L4936NH datasheet(7/11 Pages) STMICROELECTRONICS

7 / 11 page

RESET CIRCUIT

The block circuit diagram of the reset circuit is shown

in fig.4. The reset circuit supervisesthe standby out-

put voltage. The reset threshold of 4.7V is defined

by the internal reference voltage and the standby

output divider.

The reset pulse delay time tRD, is defined by the

charge time of an external capacitor CT :

CT x2V

tRD =

µA

The reaction time of the reset circuit dependson the

discharge time limitation of the reset capacitor CT

and is proportional to the value of CT.

The reaction time of the reset circuit increases the

noise immunity. In fact, if the standbyoutput voltage

drops below the reset threshold for a time shorter

than the reaction time tRR, no reset output variation

occurs. The nominal reset delay is generated for

standby output voltage drops longer than the time

necessary for the complete discharging of the ca-

pacitor CT. This time is typically equal to 50

µsif

CT = 100nF.The typical resetoutputwaveforms are

shown in fig. 5.

SENSE COMPARATOR

This circuit compares an input signal with an internal

voltage reference of typically 1.23V. The use of an

external voltage divider makes the comparator very

flexible in the application.This function can be used

to supervisethe input voltage - either before or after

the protection diode- andto give additionalinforma-

tion to the microprocessor such as lowvoltage warn-

ings.

Figure 4 :Block Diagram of the Reset Circuit.

L4936N/NH

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