ISL5585 Intersil Corporation, ISL5585 Datasheet - Page 15

• Manufacturer Part Number: ISL5585
• Description: 3.3 Volt Ringing Slic Family For Voice Over Broadband (VOB)
• Manufacturer: Intersil Corporation

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
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.
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
provide the current sensing. The diagram below illustrates
the concept.
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
R
resistors. The internal sense resistor, R
than the gain resistors and is typically 20Ω for this device.
The feedback mechanism, K
configuration providing the negative feedback.
V
C
RING
CS
FIGURE 9. VOLTAGE FEED CURRENT SENSE DIAGRAM
) providing all the performance benefits of matched
V
) are placed in series with Tip and Ring outputs to
OUT
=
V
BH
+
R
5
L
R
CS
S
15
R
B
, represents the amplifier
-
+
+
-
K
S
CS
, is much smaller
R
A
R
C
(EQ. 48)
A
PEAK
V
, R
IN
B
,
.
ISL5585
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
of the signal injected at V
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.
The resistor ratio, R
the transmit gain, G
gain is calculated using Equation 49.
Most applications set R
4-wire equals the transmit gain. Typically R
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
gain of the device, G
transmit gain requirement and R
Equation 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.
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
current, I
Equation 52.
R
G
P
P
FAQ
FA IA
B
TX
1:1
=
(
=
--------- -
G
FIGURE 10. TRANSHYBRID BALANCE INTERFACE
R
=
44
)
A
–
V
=
G
A
BH
T
ISL5585
=
24
, and the power dissipation is calculated using
A
P
FA Q
R



×
------- -
R
R
A
I
R
( )
BHQ
F
A



R



R
--------- -
R
IN
S
KNEE
+
=
TX
+
 Z


(
F
F
AUX
–
V
44



V
/R



/R
-IN
-------------------------------------- -
VTX
BL
R
(V
------------------------------------------- -
(
BL
L
Z
. R
S
F
A
B
, the device is providing constant
xI
2w
L
+
, provides the final adjustment for
, is determined by the transhybrid
= R
IN
×
+
F
2R
A
R
Z
I
)
. The echo must be cancelled to
IN
to PCM, Figure 18). The transmit
2R
BLQ
is previously defined by the
O
Z
–
A
P
O
(
P
, hence the device 2-wire to
+
R
+
+
Z
B
LOOP
O
V
Z
R
is calculated using



CC
O
A
)
R
 R


xI
×



B
------- -
R
I
R
2
CCQ
A
F
A
F



)
A
is greater than
+2.4V
-
+
CODEC
TX
TX IN
(EQ. 51)
(EQ. 52)
RX OUT
(EQ. 49)
(EQ. 50)
output