LP3470A438 datasheet(9/16 Pages) TI1

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Reference

GND

VCC

RESET

GND

RESET

LOGIC

TIMER

SRT

VCC1

SRT

LP3470A

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SNVSBF5B – JULY 2019 – REVISED NOVEMBER 2019

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8 Detailed Description

8.1 Overview

The LP3470A micropower voltage supervisory circuit provides a simple solution to monitor the power supplies in

microprocessor and digital systems and provides a reset controlled by the factory-programmed reset threshold

on the VCC supply voltage pin. When the voltage declines below the reset threshold, the reset signal is asserted

and remains asserted for an interval programmed by an external capacitor after VCC has risen above the

threshold voltage. The reset threshold options are 2.63 V, 2.75 V, 2.93 V, 3.08 V, 3.65 V, 4 V, 4.38 V, 4.63 V.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 RESET Time-Out Period

The reset time delay can be set to a minimum value of 50 µs by leaving the SRT pin floating, or a maximum

value of approximately 6.2 seconds by connecting 10 µF delay capacitor. The reset time delay (tD) can be

programmed by connecting a capacitor no larger than 10 µF between SRT pin and GND.

The relationship between external capacitor (CSRT) in Farads at SRT pin and the time delay (tD) in seconds is

given by Equation 1.

tD = -ln (0.29) x RSRT x CSRT + tD (no cap)

(1)

Equation 1 is simplified to Equation 2 by plugging RSRT and tD(no cap) given in Electrical Characteristics section:

tD = 618937 x CSRT + 50 µs

(2)

Equation 3 solves for external capacitor value (CSRT) in units of Farads where tD is in units of seconds

CSRT = (tD- 50 µs) ÷ 618937

(3)

The reset delay varies according to three variables: the external capacitor variance (CSRT), SRT pin internal

resistance (RSRT) provided in the Electrical Characteristics table, and a constant. The minimum and maximum

variance due to the constant is shown in Equation 5 and Equation 6.

tD (minimum) = -ln (0.36) x RSRT (min) x CSRT (min) + tD (no cap, min)

(4)

tD (maximum) = -ln (0.26) x RSRT (max) x CSRT (max) + tD (no cap, max)

(5)

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