ISL1532IRZ Datasheet, PDF (14/19 Pages) Intersil Corporation – Dual Channel Differential DSL Line Driver

ISL1532
output swing. There is a trade off, however, with the selection
of transformer ratio. As the ratio is increased, the receive
signal available to the receivers is reduced.
Once the user has selected the transformer ratio, the
dissipation in the output stages can be selected with the
following equation:
P_Dtransistors
=
2
×
I_O
×
⎛
⎝
V_-----S--
2
-
V_{O⎠⎞}
(EQ. 3)
where:
• V_S is the supply voltage (V_S+ to V_S-)
• V_O is the average output voltage per channel
• I_O is the average output current per channel
The overall power dissipation (P_DISS) is obtained by
summing P_Dquiescent and P_Dtransistor.
Estimating Line Driver Power Dissipation in
ADSL2+ CO Applications
Figure 46 shows a typical ADSL CO line driver
implementation. The average line power requirement for the
ADSL2+ CO application is 20dBM (100mW) into a 100Ω line
which translated to 3.16V_RMS line voltage. The ADSL2+
DMT peak to average ratio (crest factor) of 5.3 implies peak
voltage of 16.7V into the line. Using a differential drive
configuration and transformer coupling with standard back
termination, a transformer ratio of 1:1 is selected. With 1:1
transformer ratio, the impedance across the driver side of
the transformer is 100Ω, the average voltage is 3.16V_RMA
and the average current is 31.6mA. The power dissipated in
the ISL1532 is a combination of the quiescent power and the
output stage power when driving the line:
P_D = P_quiescent + P_output-stage
P_D = V_S × (I_S - 2I_X ) + (V_S - 2 × V_OUT-RMS ) × I_OUT-RMS
(EQ. 4)
In the full power mode and with 1.5k R_ADJ resistor, the
ISL1532 consumes typically 2.7mA quiescent current per
amplifier and still able to maintain very low distortion. The
distortion results are shown in typical performance section of
the data sheet. When driving a load, a large portion (about
50%) of the quiescent current becomes output load current.
The total power dissipation per channel is:
P_D = 24 × (2.7mA × 2 × 50% ) + 2 × 31.6mA × (12 - 3.16 )
(EQ. 5)
where:
P_D = 623mW
The total power dissipation for dual channel is:
P_Dtotal
= 2 x P_D
= 1247mW
The θ_JA requirement needs to be calculated. This is done
using the equation:
Θ_JA
=
T_{----J---U-----N----C----T-----}-_-----T_{---A----M-----B--}
P_DISS
(EQ. 6)
where:
• T_JUNCT is the maximum die temperature (+150°C)
• T_AMB is the maximum ambient temperature (+85°C)
• P_DISS is the dissipation calculated above
• θ_JA is the junction to ambient thermal resistance for the
package when mounted on the PCB
Θ_JA
=
-¹----⁵---⁰------^-----⁸---⁵---
1247 m W
=
52 ° C / W
(EQ. 7)
T_X+
V_S+
+
R_T
From
- _VS-
R_F
50 _{0.22µF TXFR}
1:1
AFE
2R_G
3k
100
1.5kΩ
T_X-
V_S+
+
R_T
- _{VS- 50 0.22µF}
R_F
3k
FIGURE 46. TYPICAL ADSL CO LINE DRIVER
IMPLEMENTATION
PCB Layout Considerations for QFN and HTSSOP
Packages
The ISL1532 die is packaged in two thermally-efficient
packages: a 24 Ld QFN (leadless plastic) and 20 Ld
HTSSOP packages. Both have the thermal pads underneath
the package and can use PCB surface metal vias areas and
internal ground planes to spread heat away from the
package. The larger the PCB area, the lower the junction
temperature of the device will be. In ADSL applications,
multiple layer circuit boards with internal ground plane are
generally used. 13 mil vias are recommended to connect the
metal area under the device with the internal ground plane.
Examples of the PCB layouts for the QFN and HTSSOP
packages are shown in Figures 47 and 48 respectively.
37°C/W (QFN package) and +35°C/W (HTSSOP package)
are obtained with the ISL1532 demoboard. The demoboard
is a 4-layer board built with 2 oz. copper and has a
dimension of 4 in².
A separate application note for the QFN package and layout
recommendation is available. If using the QFN package, the
layout and manufacturing process recommendations should
be carefully reviewed.
14
FN6173.3
December 4, 2006