LDO package thermal limitations are a key factor when determining the functionality of a device. Many applications require a device to handle large differences between the input and output voltage, leading to a significant increase in junction temperature, T_J, due to the power dissipated, P_D, in the system. High junction temperature can affect the lifetime reliability and accelerate common failures (LDOs Thermal Performance in Small SMD Packages, application note). Additionally, the region of operation of the LDO can become limited due to thermal shutdown being triggered from a rise in T_J. Consequently, the device can become unable to perform as intended. Reducing the thermal resistance, R_ϴJA , is critical to extending device lifetime and operation. The thermal resistance is a measure of a package’s ability to dissipate heat to the ambient environment ,T_A , which is typically calculated using Equation 1.

Additionally, the LDO die size can contribute to thermal limitations. By spreading the same amount of heat over a smaller area, peak temperature and the resulting R_ϴJA parameter can increase. A well designed die can efficiently spread heat generated by power dissipation of the LDO. In a LDO, the pass FET is the main source of heat in the device. The die helps to distribute and spread heat throughout the bulk of the silicon. Heat is then transferred to the path of least thermal resistance or the path with the highest thermal conductivity. In a package with a large thermal pad, heat flows through the silicon die to the die attach material. After reaching the die attach material, heat transfers to the thermal pad, and then to the PCB, where the heat finally flows to the ambient, otherwise known as the external environment.

Thermal performance can also be impacted through the LDO's pass FET design. A properly laid out pass FET can maximize the FET area, perimeter, and aspect ratio to spread the heat across a larger area through the bulk of the silicon die.

When redesigning a LDO to reduce the die size, alternate FET shapes can modify the electrical performance. The thermal performance can be optimized to make sure that the electrical performance of the LDO still meets data sheet specifications. Ultimately, the TPS74801 die area was decreased by over 72% while also maintaining the R_ϴJA specification, which has allowed for an improved ability to maintain device thermal specification and accuracy.