OPA690 datasheet(20/40 Pages) TI

20 / 40 page

THS4221, THS4225

THS4222, THS4226

SLOS399G − AUGUST 2002 − REVISED JANUARY 2004

www.ti.com

0.060

0.040

0.075

0.025

0.205

0.010

vias

Pin 1

Top View

0.017

0.035

0.094

0.030

0.013

Figure 37. PowerPAD PCB Etch and Via

Pattern

PowerPAD PCB LAYOUT CONSIDERATIONS

1. Prepare the PCB with a top side etch pattern as shown

in Figure 37. There should be etch for the leads as well

as etch for the thermal pad.

2. Place five holes in the area of the thermal pad. These

holes should be 13 mils in diameter. Keep them small

so that solder wicking through the holes is not a

problem during reflow.

3. Additional vias may be placed anywhere along the

thermal plane outside of the thermal pad area. They

help dissipate the heat generated by the THS4222

family IC. These additional vias may be larger than the

13-mil diameter vias directly under the thermal pad.

They can be larger because they are not in the thermal

pad area to be soldered, so that wicking is not a

problem.

4. Connect all holes to the internal ground plane.

5. When connecting these holes to the ground plane, do

not use the typical web or spoke via connection

methodology. Web connections have a high thermal

resistance connection that is useful for slowing the

heat transfer during soldering operations. This

resistance makes the soldering of vias that have plane

connections easier. In this application, however, low

thermal resistance is desired for the most efficient

heat transfer. Therefore, the holes under the THS4222

family PowerPAD package should make their

connection to the internal ground plane, with a

complete connection around the entire circumference

of the plated-through hole.

6. 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.

7. Apply solder paste to the exposed thermal pad area

and all of the IC terminals.

8. 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 38 and is calculated by the equation 5:

PD +

Tmax * TA

qJA

where:

PD = Maximum power dissipation of THS4222 (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 THS4222 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.

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.

(3)

OPA690 Datasheet(PDF) 20 Page - Texas Instruments

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