DC-DC Converter IC Integrates On-Chip 6A MOSFET

Jan 1, 2002 12:00 PM
By Rich Nowakowski, Ram Anant, and David Daniels, Texas Instruments Inc., Dallas

With the core functionality of an entire dc-dc converter on-chip, a power IC addresses the needs of present and future lower voltage processors.

Processor core voltages are starting to tumble as process technology advances, and roadmaps project voltages as low as 0.9V. In addition, processors continue to draw more current as they execute more instructions per second. The bus voltage rail in many performance point-of-load (POL) applications is migrating from a 5V to a 3.3V bus and is expected to go lower in several years. Therefore, switching regulators must efficiently support these lower supply voltages.

Compounding the problem for power supply designers, processors in embedded and space-constrained applications (such as optical networking, telecommunication infrastructure, and notebook PCs) are allotted less real estate on a p. c. board. With the highly competitive nature of today's markets, design teams are under pressure to accelerate normal design cycles and bring products to market in several months, instead of a year or more. Many designers must cope with analog and digital circuitry, leaving less time for power management at the end of the design cycle. Issues such as voltage sequencing, transient response, and frequency synchronization suddenly become critical for system performance.

The TPS5461x family of synchronous buck dc-dc converters (Fig. 1) addresses the market needs of low-input voltage, low-output voltage, high current, and efficiency. With the input voltage range of 3V to 6V, eight different dc-dc converters are available with either fixed or adjustable output voltage versions that source and sink a continuous 6A output. Fixed output voltage versions of 3.3V, 2.5V, 1.8V, 1.5V, 1.2V, and 0.9V feature internal feedback compensation and allow for an easy, straightforward design with as few as six external components.

For greater flexibility in terms of transient response and size, you can use the externally compensated version (the TPS54610) for an adjustable voltage down to 0.891V. Its initial accuracy is 1% over the full operating input voltage, output voltage, and junction temperature ranges — making it ideal for powering high-performance processors. For tracking regulator applications, the TPS54672 is available in which both input terminals of the error amplifier are pinned out (Fig. 2, on page 36). Other than this one difference , the TPS54672 performs identically to the TPS54610 and is used in DDR memory and SSTL/GTL bus termination applications.

Fig. 3, on page 39, shows a plot of efficiency vs. output current. With the efficiency ratings up to 95% and thermally enhanced packaging, these devices can cope with the thermal issues inherent to high-density applications. In addition, a very small footprint (9.7 mm×6.4 mm) and lower parts count resulting from power and control stage integration means that an entire processor power supply can be implemented with as little as 1.33 in.×0.445 in. of board space. To improve time to market, the TPS5461x comes with a software development tool, which selects the external components based on the user-defined operating requirements of the power supply circuit.

By combining high-side and low-side N-channel lateral MOSFETs with an advanced linear BiCMOS process, the TPS5461x family achieves a high level of integration. Integrating MOSFET drivers on-chip enhances the efficiency of a synchronous buck topology. The integration greatly reduces the typical parasitics that power supply designers face when the MOSFETs are discrete from the rest of the supply's analog control and drive circuitry. Compressed-drain lateral MOSFETs allow easy integration of the power MOSFET with the controller and driver circuitry on the same die in a small area. With a 3V, 6A output, the R_DS(on) of the high-side and low-side MOSFETs are 36mΩ typically (26mΩ with 5V input), allowing lower conduction losses and resulting higher efficiencies.

High-Level Integration

The integration of the power MOSFETs, MOSFET drivers, and controls results in a simplified board design. The driver and analog control logic circuits are matched appropriately to enhance the performance of the integrated MOSFETs. When implementing discrete MOSFETs with a dc-dc controller, more attention must be paid to the board layout and parasitic issues, which results in a more complicated and time consuming design. The close proximity of the circuitry on the same die reduces parasitic inductances and capacitances. The TPS5461x achieves higher levels of efficiency than non-synchronous converters by adding a low R_DS(on) MOSFET switch instead of the diode rectifier on the lower side.

