A matter of taste in regulator choice
01 November 2006
Recent technical advancements in the performance of switching regulators and innovative packaging methods allow a new generation of point-of-load DC/DC regulators, including all the circuit components, to be shrunk and resemble surface-mount ICs.
These high-end point-of-load regulator modules are complete with DC/DC controller, MOSFETs, inductor, capacitors and the compensation circuitry. The encapsulation protects the device and increases the reliability of the total solution.
Although these are complete circuits in encapsulated packages that mount on a PCB, it is difficult to distinguish these tiny regulator modules from other ICs such as FPGAs or graphics processors. Despite their small size, Linear's uModule DC/DC regulators convert supplies from 4.5V to 28V to lower output voltages as low as 0.6V.
Systems such as automotive telematics, industrial test equipment or telecomms routers that incorporate FPGAs, microcontrollers, memory modules and other sophisticated digital ICs, have specific requirements for DC/DC regulators. For example, systems such as data acquisitions that may be sensitive to noise, require the switching frequency of the DC/DC regulator to match a certain clock frequency so filters can attenuate more of the switching noise. These regulators must offer phase-lock loop (PLL) capability for synchronisation to an external clock. Furthermore, these complex systems need several different point-of-load (PoL) voltages to satisfy a variety of board components such memory, I/O, core and even operational amplifiers. With multi-rails, it is often necessary for proper start-up of the system that these rails follow a set sequence during power-up. As a result, a DC/DC PoL converter must be equipped with precision tracking and sequencing circuitry to ensure a well controlled power up and power down of the rails.
Family ties
New functions and power levels have been added to the LTM4600 (10A DC/DC uModule regulator) family to satisfy power requirements for a variety of systems. For example, the LTM4601, the new uModule regulator delivers 20 per cent higher current in an identical package size of 15mm x 15mm x 2.8mm LGA. The LTM4601 also provides more functions such as tracking, PLL and remote sensing. In addition, to simplifying the task of layout and copying the uModule converter's layout, the lower output current version of each uModule regulator (LTM4602 and LTM4603 6A output) is offered in the same footprint and pin function as its higher current version (LTM4600 and LTM4601, respectively). Other new features are remote sensing for precision regulation and current sharing for higher output power by paralleling multiple uModule converters.
These uModule converters occupy approximately 50 per cent less board space than a discrete board mounted solution with similar power handling, voltage range and performance. This compact design is possible because of high efficiency synchronous operation, fast switching frequency, and the use of a high performance, thermally enhanced package.
Thermal Performance
The challenge of creating high-power density DC/DC modules is due in part to the integration of MOSFETs, inductors and other components into an enclosed plastic moulded package. The most difficult challenge is the removal of heat from the device and transferring it efficiently to the ambient environment such as PCB, air, or heatsink. Without clever packaging and assembly methods, such a compact device would easily overheat and the user would be forced to significantly derate the output power performance of the product. If the device had to operate in environments such as industrial applications where the ambient temperature can be relatively high, the DC/DC module's poor heat transfer capability would severely impact its use.
The uModule's package is designed to dissipate heat from both the bottom and top. The substrate, soldering techniques, thermal planes and layout of contents were calculated to provide extremely low thermal resistance in a thin, compact package. As a result, the junction-to-ambient and junctionto- case thermal resistance values are only 15 C/W and 6 C/W, respectively. With these values, the uModules can reliably operate at their defined output power capability, although each is housed in a small package.
By encapsulating all the major power components and supporting circuitry in a plastic LGA package, a uModule converter can provide outstanding thermal performance. Among the techniques used to achieve this goal is the use of solder to attach components to the PCB substrate instead of an epoxy attach method. The use of lead-free solder has two benefits. First, it provides a low thermal resistance so that heat from the on-board MOSFETs (in die form) can easily and quickly transfer from the backside of the die to the substrate into the system PCB. Secondly, it allows a very thin layer of solder. If attaching epoxy were to be used, its thickness would have been more than four times greater.
Thermal modelling
Another approach used to lower the thermal resistance of the uModule involves incorporation of generous amounts of copper in the substrate. The ground, input and output paths, where high current is directed, all use copper planes that with the solder mask defined PADs on the bottom of the package remove the heat from the inside of the package to the PCB.
In figure 1, the heat generated from the ¥ìModule flows to top and bottom sides. For the topside heat path, Rjt is used to represent the thermal resistance from junction to top surface, while Rta represents the resistance from top surface to ambient. Similarly, for the bottom side, Rjb is the thermal resistance from junction to bottom surface and Rba is the resistance from bottom surface to ambient. The double-sided cooling scheme can be realised using a heatsink for the top.
One example would be a 12V to 3.3V at 10A (33W) design with 91 per cent efficiency has about 3W power loss. This loss is attributed to the power dissipation in the DC/DC controller section, and transition losses in the internal top MOSFET. Figure 2 shows a thermal image of this design with several data points. Surprisingly, the maximum temperature is only 66ºC on the μModule with 3W of dissipation. These measurements are taken without the use of heatsink on the top of the μModule. A heatsink would further improve heat dissipation.
Anatomy of a converter
The architecture and performance of the controller IC and MOSFETs (see figure 3) enables the μModule to operate at high efficiency over a wide range of input and output voltages and load currents and also allows the use of small discrete components. Combined, this permits the complete solution to fit a 15mm x 15mm footprint with only 2.8mm of thickness.
The DC/DC controller features valley current mode synchronous switching architecture. This architecture allows very low duty cycle operation for high-input to lowoutput voltage DC/DC conversion as well as very fast transient response to load current changes (see figure 4). Where other controllers must wait one clock cycle before responding to a load transient, a μModule DC/DC converter reacts almost instantaneously because there is no clocklatency operation. No clock-latency and faster transient response means less dependency on output capacitance and, therefore, fewer output capacitors. As a result, the internal output capacitor in the package is sufficient to the requirements of most load transients.
To incorporate a small inductor and capacitors, the internal DC/DC controller switches at approximately 800kHz to 1MHz. At this switching frequency, the size and height of the inductor can be reduced. The inductor is shielded to reduce magnetic interference with surrounding components and devices. High switching frequency also permits the use of smaller capacitors. The μModule includes small signal capacitors for both the input and output.
High voltage MOSFETs were manufactured using proprietary DMOS technology to reduce reverse-transfer capacitance and RDS(ON) so that the devices safely deliver the anticipated power while the entire power supply circuit is enclosed in a tiny moulded package. The MOSFETs used are in die-form and are solder-attached to the substrate. The combination of die-form and low thermal resistance of the solder creates an extremely efficient way for the heat to be removed from the inside of the μModule to the junction and then to the PCB or surrounding air.
Increase functions
Some members of the family include functions such as PLL, remote sensing, tracking and margining. The PLL synchronises the clock frequency of a μModule to an external clock for better noise performance. Another use is to parallel multiple μModule converters to current-share in an out-of-phase fashion for higher output power applications. For example, four LTM4601s can be used in parallel to current share and deliver more than 40A to a load. The differential remote sensing in the LTM4601 guarantees the best voltage regulation at the load even at high load currents. An op amp is used to sense the voltage at the location of the load (FPGA, for example) thus compensating for the voltage drop imposed by the trace
impedance of a PCB.
AFSHIN ODABAEE is product marketing engineer, Power Products, Linear Technology.
Contact Details and Archive...
Related Articles...
Most Viewed Articles...