EL1503A
Then, the θ requirement needs to be calculated. This is
technique, but several aspects of board layout should be
JA
done using the equation:
noted. First, the heat should not be shunted to internal
copper layers of the board nor backside foil, since the
feedthroughs and fiberglass of the board are not very
thermally conductive. To obtain the best thermal resistance
(T
– T
)
AMB
JUNCT
-------------------------------------------------
θ
=
JA
P
DISS
of the mounted part, θ , the topside copper ground plane
JA
where:
should have as much area as possible and be as thick as
practical. If possible, the solder mask should be cut away
from the EL1503A to improve thermal resistance. Finally,
metal heatsinks can be placed against the board close to the
part to draw heat toward the chassis.
T
T
is the maximum die temperature (150°C)
JUNCT
is the maximum ambient temperature
is the dissipation calculated above
AMB
P
DISS
θ
is the junction to ambient thermal resistance for the
JA
package when mounted on the PCB
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. This allows safely driving 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.
This θ value is then used to calculate the area of copper
needed on the board to dissipate the power. The graph
JA
below show various θ for the SO20 mounted on different
JA
copper foil areas.
55
Note: 2oz. COPPER USED
50
TOP FOIL ONLY-WITH SOLDER MASK
2
TOP FOIL-WITH 0.45IN BOTTOM
FOIL WITH MANY FEEDTHROUGHS
45
40
35
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.
TOP FOIL ONLY-NO SOLDER MASK
Power Supplies
30
The power supplies should be well bypassed close to the
EL1503A. A 3.3µF tantalum 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 EL1503A
demonstration board, and documentation can be obtained
from the factory.
0
1
2
3
4
5
6
7
8
9
10
2
AREA OF CIRCUIT BOARD HEAT SINK (in )
FIGURE 41. THERMAL RESISTANCE of 20 Ld SOIC (0.300")
EL1503A vs BOARD COPPER AREA
A separate application note details the 24 Ld QFN PCB
design considerations.
Single Supply Operation
The EL1503A can also be powered from a single supply
voltage. When operating in this mode, the GND pins can still
be connected directly to GND. To calculate power
Feedback Resistor Value
dissipation, the equations in the previous section should be
used, with V equal to half the supply rail.
S
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:
EL1503A PCB Design
A separate application note details the 24 Ld QFN PCB
design considerations. The SOIC power packages
(20 leads) are designed so that heat may be conducted
away from the device in an efficient manner. To disperse this
heat, the center leads (4 per side for the 20 lead and 2 per
side for the 16 lead) are internally connected to the mounting
platform of the die. Heat flows through the leads into the
circuit board copper, then spreads and convects to air. Thus,
the ground plane on the component side of the board
becomes the heatsink. This has proven to be a very effective
TABLE 1. OPTIMUM DRIVER FEEDBACK RESISTOR for
VARIOUS GAINS and SUPPLY VOLTAGES
DRIVER VOLTAGE GAIN
SUPPLY
VOLTAGE
2.5
5
10
±5V
±12V
2.7k
2.2k
2.2k
2.0k
2.0k
2.0k
FN7039.2
March 26, 2007
14
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