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NCV8518B

Low Dropout Linear Regulator with Watchdog, Wake Up, RESET, and ENABLE

ON Semiconductor 

NCV8518B

OPERATING DESCRIPTION

General

The NCV8518B 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 NCV8518B

ideal for use in both battery operated and automotive

applications.

The NCV8518B 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 NCV8518B 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 RDELAY. 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 NCV8518B 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 VOUT = 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 VIN, 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) * RDELAY (W)

RESET Delay Time (seconds) = (5.21 10−8) * RDELAY (W)

RESET High to Wakeup Delay Time (seconds) =

(2.08 10−7) * RDELAY (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.

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