HC5513BIM 查看數據表（PDF） - Intersil
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HC5513BIM Datasheet PDF : 18 Pages
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HC5513
Transhybrid Circuit
The purpose of the transhybrid circuit is to remove the
receive signal (VRX) from the transmit signal (VTX), thereby
preventing an echo on the transmit side. This is
accomplished by using an external op amp (usually part of
the CODEC) and by the inversion of the signal from the
4-wire receive port (RSN) to the 4-wire transmit port (VTX).
Figure 17 shows the transhybrid circuit. The input signal will
be subtracted from the output signal if I1 equals I2. Node
analysis yields the following equation:
R-V----TT----XX-- + -V--Z--R--B--X-- = 0
(EQ. 21)
The value of ZB is then:
ZB = –RTX • V-V----RT----XX--
Where VRX/VTX equals 1/ A4-4.
Therefore:
ZB = RTX • Z---Z-R---T--X- • -1-------Z0------0--T-----Z-0----L--+---+--2---2R----R-F---F--+-----Z---L--
(EQ. 22)
(EQ. 23)
Example:
Given: RTX = 20kΩ, ZRX = 280kΩ, ZT = 562kΩ (standard
value), RF = 20Ω and ZL= 600Ω,
The value of ZB = 18.7kΩ.
RFB
I2
VTX
RTX
HC5513
ZT
I1
ZB
RSN
ZRX
-
+
+
VTX
-
+
VRX
-
CODEC/
FILTER
FIGURE 17. TRANSHYBRID CIRCUIT
Supervisory Functions
The loop current, ground key and the ring trip detector
outputs are multiplexed to a single logic output pin called
DET. See Table 1 to determine the active detector for a given
logic input. For further discussion of the logic circuitry see
section titled “Digital Logic Inputs”.
Before proceeding with an explanation of the loop current
detector, ground key detector and later the longitudinal
impedance, it is important to understand the difference
between a “metallic” and “longitudinal” loop currents. Figure
18 illustrates 3 different types of loop current encountered.
Case 1 illustrates the metallic loop current. The definition of
a metallic loop current is when equal currents flow out of tip
and into ring. Loop current is a metallic current.
Cases 2 and 3 illustrate the longitudinal loop current. The
definition of a longitudinal loop current is a common mode
current, that flows either out of or into tip and ring
simultaneously. Longitudinal currents in the on-hook state
result in equal currents flowing through the sense resistors
R1 and R2 (Figure 18). And longitudinal currents in the off-
hook state result in unequal currents flowing through the
sense resistors R1 and R2. Notice that for case 2,
longitudinal currents flowing away from the SLIC, the current
through R1 is the metallic loop current plus the longitudinal
current; whereas the current through R2 is the metallic loop
current minus the longitudinal current. Longitudinal currents
are generated when the phone line is influenced by
magnetic fields (e.g., power lines).
Loop Current Detector
Figure 18 shows a simplified schematic of the loop current
and ground key detectors. The loop current detector works
by sensing the metallic current flowing through resistors R1
and R2. This results in a current (IRD) out of the
transconductance amplifier (gm1) that is equal to the product
of gm1 and the metallic loop current. IRD then flows out the
RD pin and through resistor RD to VEE. The value of IRD is
equal to:
IRD = --I--T----I-P-----6–---0--I-0-R----I--N----G---- = 3---I-0-L--0--
(EQ. 24)
The IRD current results in a voltage drop across RD that is
compared to an internal 1.25V reference voltage. When the
voltage drop across RD exceeds 1.25V, and the logic is
configured for loop current detection, the DET pin goes low.
The hysteresis resistor RH adds an additional voltage
effectively across RD, causing the on-hook to off-hook
threshold to be slightly higher than the off-hook to on-hook
threshold.
Taking into account the hysteresis voltage, the typical value
of RD for the on-hook to off-hook condition is:
RD = I--O-----N-----–----H----O-----O----K----4--t-6-o---5--O-----F---F-----–----H----O----O-----K--
(EQ. 25)
Taking into account the hysteresis voltage, the typical value
of RD for the off-hook to on-hook condition is:
RD = I--O-----F----F----–----H----O-----O----K-3---7--t--5o-----O-----N-----–----H----O----O-----K--
(EQ. 26)
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