The TPS5461x further minimizes power losses by implementing adaptive dead time control. The adaptive dead time feature of the TPS5461x accurately controls turn on and turn off times of the synchronous MOSFETs to prevent shoot through currents, inherently resulting in better efficiency since both MOSFETs are never simultaneously on. Dead time is the short period when both the high-side and low-side MOSFETs in a synchronous topology are in the off state. The dead time prevents the input power source from experiencing a short and dissipating power. Excessive dead time results in reverse current flowing through the body diode of the lower MOSFET for a longer than necessary time, and affects overall converter efficiency.

Voltage Mode PWM Control

The TPS5461x features a high performance error amplifier with a typical open loop gain of 110 dB and a typical unity gain bandwidth of 5 MHz. The switching frequency of the TPS5461x is pin selectable to either 350 kHz or 550 kHz. The switching frequency can also be programmed from 280 kHz to 700 kHz by choosing an appropriately sized external resistor connected between the RT pin and ground. All of the internal control circuitry is designed for high frequency operation.

Not only does the versatility of an adjustable frequency range permit a specific switching frequency to be avoided in an application, it also gives the designer the option of designing a power supply for higher performance and compact size. The TPS54610 allows synchronization to an external frequency.

The TPS54611 through TPS54616 feature an internal feedback compensation loop which eliminates the need for selecting the feedback compensation component values. The gain of the error amplifier is limited internally, allowing a stable power supply to be quickly and easily designed, saving valuable development time. The externally compensated versions allow greater freedom in optimizing the design, supporting a wider range of output filter components. The TPS54672 design can be optimized for size and transient performance using conventional frequency compensation methods. Fig. 4 shows the transient response achievable with a switching frequency of 700 kHz. Similar results are obtainable with the TPS54610.

The TPS5461x also includes several features and capabilities that eliminate a designer's concerns when developing a power system for high-performance processor applications, including:

• Undervoltage lockout
• Overcurrent protection
• Power Good output
• Thermal shutdown
• Internal or external slow-start
• Output enable

The slow start/enable (SS/ENA) pin has two functions. First, the pin functions as an enable by keeping the device off until the voltage exceeds the start threshold voltage of approximately 0.9V. Second, the pin functions as an external slow start circuit. By placing a small value ceramic capacitor on the SS/ENA pin to ground, you can adjust the output voltage to ramp more slowly than the internal slow start rate. Sequencing of multiple TPS5461x devices is possible with three additional components.

The TPS5461x comes in a 28-pin thin scale small outline package (TSSOP), which includes the thermally enhanced PowerPad^™ feature. The PowerPad package exposes the lead frame at the base of the package and can help dissipate power from the device. The 28-pin TSSOP PowerPad package can dissipate up to 3.58W at 25°C ambient and 1.43W at 85°C ambient when the thermal pad is soldered to the p. c. board. PowerPad devices are simple to install and require no extraordinary assembly techniques beyond the normal surface mount flow soldering. Most digital p. c. boards utilize four layers or more with 1 oz. of copper for the bottom and top layers and 0.5 oz. for inner layers. In this case, mounting of the thermal pad directly to the ground plane or through thermal vias is adequate for proper thermal operation.

Power Supply Tools

The SWIFT^™ (switcher with integrated FET technology) Designer provides simplified step-by-step guidelines for designers who may not be seasoned power supply designers. Fig. 5 shows a typical input page screen shot. It supports internally and externally compensated versions. The software tool configures the power supply design specified by the designer and contains a customizable database of components such as the inductors, resistors, and capacitors necessary to complete the design. The output of the development tool is a complete bill of materials, schematic, efficiency graph, stress analysis, and loop compensation (gain and phase) plots of the power supply that meet the designer's requirements. The tool reconfigures the design after changing any of the design's parameters.

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