

ΩresistorconnectedtotheRESEToutput

pin (Figure 13) provides an alternative for manual reset.

Connecting a 0.1

µF capacitor to the RESET pin debounces

the push-button input.

The 100

Ω resistor in series with the push-button is

required to prevent the ringing, due to the capacitance and

lead inductance, from pulling the RESET pins of the MPU

and LTC69X below ground.

If a dedicated pushbutton reset input is desired, the

LTC1235 is a good choice (Figure 14). It has all the

functions of the LTC695 and provides push-button reset

as an extra feature. Its push-button is internally debounced

and invokes the normal 200ms reset sequence. This

eliminates the need for the 100

Ω resistor and 0.1µF

capacitor. It also provides a more consistent reset pulse.

5V

100

Ω

LTC690/LTC691

LTC694/LTC695

GND

690 F13

RESET

0.1

µF

MPU

(e.g. 6805)

RESET

5V

LTC1235

GND

690 F14

RESET

MPU

(e.g. 6805)

RESET

PBRST

184,000

C(pF) • 1025