Innovative SoC Packaging with Integrated Heat Sink

By Eddie Beville, Afshin Odabaee & Mike Stokowski, Linear Technology Corp.

Each generation of high end processors, FPGAs and ASICs burdens power supplies with heavier loads, but system designers rarely allocate precious additional system board space to correspond to the power inflation. The squeeze on power supplies is compounded by the widespread requirement for greater numbers of dedicated board-mount power supplies, which provide POL (point-of-load) regulation for multiple voltage rails. Individual rails must increasingly support from tens to over a hundred amps at low voltages, under ≤1V, requiring an initial accuracy of ~1% and superb load transient deviation of less than a few percent. So the challenge is to find power supply solutions that are accurate, can deliver high load currents at low voltages, while taking little system board space.
Once a suitably powerful regulator solution is found, it must be evaluated for power loss and thermal resistance. These two parameters can break an otherwise good regulator solution if it can’t meet system heat requirements, especially when the system must operate in an elevated ambient temperature environment. Obviously, conversion efficiency must be high in order to limit power loss, and the package design must feature low internal thermal resistance and a low thermal resistance connection to the ambient environment. As solutions shrink, the thermal resistance between the regulator and the board decreases in area, making it increasingly difficult to keep the board cool because the power regulator usually conducts most of the power loss back into the system board, significantly increasing the internal system temperature.

The Real Issue: Heat & Cost of Cooling
System and thermal engineers spend a lot of time modeling and evaluating these complex electronic systems in order to design solutions that remove power loss in the form of heat. Air flow and heat sinks are typically used to remove this unwanted heat. The real issue is that modern processors, FPGAs, and custom ASICs usually dissipate significantly more power as the internal system temperature increases. This unfortunately requires more power from the power regulators, and will increase their internal power loss thus increasing system temperature even further. So eliminating power loss and heat is very important, and high density power solutions must limit power loss and remove heat effectively. But most compact packaged power solutions either dissipate too much power or cannot effectively remove the heat and therefore cannot operate at elevated temperature without significant de-rating. A reasonable solution is needed to help alleviate the real issue.
It’s no surprise that to keep the temperature of a high power design to reasonable levels attention to cooling methods is crucial. Installation of fans, cold plates, heat sinks and sometime submerging the system in special liquids are examples of approaches that some designers are forced to implement. All are costly, but necessary. However, if a high power point-of-load regulator could deliver the required power while dissipating heat evenly and efficiently, the requirements for cooling that portion of the circuit will be reduced saving on cooling size, weight, maintenance and cost.

Power Density is Misleading
The topic of high power density DC/DC regulators is misleading because it does not address the behavior of the temperature of the device. System designers should be educated to seek more information from the device’s data sheet once they decide on a product that meets the system’s electrical, physical and power requirements for a DC/DC regulator. Here is an example: if a DC/DC regulator in 2cm x 1cm delivers 54W to a load, its power density is rated as 27W/square cm. This number may impress a few designers and satisfy their search: desired power, desired size and desired price. However, what’s forgotten is heat which finally translates into temperature. The key piece of information is to study the DC/DC regulator’s thermal impedance looking for values for the package’s junction to case, junction to air and junction to PCB.
Continuing with the above example, the device has another attractive attribute. It operates at an impressive efficiency of 90%. It dissipates 6W while delivering 54W output in a package with 20ºC/W junction-to-air thermal impedance. Multiply 6W by 20ºC/W and the result is 120ºC rise on ambient temperature. At 45ºC ambient temperature, junction temperature of the package of this seemingly impressive DC/DC regulator is calculated at 165ºC. 165ºC is not an impressive value for two reasons: a) its above maximum temperature of most silicon ICs which is roughly 120ºC and b) it requires special attention to keep the junction temperature at a safer value below 120ºC.
Above simple calculation is sometimes ignored. A DC/DC regulator that seemed to address all the electrical and power requirements failed to meet thermal guidelines of the system or proved too costly to use due to additional measures to operate at a safe temperature environment. It’s important to remember to study thermal performance of a DC/DC regulator as one first becomes involved in evaluating attributes such volts, amps and centimeters.
This article will describe a new high density and scalable LTM4620 µModule® regulator. The discussion will comprise of the electrical, mechanical/package and thermal performance along with different scalable power designs. The goal is to show a new high density scalable power regulator that has excellent electrical performance, low power loss and a unique thermally enhanced package design to help resolve high power density challenges.

