NCV4279B

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7

CIRCUIT DESCRIPTION

REGULATOR CONTROL FUNCTIONS

The NCV4279B contains the microprocessor compatible

control function RESET (Figure 11).

Figure 11. Reset and Delay Circuit Wave Forms

RESET

DELAY

Threshold

DELAY

Threshold

RESET

RESET Function

A RESET signal (low voltage) is generated as the IC

than 8.0% below the regulated output voltage. Hysteresis is

included in the function to minimize oscillations.

The RESET output is an open collector NPN transistor,

controlled by a low voltage detection circuit. The circuit is

functionally independent of the rest of the IC thereby

as 1.0 V.

Adjustable Reset Function

The reset threshold can be made lower by connecting an

lead, as displayed in Figure 12. This lead is grounded to

select the default value of 4.6 V.

Figure 12. Adjustable RESET

to

µP and

System

Power

RESET

Delay

NCV4279B

to

µP and

RESET

Port

DELAY Function

The reset delay circuit provides a programmable (by

external capacitor) delay on the RESET output lead.

The DELAY lead provides source current (typically

2.5

mA) to the external DELAY capacitor during the

following proceedings:

1. During Power Up (once the regulation threshold

has been verified).

2. After a reset event has occurred and the device is

back in regulation. The DELAY capacitor is

discharged when the regulation (RESET threshold)

has been violated. This is a latched incident. The

capacitor will fully discharge and wait for the

device to regulate before going through the delay

time event again.

FLAG/Monitor Function

An on−chip comparator is provided to perform an early

warning to the microprocessor of a possible reset signal. The

reset signal typically turns the microprocessor off

instantaneously. This can cause unpredictable results with

the microprocessor. The signal received from the FLAG pin

will allow the microprocessor time to complete its present

task before shutting down. This function is performed by a

comparator referenced to the bandgap reference. The actual

trip point can be programmed externally using a resistor

divider to the input monitor (MON) (Figure 13). The typical

threshold is 1.20 V on the MON Pin.

Figure 13. FLAG/Monitor Function

MON

I/O

RESET

µP

FLAG

RESET

GND

Delay

NCV4279B