HT7022A datasheet(4/12 Pages) HOLTEK

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HT7022A

Rev. 1.10

April 20, 2004

Functional Description

The HT7022A is a voltage detector equipped with a high

stability voltage reference which is connected to the

negative input of a comparator

¾ denoted as VREF in the

following figure for NMOS output voltage detector.

When the voltage drop to the positive input of the com-

parator (i,e,VB) is higher than VREF, VOUT goes high,

M1 turns off, and VB is expressed as VBH=VDD

(RB+RC)/(RA+RB+RC). If VDD is decreased so that VB

falls to a value less than VREF, the comparator output in-

verts from high to low, VOUT goes low, VC is high, M1

turns on, RC is bypassed, and VB becomes:

VBL=VDD

´RB /(RA+RB), which is less than VBH. By so

doing, the comparator output will stay low to prevent the

circuit from oscillating when VB

» VREF.

If VDD falls below the minimum operating voltage, the

output becomes undefined. When VDD goes from low to

VDD

´ RB /(RA+RB)>VREF, the comparator output and

VOUT goes high.

The detectable voltage is defined as:

VDET (

-)=

R+ R + R

R+ R

´ VREF

The release voltage is defined as:

VDET (+) =

R+ R

´ VREF

The hysteresis width is:

VHYS =VDET (+)

- VDET (-)

The figure demonstrates the NMOS output type with

positive output polarity (VOUT is normally open, active

low).

V S S

V D D

R A

R B

R C

M 1

M N

C o m p a r a t o r

V O U T

R L

V R E F

V B

V C

NMOS Output Voltage Detector

Application Circuits

Microcomputer Reset Circuit

Normally a reset circuit is required to protect the micro-

computer system from malfunctions due to power line

interruptions. The following examples show how differ-

ent output configurations perform a reset function in var-

ious systems.

· NMOS open drain output application for separate

power supply

· NMOS open drain output application with R-C delay

H T 7 0 2 2 A

V D D

V S S

V O U T

M i c r o -

c o m p u t e r

V D D 2

V D D 1

R E S E T

H T 7 0 2 2 A

V S S

V O U T

M i c r o -

c o m p u t e r

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