When designing power-management circuitry, the goal is to provide the proper power to the rest of the electrical system. This sounds easy enough but, in addition to the power being provided to the actual load, system designers must also worry about unwanted by-products, including heat. Thermal considerations can become paramount in portable designs where cooling fans, large heat sinks and Printed Circuit Boards (PCBs) with lots of copper may not be realistic options. In these situations, the designer must be extra careful to not only select the proper power-management circuit topology, but must also be aware of the various integrated-circuit (IC) packaging options and their thermal characteristics.
Power-Management Circuit Topology
The emergence of portable electronics has forced system designers to focus more and more on the design of power-management circuitry. Todayís designer must find ways to extend battery life while still maintaining performance, not to mention fitting designs into smaller and smaller form factors. These constraints have made thermal management a critical design component.
For portable applications, dc-to-dc conversion is a large portion of the power-management circuitry. As the name implies, a dc-to-dc converter converts a direct current at one voltage potential to a direct current at a different voltage potential. This is a critical function in battery-powered applications, where the voltage across the battery or batteries can change as the cell is depleted. A dc-to-dc converter may be required to convert this battery voltage to a different voltage potential, in order to operate various ICs in the system. The selection of the dc-to-dc converter topology can drastically affect the thermal characteristic of the power-management circuit. In general, designers have two main options for addressing dc-to-dc converters ó either switching or linear regulators.
Both types of dc-to-dc converter have their advantages and disadvantages. While switching regulators typically achieve higher levels of efficiency, the larger overall solution size can become a limiting factor in small-form-factor circuit designs. With this in mind, this article will focus on thermal management issues with regard to linear regulators.
A linear regulator uses a voltage-controlled current source to create a given output voltage. This architecture relies on adjusting a resistive element to create a given output voltage. The resistive element dissipates power in the form of heat. A system designer must be aware of the input and output voltage and current requirements. Linear regulators may not be a viable option for systems that require a large voltage drop and/or a large output current, due to the issues caused by self heating.
Letís take a closer look at the power dissipation within a linear regulator. The law of energy conservation states that energy cannot be created or destroyed. So, the power that goes into a linear regulator must come out, either in the form of power being applied to the load or as heat dissipation. Figure 1, above, shows a basic 3-terminal linear regulator, which has an input, an output and a ground pin.
The current that flows into the linear regular (depicted as IIN in Figure 1) either flows through the regulatorís pass transistor and out to the load (via the VOUT pin) or through the regulator to the ground pin (depicted as IGND in Figure 1). The power dissipation of the linear regulator, due to the ground current, is typically insignificant. So, the total heat dissipation of the regulator can be simplified to the change in voltage between the input and the output, multiplied by the output current.
For example, assume a circuit is using Microchipís MCP1700 linear regulator. The input is at 3.6V (a nominal voltage for a single-cell Li-Ion battery) and the output is a regulated 1.2V. The maximum load current is 200 mA under these conditions for the MCP1700. In this case, the power dissipation of the linear regulator is 480 mW. The power (in the form of heat) that must be dissipated by the linear regulator will increase as the voltage drop between the input and the output increases and as the load current increases.
Convection Versus Conduction
So, how does this generated heat leave the linear regulator? One way is through convection, during which the heat gener-ated in the silicon die is transferred through the plastic packaging to the surrounding ambient air. This transfer of heat energy is dependent upon various factors, including the size of the die, the size of the package and the thermal characteristics of the molding compound. The thermal resistance from the junction of the silicon die to the ambient air is listed in the device datasheet as ?JA and is measured in units of degree Celsius per Watt. Heat sinks and cooling fans can help to decrease the ?JA, but for portable applications, this is typically not an option. So, the design must factor in the thermal limitations of the packaging.
Another way heat can dissipate out of an IC is through conduction. In this case, heat is transferred from the IC to the PCB. The amount of conduction is dependent, once again, upon the packaging type, whether any thermal film is used, and on the PCB, itself. Copper is a great conduit for conducting heat, and the more copper available, the better. In an ideal situation, the PCB will have an entire copper layer with lots of thermal vias, to help reduce the thermal resistance. However, as the number of layers increases, so does the cost. Most electronics attempt to reduce the amount of layers needed, in order to save on manufacturing costs; but the trade-off is a higher thermal resistance. IC manufacturers are continuing to find new ways to reduce these thermal concerns with thermally enhanced surface-mount packages that contain power pads, such as the Dual Flat No-lead (DFN) packages shown in Figure 2, above.
Letís go back to the original example in order to get a better understanding of how these thermal issues can affect package selection. Previously, we investigated the MCP1700 linear regulator in an application that was running off a Li-Ion battery with a nominal voltage of 3.6V. The regulator then regulated this voltage down to 1.2V and supplied a maximum of 200 mA to the load. Let us also assume that this regulator will be placed in a small enclosed area, with a maximum ambient temperature of 85C.
Reviewing the MCP1700 datasheet reveals that the maximum junction temperature (this is the temperature at the actual silicon die) cannot exceed 150C. We also note that the MCP1700 is available in a 3-pin SOT-23 package, as well as a 3-pin SOT-89 package. The SOT-23 package is much smaller and is therefore more attractive for this space-constrained application. But, can this package handle the thermal requirements? Table 1, above, right, summarizes the relevant data for this example.
As discussed previously, heat dissipation not only happens via convection through the plastic packaging, but also through conduction to the PCB. The design of the PCB will affect the ?JA for a given device, based on trace width, number of layers, amount of copper, etc. The ?JA numbers listed in Table 1 are used for this example and will vary based upon design.
Given this information, the junction temperature of the die can be found. Once thermal equilibrium has been reached, the silicon die will be at the same temperature as the ambient air. Additionally, we need to factor in the power dissipated by the regulator, times the thermal resistance (?JA) for the given package:
TJ = TA + (PD x ?JA)
For the SOT-23 package:
TJ = 85C + (0.48W x 336C/W)
TJ = 246.3C
A junction temperature of 246.3C exceeds the maximum limit of 150C for this device and may cause the regulator to fail.
For the SOT-89 package:
TJ = 85C + (0.48W x 52C/W)
TJ = 110C
In this case, the junction temperature is below the specified maximum level and therefore will be fine in this application. For these given conditions, a SOT-23 package will not meet the thermal requirements. However, the larger SOT-89 package, which has a power tab, has a much lower thermal resistance and satisfies the thermal requirements for this application.
Todayís system designer faces many challenges when designing power-management circuitry. In an effort to get more performance out of smaller and smaller devices, thermal management is a critical design consideration. There are many factors to consider when it comes to thermal management, but device packaging is one area that should not be overlooked.
Kevin Tretter is a senior product marketing engineer with Microchip Technologyís Analog and Interface Products Div. in Chandler, AZ. He is responsible for tactical marketing support for Microchipís analog and interface products in the Eastern and South Central U.S., as well as strategic marketing of the companyís operational amplifiers, comparators and programmable-gain amplifiers (PGAs).