THS6226A Datasheet, PDF (20/32 Pages) Texas Instruments – THS6226A Gated-Class H, Dual-Port VDSL2 Line Driver

THS6226A
SBOS643A – APRIL 2014 – REVISED MAY 2014
Typical Application (continued)
8.2.1 Design Requirements
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Table 6. Design Requirements
DESIGN REQUIREMENT
AC-coupling capacitors
Synthesis factor
Output transformer ratio
Surge protection circuit
CONDITION
0.1 µF
5
1:1.4
Not shown
8.2.2 Detailed Design Procedure
The input capacitor forms a high-pass filter with the device input impedance. This pole must be set at a
frequency low enough to not interfere with the desired signal.
The output transformer can be changed to any transformer-turn ratio. Note that the load is expected to be a
transmission line with 100-Ω characteristic impedance. Referred to the transformer secondary, the load detected
by the amplifier is 1 / n² with 1:n being the transformer-turn ratio.
Practical limitations force the transformer-turn ratio to be between 1.4:1 and 1:2. At the lighter load detected by
the amplifier (1.4:1), the voltage swing is limited by the class H and the maximum achievable swing of the
amplifier. At the heaviest load (1:2), the voltage swing is limited by the current drive capability of the amplifier. To
satisfy the synthesis impedance factor and the loading, the series resistance (R_S) can be set to R_S = R_L / 5 =
100 Ω / (5 × n²).
For the charge pump, consider the thermal characteristic of the capacitor (X7R, X5R, or Y5V can be used for the
charge pump).
For the power-supply bypass, consider using only X7R or X5R because of the better stability of these materials
over temperature.
For surge protection, consider adding 47-Ω resistors in series on the positive feedback path. The secondary
protection is also normally added after the series resistance on the secondary transformer.
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