NCV8518A
http://onsemi.com
10
OPERATING DESCRIPTION
General
The NCV8518A is a precision micropower voltage
regulator featuring low quiescent current (100 mA typical at
250 mA load) and low dropout voltage (450 mV typical at
150 mA). Integrated microprocessor control functions
include Watchdog, Wakeup and RESET
. An Enable input is
provided for logic level control of the regulator state. The
combination of low quiescent current and comprehensive
microprocessor interface functions make the NCV8518A
ideal for use in both battery operated and automotive
applications.
The NCV8518A is internally protected against short
circuit and thermal runaway conditions. No external
components are required to engage these protective
mechanisms. The device continues to operate through 45
volt input transients, an important consideration in
automotive environments.
Wakeup and Watchdog
To reduce battery drain, a microprocessor or
microcontroller can transition to a low current consumption
(“sleep”) mode when code execution is suspended or
complete. The NCV8518A Wakeup signal is generated and
output periodically to interrupt sleep mode. The nominal
Wakeup output is a 5 volt square wave (generated from
VOUT) with a duty cycle of 50%, at a frequency determined
by external timing resistor R
DELAY
. In response to the rising
edge of the Wakeup signal, the microprocessor will
subsequently output a Watchdog pulse and check its inputs
to decide if it should resume normal operation or remain in
sleep mode.
The NCV8518A responds to the falling edge of the
Watchdog signal, which it expects at least once during each
Wakeup period. When the correct Watchdog signal is
received, the Wakeup output is forced low. Other Watchdog
pulses received within the same cycle are ignored. 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 be output at the end
of the Wakeup cycle (see Figure 4).
RESET
As output voltage falls, the RESET output will maintain
its current state down to V
OUT
= 1 V. A Reset signal (active
low) is asserted for any of four conditions:
1. During power up, RESET
is held low until the
output voltage is in regulation.
2. During operation, if the output voltage falls below
the Reset threshold, RESET
switches low, and will
remain low until both the output voltage has
recovered and the Reset delay timer cycle has
completed following that recovery.
3. RESET
will switch low if the regulator does not
receive a Watchdog input signal within a Wakeup
period.
4. Regardless of output voltage, RESET
will switch
low if the regulator input voltage V
IN
, falls below
a level required to sustain the internal control
circuits. The specific voltage is temperature
dependent, and is approximately 4.75 V at 20°C.
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, according to the following equations:
Wakeup Period (seconds) = (4.17 10
−7
) * R
DELAY
(W)
RESET
Delay Time (seconds) = (5.21 10
−8
) * R
DELAY
(W)
RESET
High to Wakeup Delay Time (seconds) =
(2.08 10
−7
) * R
DELAY
(W)
The voltage present at the Delay pin is a buffered bandgap
voltage (~1.25 V) and can be used as a reference for an
external tracking regulator.
Enable
This is a standard TTL and CMOS logic compatible input
that can be used to turn the regulator on or off. Logic high
enables the regulator; logic low disables it (also called
shutdown). In the disabled/shutdown state, the pass
transistor is off and total quiescent current is less than 1 mA.
