NCV8508(2004) 데이터 시트보기 (PDF) - ON Semiconductor

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NCV8508
(Rev.:2004)

5.0 V, 250 mA LDO with Watchdog and RESET

ON Semiconductor 

NCV8508

CIRCUIT DESCRIPTION

Functional Description

To reduce the drain on the battery a system can go into a

low current consumption mode when ever its not performing

a main routine. The Wakeup signal is generated

continuously and is used to interrupt a microcontroller that

is in sleep mode. The nominal output is a 5.0 volt square

wave (voltage generated from VOUT) with a duty cycle of

50% at a frequency that is determined by a timing resistor,

RDelay.

When the microprocessor receives a rising edge from the

Wakeup output, it must issue a Watchdog pulse and check its

inputs to decide if it should resume normal operations or

remain in the sleep mode.

The first falling edge of the Watchdog signal causes the

Wakeup to go low within 2.0 ms (typ) and remain low until

the next Wakeup cycle (see Figure 17). Other Watchdog

pulses received within the same cycle are ignored (Figure 3).

During power up, RESET is held low until the output

voltage is in regulation. During operation, if the output

voltage shifts below the regulation limits, the RESET

toggles low and remains low until proper output voltage

regulation is restored. After the RESET delay, RESET

returns high.

The Watchdog circuitry continuously monitors the input

Watchdog signal (WDI) from the microprocessor. The

absence of a falling edge on the Watchdog input during one

Wakeup cycle will cause a RESET pulse to occur at the end

of the Wakeup cycle. (see Figure 4).

The Wakeup output is pulled low during a RESET

regardless of the cause of the RESET. After the RESET

returns high, the Wakeup cycle begins again (see Figure 4).

The RESET Delay Time, Wakeup signal frequency and

RESET high to Wakeup delay time are all set by one external

resistor RDelay.

Wakeup Period = (4.17 × 10−7)RDelay

RESET Delay Time = (5.21 × 10−8)RDelay

RESET HIGH to Wakeup Delay Time = (2.08 × 10−7)RDelay

Resistor temperature coefficient and tolerance as well as

the tolerance of the NCV8508 must be taken into account in

order to get the correct system tolerance for each parameter.

WDI

Wakeup

Wakeup

Response

to WDI

Figure 17. Wakeup Response to WDI

RESET

Wakeup

Wakeup

Response

to RESET

Figure 18. Wakeup Response to RESET (Low

Voltage)

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