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 PCB. Maximum power dissipation for a given package is calculated using the following equation:

Where:

• P_Dmax 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)

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. The data for the PowerPAD packages assume a board layout that follows the PowerPAD layout guidelines referenced above and detailed in the PowerPAD™ Thermally Enhanced Package application note. If the PowerPAD is not soldered to the PCB, then the thermal impedance increases substantially, which can cause serious heat and performance issues. Be sure to always solder the PowerPAD to the PCB to optimize performance.

When determining whether or not the device satisfies the maximum power-dissipation requirement, consider not only quiescent power dissipation, but also dynamic power dissipation. Often times, ynamic power 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.