Power Electronics

New Breed of MCMs Optimize System Performance and Cost

Although MCM circuit costs are higher than their monolithic cousins, they offer advantages from a total system cost standpoint.

The acronym MCM usually refers to multi-chip modules: sealed packages containing high-density, interconnected bare and/or packaged semiconductors. The term has also come to refer to multi-component modules, packages that contain miscellaneous components as well as ICs.

With some of these modules it is difficult to tell by looking at the package whether it is an IC or multi-component module. A major advantage for either type of MCM is that they enable individual power MOSFET performance to be optimized as a separate chip, whereas an IC with a monolithic MOSFET may exhibit higher on-resistance or be compromised in some other way due to the single process technology employed. Further, MCMs enable the integration of components, for example power inductors, which are not feasible to manufacture in monolithic form.

The cost of an MCM may be higher than multiple monolithic and discrete solutions with similar functions. However, the user should take overall cost into account, looking at them from a system viewpoint, because an MCM:

  • Saves board space
  • Minimizes potential noise problems
  • Eliminates layout errors that can occur if the circuit is user-designed
  • Minimizes circuit test requirements because the manufacturer tests the MCM before delivery
  • Reduces the BOM (bill of materials), which shortens purchasing time
  • Reduces system production time and cost because there are fewer devices to assemble
  • Shortens design time and time-to-market for the system being manufactured

One of the latest examples of a multi-component module is Linear Technology's LTM2881, an isolated RS485 µModule® transceiver that guards against large ground-to-ground differentials and common-mode transients (Fig. 1 and Fig. 2). Galvanic isolation splits the part into two.

MULTI-COMPONENT MODULES

On the left — the non-isolated side — the isolated communications interface communicates with the external digital controller in the application; this communications interface is also responsible for encoding and decoding signals and communicating the information across the isolation barrier. Similarly, on the right — the isolated side (the field side) — the other isolated communications interface is responsible for the same handling of signals and communicating with the RS485 driver and receiver. The traditional names and functions of the RS485 transceiver pins are DI, DE, RE, RO on the non-isolated side and A, B, Y, and Z on the isolated side. The TE pin enables and disables, across the barrier, the 120-Ω termination that appears between bus pins A and B.

In practical RS485 systems, ground potentials vary widely from node to node, often exceeding the tolerable range, which can result in an interruption of communications or destruction of a transceiver. The LTM2881 breaks ground loops by isolating the logic-level interface and line transceiver using internal inductive signal isolation that allows a much larger common-mode voltage range plus superior common-mode rejection of >30 kV/µsec. With 2,500 VRMS of galvanic isolation, onboard secondary power, and a fully compliant RS485 transmitter and receiver, the LTM2881 requires no external components, ensuring a complete, small µModule® product for isolated serial data communications.

Features of the LTM2881 make it suitable for a wide range of applications that work with large common-mode voltages, and when using multiple line taps that are not terminated. The ±15-kV ESD-protected transceiver operates at 20 Mbps or low-EMI 250 kbps, in half-duplex or full-duplex configurations. The driver maintains high impedance over the entire common-mode range and has a short-circuit current limit as well as slew-rate control for minimizing EMI. Its counterpart, the failsafe receiver, allows up to 256 devices to be connected on the same lines and defaults to a logic ‘1’ if the receiver inputs are open, shorted, or terminated but not driven — all without the need to pre-bias the network.

Integrated selectable termination allows cables to be properly terminated to avoid signal reflections and distorted waveforms, with the flexibility to add or remove terminations anywhere onto the bus via a software switch. The device also features thermal shutdown protection, disabling the transceiver outputs in case of excessive power dissipation. The self-powered LTM2881 takes many precautions to guarantee safe and reliable communications in RS485 or RS422 systems.

The LTM2881 is available in two input supply-voltage options: -3.3 V or 5 V, which is applied to the VCC pin. Then, a low-EMI dc-dc converter running at 1.6 MHz — along with some rectification components and an LDO — power all the internal components and provide a regulated 5-V output supply with up to 500 mW surplus power via the VCC2 pin, to support other circuits the application might require, such as network activity LEDs.

This internal power solution is sufficient to support the transceiver interface at its maximum specified load and data rate, and external pins are supplied for extra optional decoupling. The slew-rate mode (SLO) pin toggles the low-EMI edge filters for a 250-kbps data rate on and off. Also, the DOUT and DIN pins, which are general-purpose output and input respectively, provide one-way communication to the system's logic. The LTM2881 also provides VL, the 1.62- to 5.5-V logic supply pin for convenient level shifting to logic interfacing.

