LTC690 查看數據表（PDF） - Linear Technology

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LTC690

Microprocessor Supervisory Circuits

Linear Technology 

LTC690/LTC691

LTC694/LTC695

APPLICATIONS INFORMATION

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

WATCHDOG TIME-OUT PERIOD

OSC SEL

OSC IN

NORMAL

(Short Period)

IMMEDIATELY

AFTER RESET

(Long Period)

Low

External Clock Input

1024 clks

4096 clks

RESET ACTIVE TIME

LTC691

512 clks

LTC695

2048 clks

Low

External Capacitor*

1.6sec •C

70pF

200ms •C

70pF

800ms •C

70pF

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

*The nominal internal frequency is 10.24kHz. The nominal oscillator frequency with external capacitor is fOSC (Hz) = C (pF)•1025

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.

5V

VCC

RESET

LTC690/LTC691

LTC694/LTC695

GND

0.1μF

100Ω

RESET

MPU

(e.g. 6805)

690 F13

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.

Figure 13. The External Pushbutton Reset

5V

VCC

RESET

LTC1235

PBRST

GND

RESET

MPU

(e.g. 6805)

690 F14

Figure 14. The External Pushbutton Reset with the LTC1235

690fe

15

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