UNLESS OTHERWISE NOTED, this document contains PRODUCTION DATA.
Achieving optimum performance with a high-frequency amplifier, like the THS309x, requires careful attention to board layout parasitic and external component types.
Recommendations that optimize performance include:
The THS309x are available in a thermally-enhanced PowerPAD family of packages. These packages are constructed using a downset leadframe on which the die is mounted [see Figure 83(a) and Figure 83(b)]. This arrangement results in the lead frame being exposed as a thermal pad on the underside of the package [see Figure 83(c)]. Because this thermal pad has direct thermal contact with the die, excellent thermal performance can be achieved by providing a good thermal path away from the thermal pad. Note that devices such as the THS309x have no electrical connection between the PowerPAD and the die.
The PowerPAD package allows for both assembly and thermal management in one manufacturing operation. During the surface-mount solder operation (when the leads are being soldered), the thermal pad can also be soldered to a copper area underneath the package. Through the use of thermal paths within this copper area, heat can be conducted away from the package into either a ground plane or other heat-dissipating device.
The PowerPAD package represents a breakthrough in combining the small area and ease of assembly of surface mount with the, heretofore, awkward mechanical methods of heatsinking.
Although there are many ways to properly heatsink the PowerPAD package, the following steps illustrate the recommended approach.
The THS309x 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. But, for maximum performance and reliability, the designer must ensure 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 PC board. Maximum power dissipation for a given package can be calculated using the following formula.
For systems where heat dissipation is more critical, the THS3091 and THS3095 are offered in an 8-pin SOIC (DDA) with PowerPAD package. The thermal coefficient for the PowerPAD packages are substantially improved over the traditional SOIC. Maximum power dissipation levels are depicted in the graph for the available packages. The data for the PowerPAD packages assume a board layout that follows the PowerPAD layout guidelines referenced above and detailed in the PowerPAD application note (SLMA002). If the PowerPAD is not soldered to the PCB, the thermal impedance will increase substantially which may cause serious heat and performance issues. Be sure to always solder the PowerPAD to the PCB for optimum performance.
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 times, this 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.