ISL5585_04 Datasheet, PDF (16/24 Pages) Intersil Corporation – 3.3V Ringing SLIC Family for Voice Over Broadband (VOB)

ISL5585
Most applications do not specify charging current
requirements during standby. When specified, the typical
charging current may be as high as 5mA.
Forward Active
Overview
The forward active mode (FA, 001) is the primary AC
transmission mode of the device. On hook transmission, DC
loop feed and voice transmission are supported during forward
active. Loop supervision is provided by either the switch hook
detector (E0 = 1) or the ground key detector (E0 = 0). The
device may be operated from either high or low battery for on-
hook transmission and low battery for loop feed.
On-Hook Transmission
The primary purpose of on hook transmission will be to
support caller ID and other advanced signalling features.
The transmission over load level while on hook is 1 V_PEAK.
When operating from the high battery, the DC voltages at Tip
and Ring are MTU compliant. The typical Tip voltage is -4V
and the Ring voltage is a function of the battery voltage for
battery voltages less than -60V as shown in Equation 48.
V_RING = V_BH + 5
(EQ. 48)
Loop supervision is provided by the switch hook detector at
the DET output. When DET goes low, the low battery should
be selected for DC loop feed and voice transmission.
Feed Architecture
The design implements a voltage feed current sense
architecture. The device controls the voltage across Tip and
Ring based on the sensing of load current.Internal resistors
(R_CS) are placed in series with Tip and Ring outputs to
provide the current sensing. The diagram below illustrates
the concept.
R_B
R_A
V_IN
V_OUT
R_CS
-
+
R_L
R_C
-
+
K_S
FIGURE 9. VOLTAGE FEED CURRENT SENSE DIAGRAM
By monitoring the current at the amplifier output, a negative
feedback mechanism sets the output voltage for a defined
load. The amplifier gains are set by resistor ratios (R_A, R_B,
R_C) providing all the performance benefits of matched
resistors. The internal sense resistor, R_CS, is much smaller
than the gain resistors and is typically 20Ω for this device.
The feedback mechanism, K_S, represents the amplifier
configuration providing the negative feedback.
Transhybrid Balance
The final step in completing the impedance synthesis design
is calculating the necessary gains for transhybrid balance.
The AC feed back loop produces an echo at the V_TX output
of the signal injected at V_IN. The echo must be cancelled to
maintain voice quality. Most applications will use a summing
amplifier in the CODEC front end as shown in Figure 10 to
cancel the echo signal.
R AUX
R
1:1
T_A
VTX
R_S
-IN ^RIN
R_A ^RF
R_B
-
+
TX IN
+2.4V
ISL5585
RX OUT
CODEC
FIGURE 10. TRANSHYBRID BALANCE INTERFACE
The resistor ratio, R_F/R_A, provides the final adjustment for
the transmit gain, G_TX(V_2w to PCM, Figure 18). The transmit
gain is calculated using Equation 49.
G_TX
=
–G₂₄



R^-R----_AF^--_{}
=

–

(^---Z^----_L^----+^-----2^---Z-R^---O-_P^-----+^-----Z^----_O^-----)



R^-R----_AF^--_{}
(EQ. 49)
Most applications set R_F = R_A, hence the device 2-wire to
4-wire equals the transmit gain. Typically R_A is greater than
20kΩ to prevent loading of the device transmit output. The
value of the RF resistor should greater than the minimum
load spec of the CODEC’s internal amplifier (typical value
30.1kΩ).
The resistor ratio, R_F/R_B, is determined by the transhybrid
gain of the device, G₄₄. R_F is previously defined by the
transmit gain requirement and R_B is calculated using
Equation 50.
R_B=
--^R----^A----
G₄₄
=
R_A



R_{--R---I-S-N--}



Z_----L-----+_-----2_--Z--R_--O--P-----+_-----Z_{----O--}
(EQ. 50)
Power Dissipation
The power dissipated by the device during on hook
transmission is strictly a function of the quiescent currents
for each supply voltage during Forward Active operation.
P_FAQ= V_BH × I_BHQ + V_BL × I_BLQ + V_CC × I_CCQ
(EQ. 51)
Off hook power dissipation is increased above the quiescent
power dissipation by the DC load. If the loop length is less
than or equal to R_KNEE, the device is providing constant
current, I_A, and the power dissipation is calculated using
Equation 52.
P_FA(IA) = P_FA(Q) + (V_BLxI_A) – (R_LOOPxI^2A)
(EQ. 52)
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