The LTM4620 Dual 13A or Single 26A µModule Regulator
Figure 1 shows a photo of the LTM4620 µModule regulator. The SIP (System-In-Package) is a 15mm x 15mm x 4.41mm LGA device. It is capable of delivering two independent outputs at 13A, or a single output at 26A. The package supports both top and bottom heat sinking for excellent thermal management.
Figure 2 shows a block diagram of the LTM4620 µmodule regulator. The LTM4620 consist of two high performance synchronous buck regulators. The input voltage range is 4.5V to 16V, and the output voltage range is 0.6V to 2.5V and 0.6V to 5.5V for the LTM4620A. The LTM4620 electrical features are ±1.5% total output accuracy, 100% tested accurate current sharing, fast transient response, multiphase parallel operation with self clocking and programmable phase shift, frequency synchronization and an accurate remote sense amplifier.
The protection features are output overvoltage protection feedback referred, fold back overcurrent protection and internal temperature diode monitoring.

The LTM4620 Unique Package Design
Figure 3 shows the side view rendering and a top view photo of an unmolded LTM4620. The package design consists of a highly thermal conductive BT substrate with adequate copper layers for current carrying capacity and low thermal resistance to the system board. A proprietary lead frame power MOSFET stack is utilized to provide high power density, low interconnect resistance, a high thermal conductivity to both the top and bottom of the device. The proprietary heat sink design attaches to the power MOSFET stacks and the power inductors to provide an effective top side heat sinking. An external heat sink can be applied to the topside exposed metal to remove heat with air flow. Airflow alone with no heat sink removes heat from the topside due to construction of the heat sink and the mold encapsulation.
Figure 4 shows a LTM4620 thermal image and a de-rating curve for a 12V to 1V at 26Amp design. The temperature rise is only 35°C above ambient temperature with 200LFM of air flow, and the de-rating curve shows that the maximum load current requires no de-rating out to ~80°C. Figure 4 reveals the thermal data that shows the real merits of a thermally enhanced high density power regulator solution. The unique package design keeps the power loss as low as possible for the small size, and effectively removes heat as a function of the power loss.

The LTM4620 Electrical Performance
Figure 5 shows the LTM4620 operating in the dual output current sharing operation. This configuration provides a very high density 1.5V at 26A solution. The RUN, TRACK, COMP, VFB, PGOOD and VOUT pins are tied together to implement the parallel operation. The design shows one way of monitoring the LTM4620 internal temperature diode using a LTC2997 temperature sensor monitor. The temperature sense diode can be monitored by many different devices that monitor a diode connected transistor. Figure 6 shows the 1.5V efficiency for the 2 phase parallel output and the two channel current sharing performance. The 86% efficiency is very respectable for such high density solution, and as shown in the Figure 4 thermal data, the temperature rise is well controlled due to a low ӨJA thermal resistance after board mount. The effective top and bottom heat sinking enables the LTM4620 to operate at full power with low temperature rise. Figure 6 shows both VOUT1 and VOUT2 current sharing performance. The LTM4620 internal controller is accurately trimmed and tested for output current sharing. This makes the LTM4620 an excellent choice for high density scalable power solutions. The high efficiency and fast transient response current mode architecture fits well with the low voltage core power supply requirements needed for high performance processors, FPGAs, and custom ASICS. The outstanding output voltage initial accuracy and the differential remote sensing provide proper DC voltage regulation at the load point. The unique thermal capabilities and the excellent current sharing allows for scaling the output current capability up to 100+ Amps. No external phase shifted clock sources are needed for setting multiphase operation for each regulator channel. Each LTM4620 has a Clock In pin and a Clock Out pin with internal programmable phase shifting for clocking the paralleled channels. Either external frequency synchronization or internal on board clocking can be selected. These clocking features further enable the power scaling concept.
Figure 7 shows both a snapshot of an 8-Phase, 4 µModule regulator 100A design and the current sharing graph for all four regulators. All eight phases are clocked phase and tied together to implement the current sharing scaled 100A implementation. As noted in Figure 7, the actual µModule regulator board space is about 1.95 square inches of space to support the 100A power solution. This provides an excellent high density power solution for these high currents. A heat sink can be applied across all four modules to remove power loss with air flow. This keeps a lot of power loss from being dissipated into the system board.

Proof of Performance
To validate the performance of the LTM4620, four quick TechClip videos are provided, demonstrating the setup and measurements. These TechClip videos cover topics of short circuit protection, thermal behavior and temperature rise at 26A and 100A, heat sink attachment and precision current sharing at startup, steady state and shutdown. View these videos at http://video.linear.com/p4634-126.

Conclusion
The LTM4620 µModule regulator brings a new concept to high density power solutions. The high performance regulator housed inside a superior thermally designed package enables high power designs to be implemented in a very small form factor. The multiphase clocking features with the accurate current sharing enable scalable designs in 25 Amps, 50 Amps and 100+ Amps. The LTM4620’s unique thermal properties provide full power operation at elevated ambient temperatures. High current designs can be implemented while controlling power loss and temperature to acceptable levels.

Linear Technology (UK) ltd
Tel 01628 477066 • Email: uksales@linear.com
www.linear.com