ISL1532IRZ Datasheet, PDF (15/19 Pages) Intersil Corporation – Dual Channel Differential DSL Line Driver

TOP METAL
INTERNAL GROUND PLANE
FIGURE 47. PCB LAYOUT - QFN PACKAGE
TOP METAL
INTERNAL GROUND PLANE
FIGURE 48. PCB LAYOUT - HTSSOP PACKAGE
ISL1532
Output Loading
While the drive amplifiers can output in excess of 500mA
transiently, the internal metallization is not designed to carry
more than 100mA of steady DC current and there is no
current-limit mechanism. The device can safely drive RMS
sinusoidal currents of 2 x 100mA, or 200mA. This current is
more than that required to drive line impedances to large
output levels, but output short circuits cannot 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 a serious hazard.
The amplifiers are sensitive to capacitive loading. More than
25pF will cause peaking of the frequency response. The
same is true of badly terminated lines connected without a
series matching resistor.
Power Supplies and Component Placement
The power supplies should be well bypassed close to the
ISL1532. A 2.2µF tantalum capacitor and a 0.1µF ceramic
capacitor for each supply works well. Since the load currents
are differential, they should not travel through the board
copper and set up ground loops that can return to amplifier
inputs. Due to the class AB output stage design, these
currents have heavy harmonic content. If the ground
terminal of the positive and negative bypass capacitors are
connected to each other directly and then returned to circuit
ground, no such ground loops will occur. This scheme is
employed in the layout of the ISL1532 demonstration board,
and documentation can be obtained from the factory.
The parallel combination of the feedback resistor and gain
setting resistor and parasitic capacitance on the inverting input
node forms a pole in the feedback path. If the frequency of this
pole is low, it can lead to frequency peaking. Since the ISL1532
is a current feedback amplifier, the feedback resistor value is
predetermined by design. The only way to increase the
frequency of this pole is to reduce the parasitic capacitance on
the inverting input node. Ground plane near the inverting input
should be avoided to minimize the parasitic capacitance.
Single Supply Operation
The ISL1532 can also be powered from a single supply
voltage. When operating in this mode, the GND pins can still
be connected directly to GND and the C₀ and C₁ pins are
relative to GND. To calculate power dissipation, the
equations in the previous section should be used, with V_S
equal to half the supply rail.
Feedback Resistor Value
The bandwidth and peaking of the amplifiers varies with
supply voltage somewhat and with gain settings. The
feedback resistor values can be adjusted to produce an
optimal frequency response. Here is a series of resistor
values that produce an optimal driver frequency response
(1dB peaking) for different supply voltages and gains:
15
FN6173.3
December 4, 2006