HC55185 Intersil Corporation, HC55185 Datasheet - Page 7

• Manufacturer Part Number: HC55185
• Description: VoIP Ringing SLIC Family
• Manufacturer: Intersil Corporation

Parametric Improvements
The most significant parametric improvement of the
HC55185 is reduction in Idle Channel Noise. This
improvement was accomplished by redistributing gains in
the impedance matching loop. The impact to the application
circuit is the change in the impedance programming resistor
RS. The redistribution of gains also improves AC
performance at the upper end of the voice band.
Functional Improvements
In addition to parametric improvements, internal circuit
changes and application circuit changes have been made to
improve the overall device functionality.
Off Hook Interface
The transient behavior of the device in response to mode
changes has been significantly improved. The benefit to the
application is reduction or more likely elimination of DET
glitches when off hook events occur. In addition to internal
circuit modifications, the change of CFB value contributes to
this functional improvement.
Transient Current Limit
The drive current capability of the output amplifiers is
determined by an externally programmable output current
limit circuit which is separate from the DC loop current limit
function and programmed at the pin TL. The current limit
circuit works in both the source and sink direction, with an
internally fixed offset to prevent the current limit functions
from turning on simultaneously. The current limit function is
provided by sensing line current and reducing the voltage
drive to the load when the externally set threshold is
exceeded, hence forcing a constant source or sink current.
SOURCE CURRENT PROGRAMMING
The source current is externally programmed as shown in
Equation 1.
For example a source current limit setting of 50mA is
programmed with a 35.6k resistor connected from pin 16 of
the device to ground. This setting determines the maximum
amount of current which flows from Tip to Ring during an off
hook event until the DC loop current limit responds. In
addition this setting also determines the amount of current
which will flow from Tip or Ring when external battery faults
occur.
SINK CURRENT PROGRAMMING
The sink current limit is internally offset 20% higher than the
externally programmed source current limit setting.
If the source current limit is set to 50mA, the sink current
limit will be 60mA. This setting will determine the amount of
which flows into Tip or Ring when external ground faults
occur.
R
I
SNK
TL
=
=
------------ -
I
1780
SRC
1.20 I
SRC
4-7
(EQ. 1)
(EQ. 2)
HC55185
FUNCTIONAL DESCRIPTION
Each amplifier is designed to limit source current and sink
current. The diagram below shows the functionality of the
circuit for the case of limiting the source current. A similar
diagram applies to the sink current limit with current polarity
changed accordingly.
During normal operation, the error current (I
the output voltage is determined by the signal current (I
multiplied by the 200k feedback resistor. With the current
polarity as shown for I
with respect to half battery. Assuming the amplifier output is
driving a load at a more negative potential, the amplifier
output will source current.
During excessive output source current flow, the scaled
output current (I
forcing an error current (I
the error current subtracts from the signal current, which
reduces the amplifier output voltage. By reducing the output
voltage the source current to the load is decreased and the
output current is limited.
DETERMINING THE PROPER SETTING
Since this feature programs the maximum output current of
the device, the setting must be high enough to allow for
detection of ring trip or programmed off hook loop current,
whichever is greater.
To allow for proper ring trip operation, the transient current
limit setting should be set at least 25% higher than the peak
ring trip current setting. Setting the transient current 25%
higher should account for programming tolerances of both
the ring trip threshold and the transient current limit.
If loop current is larger than ring trip current (low REN
applications) then the transient current limit should be set at
least 35% higher than the loop current setting. The slightly
higher offset accounts for the slope of the loop current limit
function.
Attention to detail should be exercised when programming
the transient current limit setting. If ring trip detect does not
occur while ringing, then re-examine the transient current
limit and ring trip threshold settings.
I
REF
= 1.21/TL
FIGURE 1. CURRENT LIMIT FUNCTIONAL DIAGRAM
I
O
SIG
/K) exceeds the reference current (I
VB/2
SIG
ERR
I
, the output voltage moves positive
ERR
-
+
). With the polarity as shown
I
O
/K
200k
20
ERR
) is zero and
TIP or RING
REF
I
O
SIG
)
)