LTC690/LTC691

LTC694/LTC695

15

690fe

APPLICATIONS INFORMATION

Table 2. LTC691 and LTC695 Reset Active Time and Watchdog Timeout Selections

OSC SEL

OSC IN

WATCHDOG TIME-OUT PERIOD

RESET ACTIVE TIME

NORMAL

(Short Period)

IMMEDIATELY

AFTER RESET

(Long Period)

LTC691

LTC695

Low

External Clock Input

1024 clks

4096 clks

512 clks

2048 clks

Low

External Capacitor*

16

70

. sec

pF

C

•

200

70

ms

pF

C

•

800

70

ms

pF

C

•

Floating or High

Floating or High

Low

Floating or High

100ms

1.6 sec

1.6 sec

1.6 sec

50ms

50ms

200ms

200ms

184 000

1025

,

()

CpF •

Pushbutton Reset

The LTC690 family does not provide a logic input for direct

connection to a pushbutton. However, a pushbutton in

series with a 100Ω resistor connected to the RESET output

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

Connecting a 0.1μF capacitor to the RESET pin debounces

the pushbutton input.

The 100Ω resistor in series with the pushbutton 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 pushbutton reset as an extra feature.

Its pushbutton is internally debounced and invokes the

normal 200ms reset sequence. This eliminates the need

for the 100Ω resistor and 0.1μF capacitor. It also provides

a more consistent reset pulse.

5V

100Ω

LTC690/LTC691

LTC694/LTC695

GND

690 F13

RESET

0.1μF

MPU

(e.g. 6805)

RESET

5V

LTC1235

GND

690 F14

RESET

MPU

(e.g. 6805)

RESET

PBRST

Figure 13. The External Pushbutton Reset

Figure 14. The External Pushbutton Reset with the LTC1235