HT70xxA-2
Functional Description
The HT70xxA-2 series is a set of voltage detectors
equipped with a high stability voltage reference which is
connected to the negative input of a comparator¾
denoted as V
REF
in the following figure for NMOS output
voltage detector.
When the voltage drop to the positive input of the
comparator (i,e,V
B
) is higher than V
REF
, VOUT goes
high, M1 turns off, and V
B
is expressed as V
BH
=V
DD
´
(R
B
+R
C
) / (R
A
+R
B
+R
C
). If V
DD
is decreased so that V
B
falls to a value less than V
REF
, the comparator output
inverts from high to low, V
OUT
goes low, V
C
is high, M1
turns on, RC is bypassed, and V
B
becomes: V
BL
= V
DD
´
R
B
/ (R
A
+R
B
), which is less than V
BH.
By so doing, the
comparator output will stay low to prevent the circuit
from oscillating when V
B
»
V
REF
.
V
The hysteresis width is:
V
HYS
= V
DET
(+)
-
V
DET
(-)
V
D D
R
A
C o m p a ra to r
V
B
R E F
R L
V
C
V
M N
O U T
R
B
M 1
R
C
If V
DD
falls below the minimum operating voltage, the
output becomes undefined. When VDD goes from low to
V
DD
´
R
B
/ (R
A
+R
B
) > V
REF
, the comparator output and
V
OUT
goes high.
The detectable voltage is defined as:
V
DET
(-) =
R
A
+ R
B
+ R
C
´
V
REF
R
B
+ R
C
V
S S
NMOS Output Voltage Detector
The figure demonstrates the NMOS output type with
positive output polarity (V
OUT
is normally open, active
low). The HT70xxA-2 series also supplies options for
other output types with active high outputs. Application
circuits shown are examples of positive output polarity
(normally open, active low) unless otherwise specified.
The release voltage is defined as:
R
A
+ R
B
´
V
REF
V
DET
(+) =
R
B
Rev. 1.00
6
February 10, 2012
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
