2 Efficiency and Thermals Comparison

Controllers are a standard design for high current and high ambient temperature applications due to their ability to use low resistance MOSFETs and have more PCB surface area for thermal dissipation. Converters, although offering less flexibility, can achieve comparable thermal performance through a combination of large thermal DAP (die attach paddle), integrated low-resistance FET and good IC design as shown in the following images.

Efficiency curves and thermal data were generated from each devices’ respective EVM. LM5148-Q1 utilizes the same EVM as LM5149-Q1 named LM5149-Q1EVM-400 and share the same efficiency and thermal performance. LM70880QEVM was adjusted to use the same 6.8µH inductor as the LM5149-Q1EVM-400. Both devices were tested under similar conditions and LM5149 EVM utilizes MOSFETs with an RDSon values of 19.5mΩ and 8.8mΩ.

The converter design has the added benefit of easy performance characterization. A controller design requires greater simulation, calculations and design trials. Controller designs must consider the different specifications and performance per controller IC and per MOSFET chosen for the design.

As mentioned, a controller design offers increased flexibility such as MOSFET optimization. MOSFETs with lower resistance specifications can yield higher efficiency and better thermals at the expense of increased cost. External MOSFET also sometimes have wider operating temperatures up to 175C° while converters are specified up to 150C°.

A growing amount of application designs require higher voltage conversions and higher output power. Thermals can be a common challenge amongst these designs and have to be managed with a variety of designs. Controller and converter based designs both require good layout and thermal mitigation techniques to work well in high ambient temperature environments.