NCV8508
http://onsemi.com
9
CIRCUIT DESCRIPTION
Functional Description
To reduce the drain on the battery, a system can go into a
low current consumption mode whenever it is 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 V
OUT
) with a duty cycle of
50% at a frequency that is determined by a timing resistor,
R
Delay
.
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 R
Delay
.
Wakeup Period = (4.17 × 10
−7
)R
Delay
RESET Delay Time = (5.21 × 10
−8
)R
Delay
RESET HIGH to Wakeup Delay Time = (2.08 × 10
−7
)R
Delay
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.
Figure 17. Wakeup Response to WDI
Wakeup
WDI
Wakeup
Response
to WDI
Figure 18. Wakeup Response to RESET (Low
Voltage)
Wakeup
Response
to RESET
RESET
Wakeup
