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W671500 SERIES
7.2.2.4. 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.
PFAQ VBH I
×
BHQ
VBL IBLQ
×
VCC ICCQ
×
+
=
(EQ. 29)
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 RKNEE, the device is providing constant current, IA, and the power
dissipation is calculated using the following equation.
PFA IA
( )
PFA Q
( )
VBLxIA
(
)
RLOOPxI2A
(
)
+
=
(EQ. 30)
If the loop length is greater than RKNEE, the device is operating in the constant voltage, resistive feed
region. The power dissipated in this region is calculated using the following equation.
PFA IB
( )
PFA Q
( )
VBLxIB
(
)
RLOOPxI2B
(
)
+
=
(EQ. 31)
Since the current relationships are different for constant current versus constant voltage, the region of
device operation is critical to valid power dissipation calculations.
7.2.3. Reverse Active Mode
The Reverse Active mode (RA, 011) provides the same functionality as the Forward Active mode. On-
hook transmission, DC loop feed and voice transmission are supported. Likewise 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.
During Reverse Active the Tip and Ring DC voltage characteristics exchange roles. That is, Ring is
typically 4V below ground and Tip is typically 4V more positive than battery. Otherwise, all feed and
voice transmission characteristics are identical to Forward Active.
7.2.3.1. Silent Polarity Reversal
Changing from Forward Active to Reverse Active or vice versa is referred to as polarity reversal. Many
applications require slew rate control of the polarity reversal event. Requirements range from
minimizing crosstalk to protocol signalling.
The device uses an external low voltage capacitor, CPOL, to set the reversal time. Once programmed,
the reversal time will remain nearly constant over various load conditions. In addition, the reversal
timing capacitor is isolated from the AC loop, so loop stability is not impacted.
The internal circuitry used to set the polarity reversal time is shown below.
Publication Release Date: April 2004
- 19 -
Revision 1.0