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參數(shù)資料
| 型號： | HC5549 |
| 廠商： | Intersil Corporation |
| 英文描述： | Low Power SLIC with Battery Switch(用戶線接口電路) |
| 中文描述： | 低功耗用戶接口與電池開關(guān)（用戶線接口電路） |
| 文件頁數(shù)： | 9/13頁 |
| 文件大?。?/td> | 125K |
| 代理商： | HC5549 |
4-9
provide additional insight as to the expected AC node
voltages for a specific Tip and Ring load.
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
RX
. The echo must be cancelled to
maintain voice quality. Most applications will use a summing
amplifier in the CODEC front end as shown below to cancel
the echo signal.
The resistor ratio, R
F
/R
B
, provides the final adjustment for
the transmit gain, G
TX
. The transmit gain is calculated using
Equation 25.
Most applications set R
F
= R
B
, hence the device 2-wire to
4-wire equals the transmit gain. Typically R
B
is greater than
20k
to prevent loading of the device transmit output.
The resistor ratio, R
F
/R
A
, is determined by the transhybrid
gain of the device, G
44
. R
F
is previously defined by the
transmit gain requirement and R
A
is calculated using
Equation 26.
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
KNEE
, the device is providing constant
current, I
A
, and the power dissipation is calculated using
Equation 28.
If the loop lengthis greater than R
KNEE
, the deviceis operating
in the constant voltage, resistive feed region. The power
dissipated in this region is calculated using Equation 29.
Since the current relationships are different for constant
current versus constant voltage, the region of device
operation is critical to valid power dissipation calculations.
Reverse Active
Overview
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. 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.
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 cross talk to protocol signalling.
The device uses an external low voltage capacitor, C
POL
, 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, therefore loop stability is not impacted.
The internal circuitry used to set the polarity reversal time is
shown below.
During forward active, the current from source I1 charges the
external timing capacitor C
POL
and the switch is open. The
internal resistor provides a clamping function for voltages on
the POL node. During reverse active, the switch closes and
I2 (roughly twice I1) pulls current from I1 and the timing
capacitor. The current at the POL node provides the drive to
FIGURE 7. TRANSHYBRID BALANCE INTERFACE
+
-
R
F
R
B
R
A
HC5518x
CODEC
+2.4V
RX OUT
TX IN
-IN
VRX
VTX
R
R
+
-
1:1
R
S
T
A
G
TX
G
–
24
R
F
R
B
-------
=
(EQ. 25)
R
A
R
B
44
----------
=
(EQ. 26)
P
FAQ
V
BH
I
×
BHQ
V
BL
I
BLQ
×
V
CC
I
CCQ
×
+
+
=
(EQ. 27)
P
FA IA
)
P
FA Q
)
V
BL
xI
A
(
)
R
LOOP
xI
2A
(
)
–
+
=
(EQ. 28)
P
FA IB
)
P
FA Q
)
V
BL
xI
B
(
)
R
LOOP
xI
2B
(
)
–
+
=
(EQ. 29)
FIGURE 8. REVERSAL TIMING CONTROL
C
POL
POL
I
1
75k
I
2
HC5549
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|---|---|
| HC5549_04 | 制造商:INTERSIL 制造商全稱:Intersil Corporation 功能描述:Low Power SLIC with Battery Switch |
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| HC5549IM | 制造商:INTERSIL 制造商全稱:Intersil Corporation 功能描述:Low Power SLIC with Battery Switch |
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