POWER DISSIPATION AND THERMAL

2

1.5

1

0

−40

−20

0

20

2.5

3

3.5

40

60

80

TA − Ambient Temperature − °C

8-Pin DGN Package

θJA = 170°C/W for 8-Pin SOIC (D)

θJA = 58.4°C/W for 8-Pin MSOP (DGN)

ΤJ = 150°C, No Airflow

0.5

8-Pin D Package

P

Dmax

=

T

max

A

T

-

qJA

(28)

DRIVING CAPACITIVE LOADS

VS

RS

RG

RF

RF

+

RT

RISO

CL

RG

R

=10

25

-

W

ISO

VS

-VS

RISO

+

-

-

THS4504

THS4505

SLOS363D – AUGUST 2002 – REVISED MAY 2008 ......................................................................................................................................................... www.ti.com

CONSIDERATIONS

The THS4500 family of devices does not incorporate

automatic thermal shutoff protection, so the designer

must take care to ensure that the design does not

violate the absolute maximum junction temperature of

the device. Failure may result if the absolute

maximum

junction

temperature

of

+150

°C is

exceeded. For best performance, design for a

maximum junction temperature of +125

°C. Between

+125

°C and +150°C, damage does not occur, but the

performance of the amplifier begins to degrade.

The thermal characteristics of the device are dictated

by the package and the PC board. Maximum power

dissipation for a given package can be calculated

using the following formula.

Figure 91. Maximum Power Dissipation vs

Ambient Temperature

Where:

When determining whether or not the device satisfies

PDmax is the maximum power dissipation in the

the maximum power dissipation requirement, it is

amplifier (W).

important to not only consider quiescent power

TMAX

is

the

absolute

maximum

junction

dissipation, but also dynamic power dissipation. Often

temperature (

°C).

times, this is difficult to quantify because the signal

pattern is inconsistent, but an estimate of the RMS

TA is the ambient temperature (°C).

power dissipation can provide visibility into a possible

θ

JA = θJC + θCA

problem.

θ

JC

is the thermal coefficient from the silicon

junctions to the case (

°C/W).

θ

CA is the thermal coefficient from the case to

High-speed amplifiers are typically not well-suited for

ambient air (

°C/w).

driving large capacitive loads. If necessary, however,

For systems where heat dissipation is more critical,

the load capacitance should be isolated by two

the THS4500 family of devices is offered in an

isolation resistors in series with the output. The

MSOP-8 with PowerPAD. The thermal coefficient for

requisite isolation resistor size depends on the value

the

MSOP

PowerPAD

package

is

substantially

of the capacitance, but 10

Ω to 25 Ω is a good place

improved over the traditional SOIC. Maximum power

to begin the optimization process. Larger isolation

dissipation levels are depicted in the graph for the

resistors decrease the amount of peaking in the

two packages. The data for the DGN package

frequency response induced by the capacitive load,

assumes a board layout that follows the PowerPAD

but this comes at the expense of larger voltage drop

layout guidelines referenced above and detailed in

across the resistors, increasing the output swing

the PowerPAD application notes in the Additional

requirements of the system.

Reference Materialsection at the end of the data

sheet.

Figure 92. Use of Isolation Resistors with a

Capacitive Load

30

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