EL1503

FN7038 Rev 0.00

Page 15 of 16

July 17, 2001

Then, the

JA requirement needs to be calculated. This is done

using the equation:

where:

TJUNCT is the maximum die temperature (150°C)

TAMB is the maximum ambient temperature

PDISS is the dissipation calculated above

JA is the junction to ambient thermal resistance for the

package when mounted on the PCB

This

JA value is then used to calculate the area of copper

needed on the board to dissipate the power. The graph below

show various

JA for the SO20 mounted on different copper foil

areas.

A separate application note details the 24-pin LPP PCB design

considerations.

Single Supply Operation

The EL1503 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 dissipation, the

equations in the previous section should be used, with VS

equal to half the supply rail.

EL1503 PCB Design

A separate application note details the 24-pin LPP PCB design

considerations. The SO power packages (16 and 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 technique, but several aspects of

board layout should be 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 of

the mounted part,

JA, the topside copper ground plane should

have as much area as possible and be as thick as practical. If

possible, the solder mask should be cut away from the EL1503

to improve thermal resistance. Finally, metal heatsinks can be

placed against the board close to the part to draw heat toward

the chassis.

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.

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

The power supplies should be well bypassed close to the

EL1503. 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 EL1503 demonstration board,

and documentation can be obtained from the factory.

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. 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

JA

TJUNCT TAMB

–



PDISS

-------------------------------------------------

=

THERMAL RESISTANCE of 20-Pin SO (0.300")

EL1503 vs BOARD COPPER AREA

012

9

10

678

345

55

50

45

40

35

30

FIGURE 42. AREA OF CIRCUIT BOARD HEAT SINK (IN2)

Note: 2oz. COPPER USED

TOP FOIL ONLY-WITH SOLDER MASK

TOP FOIL-WITH 0.45IN2 BOTTOM

FOIL WITH MANY FEEDTHROUGHS

TOP FOIL ONLY-NO SOLDER MASK