CMX878 Ver la hoja de datos (PDF) - CML Microsystems Plc
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CMX878 Datasheet PDF : 61 Pages
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CMX878
1.4.1 Ring Detector Interface
This interface runs from the Standby Supply.
Figure 3 shows how the CMX878 may be used to detect the large amplitude Ringing signal voltage or a
Line Reversal present at the start of an incoming telephone call.
The ring signal is usually applied at the subscriber's exchange as an ac voltage inserted in series with
one of the telephone wires and will pass through either C19 and R26 or C20 and R27 to appear at the top
end of R28 (point X in Figure 3) in a rectified and attenuated form.
The signal at point X is further attenuated by the potential divider formed by R28 and R29 before being
applied to the CMX878 RD input. If the amplitude of the signal appearing at RD is greater than the input
threshold (Vthi) of Schmitt trigger 'A' then the N transistor connected to RT will be turned on, pulling the
voltage at RT to VSS by discharging the external capacitor C21. The output of the Schmitt trigger 'B' will
then go high; this output is then processed by logic circuits running from the Standby Supply.
The minimum amplitude ringing signal that is certain to be detected is:
( 0.7 + Vthi x [R26 + R28 + R29] / R29 ) x 0.707 Vrms
where Vthi is the high-going threshold voltage of the Schmitt trigger A (see section 1.7.1).
With R26-28 all 470kΩ as Figure 3, then setting R29 to 68kΩ will ensure detection of ringing signals of
30Vrms and above for SBYVDD over the range 2.9V to 3.9V.
C19 R26
From
Line
X
C22
D9 -
R28 RD
(opt.) AVSS 12
C20 R27
AVSS
CMX878
B
A
To
Standby
logic
circuits
RT
R30
SBYVDD
Ring signal
Bridge rectifier o/p (X)
Vthi
AVSS
RT
Output of
inverting
Schmitt trigger ‘B’
Vthi
AVSS
R26, 27, 28
R29
R30
470kΩ
See text
1MΩ
C19, 20
C21
D9-12
0.1µF
150nF
1N4004
Resistors ±5%, capacitors ±20%
Figure 3 Ring Signal Detector Interface Circuit
© 2002 Consumer Microcircuits Limited
10
D/878/2
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