ISL1532IRZ Datasheet, PDF (17/19 Pages) Intersil Corporation – Dual Channel Differential DSL Line Driver

ISL1532
V+
=
---------^R-----^P----¹----------
R_P1 + R_P2
×
V_X
V-
=
--------^R----^G----------
R_F + R_G
×
V_O
R_SOURCE
=
V_-----X--
I_X
=
R_BM
/
⎛
⎜1
⎝
-
---------^R-----^P----¹----------
R_P1 + R_P2
×
R_{-----F---R--+--G---}R_{----G---⎠⎟⎞}
R_SOURCE = R_L = K × RBM
K=
1⁄
1
-
⎛
⎜
⎝
-_R----_P----₁^R----+-^P----¹_R-----_P----₂-_{⎠⎟⎞}
×
⎛
⎜
⎝
R_{-----F---R--+--G---}R_{----G---⎠⎟⎞}
The overall gain of the active termination circuit is:
(EQ. 9)
⎛
⎜
⎝
V^-V----_I^-O-_N^--_{⎠⎟⎞}
=
----------------------R-----^P----²-----/----⁽--R-----^P----²----+-----R-----^P----¹---⁾----------------------
⎝⎛^{1 +} _K-1--⎠⎞
/
⎛
⎜
⎝
R-----_{G---R---+-G----R----F--⎠⎟⎞}
-
⎛
⎜
⎝
-R----_P----₂^R----+-P----¹_R-----P----₁-_{⎠⎟⎞}
(EQ. 10)
V_IN+
R_G
R_G
-
+
R_F
R
P
R^P
R_BM
V_O+
R_L
R_F
+
V_IN-
-
R_BM
V_O-
FIGURE 52. DIFFERENTIAL LINE DRIVER USING ACTIVE
TERMINATION TECHNIQUE
In an ADSL2+ system, the POTS phone line, a twisted pair
cable is used for data transmission. As shown in Figure 52,
the single ended active terminate line driver is reconfigured
to drive differential lines. The gain resistor is shared to allow
accurate gain matching between the two amplifiers. Applying
the same analysis technique as the single ended circuit, the
following relationship can be derived:
K
=
1
/
⎛
⎜1
⎝
-
R^-R----_PF^--_{⎠⎟⎞}
⎛
⎜
⎝
V_{----V-O---I-U-N---T--⎠⎟⎞}
=
-¹----+-----^R-----^F-----^/----^R-----^G-----+-----^R-----^F-----^/----^R----^P--
2
×
⎛
⎜1
⎝
-
R^-R----_PF^--_{⎠⎟⎞}
(EQ. 11)
Table 3 is a quick comparison of the reduction in voltage and
power requirements for the driver with passive or active
termination. The key specification of a ADSL2+ CO driver
are as follows: Peak output line power is 20dBM, POTS line
impedance is 100Ω and the crest factor for ADSL DMT
signal is 14.5dB. This specification translates to 16.76V
peak-to-peak voltage on the line with 5.3 peak to average
ratio PAR and 31.6mA average output current. In the passive
termination case where the load and backmatch resistors
are the same, the amplifier must provide 33.52V
peak-to-peak at its outputs. A high voltage line driver
typically needs 4V of total headroom. As a result the total
supply voltage required is 37.5V. With the necessary output
average current, that translates into 1.185W dissipated in
addition to the quiescent power of the amplifier.
TABLE 3.
PASSIVE TERMINATION
ACTIVE TERMINATION
16.5V_P-P into a 100Ω line
16.5V_P-P into a 100Ω line
V_{OUT DRIVER} = V_RBM +
V_RLOAD
V_{OUT DRIVER} = V_RBM +
V_RLOAD
RBM = R_LOAD
RBM = R_LOAD/5
V_RBM = V_RLOAD
V_RBM = V_RLOAD/5
V_{OUT DRIVER} = 33.52V
V_{OUT DRIVER} = 20.11V
V_SUPPLY = 37.52
V_SUPPLY = 24.11
I_OUT = 31.6mA
I_OUT = 31.6mA
P_{OUT DRIVER} = V_SUPPLY * I_OUT P_{OUT DRIVER} = V_SUPPLY * I_OUT
= 1.185W (plus quiescent power) = 0.714W (plus quiescent power)
In the active case, a K of 5 is assumed. This reduces the
backmatch resistor to 20% of its value in the passive case.
The peak-to-peak output voltage provided by the driver is
reduced to 20.11V which allows the use of the EL1508, a
median voltage line driver. The EL1508 requires 2.5V of
headroom. With 2.5V of supply voltage headroom, the power
supply required becomes 22.61. With the same output
current drive, the power dissipation is reduced by 39.7% to
0.714W. While it is true that additional power is dissipated in
the feedback networks, the feedback resistors are typically
much larger than the backmatch resistor and their losses are
negligible.
17
FN6173.3
December 4, 2006