THS3491 Datasheet, PDF (20/28 Pages) Texas Instruments – 900-MHz, High-Power Output, Current Feedback Amplifier

THS3491
SBOS875 – AUGUST 2017
www.ti.com
Layout Guidelines (continued)
(0.254 mm) diameter vias directly under the thermal pad 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. The PowerPAD package is electrically isolated from the
silicon and all leads. Connecting the PowerPAD package to any potential voltage such as –V_S is acceptable
because there is no electrical connection to the silicon.
5. When connecting these holes to the ground plane, do not use the typical web or spoke via connection
methodology. Web and spoke connections have a high thermal resistance that slows the heat transfer during
soldering operations. Avoiding these connection methods 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 THS3491 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 must leave the terminals of the package and the thermal pad area with its 13 holes
exposed. The bottom-side solder mask must cover the 13 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 device terminals.
8. With these preparatory steps in place, the device is simply placed in position and run through the solder
reflow operation as any standard surface-mount component. This results in a device that is properly installed.
11.1.1.2 Power Dissipation and Thermal Considerations
The THS3491 incorporates automatic thermal shutoff protection. This protection circuitry shuts down the amplifier
if the junction temperature exceeds approximately 160°C. When the junction temperature reduces to
approximately 140°C, the amplifier turns on again. However, for maximum performance and reliability, make sure
that the design does not exceed a junction temperature of 125°C. Between 125°C and 150°C, damage does not
occur, but the performance of the amplifier begins to degrade and long-term reliability suffers. The thermal
characteristics of the device are dictated by the package and the PCB. Maximum power dissipation for a given
package can be calculated using the following formula.
P_{D max}
T_max T_A
T_JA
where
• PD_max is the maximum power dissipation in the amplifier (W).
• T_max is the absolute maximum junction temperature (°C).
• T_A is the ambient temperature (°C).
• θ_JA = θ_JC + θ_CA
• 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 ambient air (°C/W).
(1)
The thermal coefficient for the PowerPAD packages are substantially improved over the traditional SOIC. The
data for the PowerPAD packages assume a board layout that follows the PowerPAD package layout guidelines
referenced above and detailed in PowerPAD™ Thermally Enhanced Package. Maximum power dissipation levels
are depicted in the graph titled Comparison of θ_JA for Various Packages. If the PowerPAD package is not
soldered to the PCB, the thermal impedance increases substantially, and may cause serious heat and
performance issues. Be sure to always solder the PowerPAD package to the PCB for optimum performance.
When determining whether or not the device satisfies the maximum power dissipation requirement, make sure to
consider not only quiescent power dissipation, but also dynamic power dissipation. Often times, this dissipation 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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