AD8390

Rev. C | Page 12 of 16

To obtain optimum thermal performance from the AD8390 in

either package, it is essential that the thermal pad be soldered to

a ground plane with minimal thermal resistance. This is par-

ticularly true for dense circuit designs with multiple integrated

circuits. Furthermore, the PCB should be designed in such a

manner as to draw the heat away from the ICs. Figure 26

illustrates the relationship between thermal resistance (°C/W)

to a 4-layer board with a given copper area.

LAYOUT, GROUNDING, AND BYPASSING

The first layout requirement is for a good solid ground plane

that covers as much of the board area around the AD8390 as

possible. The only exception to this is that the two input pins

should be kept a few millimeters from the ground plane, and

ground should be removed from inner layers and the opposite

side of the board under the input traces. This minimizes the

stray capacitance on these nodes and helps preserve the gain

flatness versus frequency.

Figure 26 can be used to help determine the copper board area

required for proper thermal management of the AD8390. The

power dissipation of the AD8390 can be computed using

Equation 11. This number can then be inserted into the

The power supply pins should be bypassed as close as possible

to the device on a ground plane common with signal ground.

Good high frequency, ceramic chip capacitors should be used.

This bypassing should be done with a capacitance value of

0.01 μF to 0.1 μF for each supply. Low frequency bypassing

should be provided with 10 μF tantalum capacitors from each

supply to signal ground. The signal routing should be short and

direct to avoid parasitic effects, particularly on traces connected

to the amplifier inputs. Wherever there are complementary

signals, a symmetrical layout should be provided to the extent

possible to maximize the balance performance. When running

differential signals over a long distance, the traces on the PCB

should be close together.

W

C

P

T

AD8390

RISE

JA

°

=

=

θ

(12)

temperature to the highest allowable die temperature. It is

generally recommended that the maximum die temperature be

limited to 125°C, and in no case should it be allowed to exceed

150°C.

POWER DISSIPATION AND THERMAL

MANAGEMENT

determine the minimum copper area required for proper thermal

dissipation of the AD8390.

The AD8390 was designed to be the most efficient class AB

ADSL/ADSL2+ line driver available. Figure 11 shows the total

power consumption (delivered line power and power consumed)

of the AD8390 driving ADSL signals at varying output powers

and power modes. To accurately determine the amount of

power dissipated by the AD8390, it is necessary to subtract the

power delivered to the load, matching losses, and transformer

losses as follows:

90

0

20

10

30

40

50

60

70

80

1

100

1000

10

10000

mW

losses

mW

load

supply,mW

AD8390

P

P

P

P

,

,

−

−

=

(11)

where:

is the total supply power in mW drawn by the AD8390.

is the power delivered into a 100 Ω twisted-pair line in mW.

is the power dissipated by the matching resistors and

the transformer in mW.

Figure 26. Thermal Resistance vs. Copper Area

While this discussion focuses mainly on ADSL applications, the

same premise can be applied to determining the power dissipa-

tion of the AD8390 in any application.