SM8223A Datasheet, PDF (9/15 Pages) Nippon Precision Circuits Inc – FSK Decoder and DTMF Receiver IC

SM8223A
FUNCTIONAL DESCRIPTION
Ring Signal Detector
The telephone tip and ring signals pass through a
protection circuit and are input to a resistor, capaci-
tor and diode bridge network, shown in figure 1.
TIP
RING
a
C2 R3
C2
R3
Db
c
b
D1
R2
RDIN
R4
R1 _RDRC
C1
RDET
d
Figure 1. Ring signal detector circuit
The diode bridge full-wave rectified output signal
(point a) is reduced in level by a resistor voltage
divider comprising R₁ and R₂ (point b), and then
input on RDIN. When the ring signal input on RDIN
exceeds the Schmitt buffer trigger voltage (0.7V_DD),
the output switches the open-drain RDRC pin. The
signal at RDRC (point c) drives a time-constant cir-
cuit comprised by resistor R₄ and capacitor C₁ con-
nected to the input of a second Schmitt buffer to
generate the detector signal output on RDET (point
d). Thus, RDET goes LOW when the ring or tip sig-
nal exceeds the level set by the resistor voltage
divider.
NIPPON PRECISION CIRCUITS—9