THS4271DR datasheet(27/38 Pages) TI

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THS4271

THS4275

SLOS397E − JULY 2002 − REVISED JANUARY 2004

www.ti.com

The top-side solder mask should leave the terminals

of the package and the thermal pad area with its five

holes exposed. The bottom-side solder mask should

cover the five holes of the thermal pad area. This

prevents solder from being pulled away from the

thermal pad area during the reflow process.

Apply solder paste to the exposed thermal pad area

and all of the IC terminals.

With these preparatory steps in place, the IC is simply

placed in position and run through the solder reflow

operation

any

standard

surface-mount

component. This results in a part that is properly

installed.

For a given

θJA , the maximum power dissipation is shown

in Figure 91 and is calculated by the equation 5:

D +

Tmax

* T

where:

PD = Maximum power dissipation of THS4271 (watts)

TMAX = Absolute maximum junction temperature (150°C)

TA = Free-ambient temperature (°C)

θJA = θJC + θCA

θJC = Thermal coefficient from junction to the case

θCA = Thermal coefficient from the case to ambient air

(

°C/W).

The next consideration is the package constraints. The

two sources of heat within an amplifier are quiescent

power and output power. The designer should never forget

about the quiescent heat generated within the device,

especially multi-amplifier devices. Because these devices

have linear output stages (Class AB), most of the heat

dissipation is at low output voltages with high output

currents.

The other key factor when dealing with power dissipation

is how the devices are mounted on the PCB. The

PowerPAD devices are extremely useful for heat

dissipation. But, the device should always be soldered to

a copper plane to fully use the heat dissipation properties

of the PowerPAD. The SOIC package, on the other hand,

is highly dependent on how it is mounted on the PCB. As

more trace and copper area is placed around the device,

θJA decreases and the heat dissipation capability

increases. For a single package, the sum of the RMS

output currents and voltages should be used to choose the

proper package.

THERMAL ANALYSIS

The THS4271 device 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.

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.

Dmax +

Tmax–TA

where:

PDmax is the maximum power dissipation in the amplifier (W).

Tmax is the absolute maximum junction temperature (°C).

TA is the ambient temperature (°C).

θJA = θJC + θCA

θJC is the thermal coefficient from the silicon junctions to the

case (

°C/W).

θCA is the thermal coefficient from the case to ambient air

(

°C/W).

For systems where heat dissipation is more critical, the

THS4271 is offered in an 8-pin MSOP with PowerPAD.

The thermal coefficient for the MSOP PowerPAD package

is substantially improved over the traditional SOIC.

Maximum power dissipation levels are depicted in the

graph for the two packages.

The data for the DGN

package assumes a board layout that follows the

PowerPAD layout guidelines referenced above and

detailed in the PowerPAD application notes in the

Additional Reference Material section at the end of the

data sheet.

1.5

−40

−20

2.5

3.5

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)

TJ = 150°C, No Airflow

Figure 91. Maximum Power Dissipation vs

Ambient Temperature

0.5

8-Pin D Package

When determining whether or not the device satisfies the

maximum power dissipation requirement, it is important to

consider not only quiescent power dissipation, but also

dynamic power dissipation. Often maximum power is

difficult to quantify because the signal pattern is

inconsistent, but an estimate of the RMS power dissipation

can provide visibility into a possible problem.

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