EL1507

FN7013 Rev 3.00

Page 14 of 16

March 26, 2007

The overall power dissipation (PDISS) is obtained by adding

PDquiescent and PDtransistor.

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 SO power packages are designed so that heat may be

conducted away from the device in an efficient manner. To

disperse this heat, the center leads 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. A separate application note details the 16 Ld QFN

PCB design considerations.

Single Supply Operation

The EL1507 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.

Output Loading

While the drive amplifiers can output in excess of 400mA

transiently, the internal metallization is not designed to carry

more than 75mA of steady DC current and there is no current-

limit mechanism. This allows safely driving rms sinusoidal

currents of 2 x 75mA, or 150mA. 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 EL1507.

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

EL1507 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 response (<1dB peaking) for different

supply voltages and gains:

Power Control Function

The EL1507 contains two forms of power control operation. Two

digital inputs, C0 and C1, can be used to control the supply

current of the EL1507 drive amplifiers. As the supply current is

reduced, the EL1507 will start to exhibit slightly higher levels of

distortion and the frequency response will be limited. The 4 power

modes of the EL1507 are set up as shown in the table below:



JA

TJUNCT TAMB

–



PDISS

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

=

TABLE 1. OPTIMUM DRIVER FEEDBACK RESISTOR FOR

VARIOUS GAINS AND SUPPLY VOLTAGES

Supply

Voltage

Driver Voltage Gain

2.5

5

10

±5V

2k

1.8k

1.5k

±12V

2k

1.8k

1.5k

TABLE 2. POWER MODES OF THE EL1507

C1

C0

Operation

00

IS Full Power Mode

01

¾-IS Power Mode

10

½-IS Power Mode

1

1

Power Down