The LTM2881 is available in low-profile 11.25 × 15 × 2.8-mm surface mount LGA and BGA packages. All ICs and passive components are housed in an RoHS-compliant µModule package.

Prior to the release of the LTM2881, Linear introduced the LTM®4616 (Fig. 3), a complete dual, two-phase 8-A-per-channel switchmode dc-dc power regulator system. The package includes the switching controller, power FETs, inductors, and all support components. Operating from an input voltage range of 2.7 to 5.5 V, the LTM4616 supports two outputs within a voltage range of 0.6 to 5 V, each set by a single external resistor. This high-efficiency design delivers up to 8 A continuous current (10 A peak) for each output. Only bulk input and output capacitors are needed, depending on ripple requirements. The part can also be configured for a two-phase single output at up to 16 A.

The LTM4616's low-profile package (2.82 mm) enables utilization of unused space on the backside of PCBs for high-density point-of-load regulation. Fault-protection features include overvoltage protection, overcurrent protection, and thermal shutdown. The power module is offered in a space saving and thermally enhanced 15 × 15 × 2.82-mm LGA package. The LTM4616 is Pb-free and RoHS compliant.

Another example of a multi-component module is Fairchild Semiconductor's FDMF6704 (Fig. 4) a fully-optimized, ultra-compact, integrated MOSFET-plus-driver power stage for high-current, high-frequency synchronous buck dc-dc applications. This MCM follows Intel's November 2004 DrMOS (“Driver-MOSFET”) document that summarizes the different requirements needed to enable a standard integrated DrMOS specification for a typical PC platform. This specification's goal is to enable features necessary to develop an integrated device that allows interoperability between various devices and controllers.

DrMOS MCM

The FDMF6704 integrates a driver IC, two power MOSFETs, and a bootstrap Schottky diode into a thermally enhanced, ultra-compact 6- × 6-mm MLP package. With an integrated approach, the complete switching power stage is optimized with regards to driver and MOSFET dynamic performance, system inductance, and RDS(ON). This greatly reduces the package parasitics and layout challenges associated with conventional discrete solutions.

The FDMF6704 uses Fairchild's high-performance PowerTrench™ 5 MOSFET technology, which dramatically reduces ringing in synchronous buck converter applications. This technology can eliminate the need for a snubber circuit in buck converter applications.

The driver IC incorporates advanced features such as SMOD for improved light-load efficiency and a Tri-State PWM input for compatibility with a wide range of PWM controllers. A 5-V gate drive, and an improved PCB interface optimized that maximizes low-side-FET exposed pad area, ensure higher performance. This product is compatible with the new Intel 6- × 6-mm DrMOS specification.

Unlike discrete solutions whose parasitic elements — combined with board layout — significantly reduce system efficiency, the FDMF6704 module can thermally and electrically minimize parasitic effects and improve overall system efficiency. In operation, the high-side MOSFET is optimized for fast switching while the low-side device is optimized for low RDS(on).

This arrangement ideally accommodates the low-duty-cycle switching requirements needed to convert the 12-V bus to supply the processor core with 1.0 V to 1.2 V at up to 30 A. Fairchild's MLP 8×8 power package extends the concept of enhanced packaging for dc-dc converter applications.

Like Linear's LTM4616 regulators, Enpirion's 6A EN5364 and the 9A EN5394 (Fig. 5) are also multi-component modules that integrate an inductor within their package. They are both voltage-mode synchronous buck dc-dc converters. These low-power switch-mode converters are complete power-system ICs that achieve up to 93% efficiency and integrate the inductor, power switches, gate drive, controller, and loop compensation in tiny 8- × 11- × 1.85-mm packages. The 9A EN5394 has a power density of 100 W per square inch (15 W per square centimeter).

INTEGRATING AN INDUCTOR

These parts meet the precise voltage and fast-transient requirements of high-performance applications such as set-top boxes/HD DVRs, LAN/SAN adapter cards, audio/video equipment, optical networking, multi-function printers, test and measurement, embedded computing, storage, and servers.

Operating the EN5364 and EN5394 power devices requires very few external components, resulting in simplified board design and layout, as well as improved manufacturability and reliability. The two parts are pin compatible, offering a scalable power solution that enables the user to easily tune to their design as the power requirements evolve. A rich set of features, including phase-lock operation and phase delay, facilitates ease of use in systems that are sensitive to ripple and beat frequencies.

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