HC5515 Intersil Corporation, HC5515 Datasheet - Page 8

• Manufacturer Part Number: HC5515
• Description: ITU CO/PABX SLIC with Low Power Standby
• Manufacturer: Intersil Corporation

Figure 14 shows the relationship between the saturation
guard voltage, the loop current and the loop resistance.
Notice from Figure 14 that for a loop resistance <1.2k
(R
feed region and for resistances >1.2k the SLIC is operating
in the resistive feed region. Operation in the resistive feed
region allows long loop and off-hook transmission by
keeping the tip and ring voltages off the rails. Operation in
this region is transparent to the customer.
R
R
L
L
SG
50
40
30
20
10
-10
-20
-30
-40
-50
0
0
= 4.0k ) the SLIC is operating in the constant current
0
100k
100k
0
RESISTIVE FEED
CONSTANT CURRENT
GUARD VOLTAGE
GUARD VOLTAGE
V
V
REGION
BAT
BAT
FEED REGION
SATURATION
SATURATION
10
FIGURE 14. V
= -48V, R
= -24V, R
4k
1.5k
LOOP CURRENT (mA)
FIGURE 13. V
I
RING
I
LOOP RESISTANCE ( )
TIP
SG
SG
V
BAT
RING
20
TIP
= 4.0k
=
2k
700
62
= -48V, I
1.2K
TR
vs I
TR
RESISTIVE FEED
L
vs R
L
HC5515
30
= 23mA, R
and R
<1.2k
<400
REGION
L
CONSTANT CURRENT
FEED REGION
SATURATION GUARD
VOLTAGE, V
SATURATION GUARD
VOLTAGE, V
I
L
TIP
SG
+
+
-
R
R
-
I
SG
SG
RING
= 4.0k
FIGURE 12. DC LOOP CURRENT
= 4.0k
=
TR
TR
= 38V
= 13V
V
V
TIP
RING
A
SATURATION GUARD
1
I
HC5515
1
-5V
LOOP CURRENT
CIRCUIT
CIRCUIT
The Saturation Guard circuit (Figure 12) monitors the tip to
ring voltage via the transconductance amplifier A
generates a current that is proportional to the tip to ring
voltage difference. I
current until the tip to ring voltage exceeds 12.5V. When the
tip to ring voltage exceeds 12.5V (with no R
supplies more current than I
A
through R
output current from A
voltage across R
decreases. This results in a corresponding decrease in the
loop current. The R
saturation guard reference voltage beyond 12.5V. Equation 3
gives the relationship between the R
the programmable saturation guard reference voltage:
where:
V
R
When the Saturation guard reference voltage is exceeded,
the tip to ring voltage is calculated using Equation 4:
where:
V
R
For on-hook transmission R
The value of R
loop length operation. This requires that the saturation guard
reference voltage be set as high as possible without clipping
the incoming or outgoing VF signal. A voltage margin of -4V
V
V
V
2
SGREF
SGREF
TR
SG
L
TR
TR
A
amplifies its input current by a factor of 12 and the current
= Loop resistance.
2
I
2
= Voltage differential between tip and ring, and
= Saturation Guard programming resistor.
=
=
-5V
R
16.66
=
L
= Saturation Guard reference voltage, and
1
12.5
becomes the difference between I
16.66
------------------------------------------------------------------------------------
+
I
RSN
R
---------------------------------- -
R
SG
+
L
SG
1
---------------------------------- -
R
+
5 10
+
decreases and the output voltage on R
should be calculated to allow maximum
R
SG
SG
5 10
+
1
1
R
+
5 10
-
2
17300
is internally set to sink all of A
+
DC1
-
. As the current from A
+
pin provides the ability to increase the
17.3k
5
17300
5
5
+
R
L
R
1
DC2
SG
= , Equation 4 reduces to:
can sink. When this happens
+
V
RSN
R
-2.5V
R
17300
TX
DC
SG
600
SG
-5V
resistor value and
R
SG
SG
R
R
R
2
RX
DC1
DC2
2
increases, the
resistor) A
and the
1
. A
1
C
’s
1
DC
(EQ. 3)
(EQ. 4)
(EQ. 5)
DC
1