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Zarlink Semiconductor Inc.
2.1.1 Component Values for Typical Application
Item Quantity Type Value Tolerance Rating
C
TX
2 X7R 0.22 µF 10% 50V
Table 1 - Parts List for Typical Application
Circuit
2.1.2 Input Considerations
The driving source impedance should be less than 100 nH to avoid any ringing or oscillation. This inductance is
equivalent to about 4" of unshielded wiring, or 6" of unterminated transmission line. Normal high-frequency
construction obviates any such problem.
2.1.3 Output Driving Considerations
While the drive amplifiers can output in excess of 450 mA peak, the internal metallization is not designed to carry
more than 100 mA of steady DC current and there is no current limit mechanism. The device can safely drive
sinusoidal currents of 2 x 100 mArms, or 200 mArms. This current is more than that required to drive line
impedance to large output levels, but output short circuits can not be tolerated. The series output resistor will
usually limit currents to safe values in the event of line shorts. Driving lines with no series resistor is not
recommended.
The amplifiers are sensitive to capacitive loading. More than 100pF may cause peaking of the frequency response.
The same is true of badly terminated lines connected without a series matching resistor.
When in power down mode, several volts of differential voltage may appear across the line driver outputs. If a DC
current path exists between the two outputs, a large DC current can flow from the positive supply rail to the negative
supply rail through the outputs. To avoid DC current flow, the most effective solution is to place DC blocking
capacitors in series at the output, as shown in the typical application circuit.
2.1.4 Power Supplies and Component Placement
The power supplies should be well bypassed close to the Le87251 device. A 2.2 µF tantalum capacitor and a 0.1 µF
ceramic capacitor for each supply is recommended. The ground terminal of the positive and negative bypass
capacitors should be connected to each other directly and then returned to circuit ground to prevent ground current
loops.
The Le87251 can also be powered from a single positive voltage supply. When operating in this mode, the VS+ pin
is connected to the positive supply. The VS- pin is connected to GND.
2.1.5 Stability
The Le87251 features improved frequency compensation for all applications, allowing stable operation at very low
power levels and eliminating any need for external “snubber” circuit. Differential circuits, such as ADSL line driver
applications, can be especially prone to common-mode oscillation. The Le87251 is specifically compensated to
eliminate this type of instability and allows for reliable operation even at very low power levels.
2.2 Cable Termination Technique
There are various techniques available. Figure 4 shows a passive termination technique. Figure 5 shows an active
termination technique. A quick comparison of the reduction in voltage and power requirements for the driver with
passive or active termination is shown in Table 2.
