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ELECTRONIC DESIGN

October 1, 1998
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Tech Insights
Exploring power-management design issues for advanced microprocessors

Ripple Regulator Takes Off On Merced's Command

Hysteretic Controller Enables Speedy And Efficient Programmable Voltage-Regulator Modules For The 64-Bit Processor.

Ashok Bindra

As microprocessor design rapidly jumps from 32- to 64-bit architectures, stringent design requirements follow right on its tail. Issues such as the load transient response and the amount of power a processor can dissipate have led to a demand for complex, low-voltage power controllers that push the performance envelope to new heights.

Intel's upcoming 64-bit Merced processor, for instance, requires about 30 A of operating current at lower voltages. This is in stark contrast to the 18 to 20 A needed by current generation 32-bit Pentium IIs and comparable processors. In addition, the power requirements for this type of processor shift rapidly with changes in load. The voltage regulator module (VRM) or dc-dc converter powering such a processor must be able to respond to these demands, while furnishing the multiple voltages needed by the processor's CPU core and I/Os. Needless to say, the new generation powercontrollers are expected to consume very little power. They've raised the efficiency bar, but can all these features come at a low cost?
Powering 64-Bit Processors
Key semiconductor manufacturers of power-supply controllers have begun addressing the strict voltage and current requirements of the high performance 64-bit Merced procesor. Texas Instruments Inc. leads the pack with an early announcement of working silicon that powers the Merced and any other foreseeable processor. TI exploited its 1.0-µm LinBiCMOS process to create the TPS5210. A programmable, synchronous-buck-regulator controller, the TPS5210 meets--and even beats--the Merced's power demands.

To comply with Intel's VRM specifications for the 64-bit processor, which is expected to be released sometime next year, the TPS5210 is in production in 28-pin SOICs, as well as in the thermally enhanced optional TSSOP packages Evaluation boards and application notes for the latest controller also are available. Because the new part is highly integrated, TI estimates that the TPS5210-based dc-dc converter solution will cost less than $9.00 (Fig. 1).

1. This typical design schematic uses the TI TPS5210 controller for a 2-V output at 19 A. The supply voltage for this circuit is 12 V. For such a dc-dc converter, TI estimates the total bill for materials can be under $9.00.

The goal was to accomplish faster load-current transient response, as well as high conversion efficiency at both lower voltages and power dissipation. TI engineers adopted a synchronous buck ripple regulator topology with a built-in hysteretic controller as opposed to standard voltage or current-mode operation. This clever topology combines the high efficiency of pulse-width regulation with the dynamic regulation capability of series or shunt dissipative regulators. In essence, the hysteretic power controller uses the output ripple as an error to control the output regulation. According to Dale Skelton, system design section manager for TI's power management products, "This creates fast transient response by allowing the switching frequency to vary in proportion to load transients."

For this type of converter, typical operating frequency ranges from 120 to 250 kHz. "The input and output voltage values, the output filter components, and the the hysteresis window set the operating frequency," explains Skelton. The device provides a user-selectable hysteresis window, which is set by two external resistors around the reference voltage (Fig. 2). The synchronous-buck-regulator controller compares the output voltage to the programmed window, turning the high-side MOSFET on and off to maintain the output voltage within the window. The maximum hysteresis setting is 60 mV. Propagation delay from the comparator inputs to the output drivers is less than 250 ns. As a result, according to Skelton, "the regulation is instantaneous."

2. The ripple regulator utilizes a hysteretic controller to accomplish ultra-fast load-current transient response, as well as over 90% efficiency over a wide output-current range.

"The user-selectable hysteretic controller and the droop compensation in the TPS5210 dramatically reduce the overshoot and undershoot caused by the load transients," notes Les Hodson, system engineering manager for TI's power management products. "Consequently," he adds, "despite high current transient rates, the controller can recover to the original output value in about 2 µs." Internal tests indicate that, at a 2.0-V output, the TPS5210 power controller can meet step load increases from 0.1 to 20.4 A in 1.0 µs (Fig. 3). With the droop compensation circuit, VOUT transient regulation does not fall outside of a 2% limit. The tests show that the transient peak-to-peak ripple is 111 mV for a 2.0-V output. Recovering to full value, however, takes about 2.2 µs

3. This load-transient waveform shows that the TPS5210 can respond to a load step of 0.1 to 20.4 A in less than 1 µs..

Higher Efficiency
According to TI, the synchronous-buck regulator controller design achieves 90% or better efficiency over a wide range of load currents. At a low load, however, the efficiency drops below 80%. The total power dissipation for the device is only 120 mW.
"Using the optimized LinBiCMOS process keeps the power dissipation low, while permitting higher functionality on-chip," states Hodson. "Plus," he adds, "the output accuracy is within ±1% over the full operating temperature range." The controller even maintains the specified accuracy at output voltages as low as 1.3 , according to TI. This accuracy is maintained over the full operating temperature range, says TI.

Other key features of this highly integrated device include active deadtime control, noise immunity circuitry, 2-A synchronous-buck drivers, an internal-drive regulator, and programmable soft-start capability. The integrated controller chip also has lossless current sensing, adjustable current limiting, overvoltage protection, VCC undervoltage lockout, an inhibit comparator, user adjustable droop compensation, 30 V rated bootstrapped high-side driver and a "power-good" output circuit (Fig. 4).

4. The higher on-chip functionality of TI's TPS5210 hysteretic controller can be achieved at low power dissipation using the semiconductor supplier's 1.0-µm LinBiCMOS process.

The deadtime control prevents shoot-through current from flowing through the main power FETs during switching transitions. To do this, it actively controls the turn-on times of the MOSFET drivers. "This period is less than 100 ns with the TPS5210," says Hodson. "Conventional designs offer twice as much. The hysteretic power controller provides improved noise immunity to make the unit less sensitive to layouts on pc boards," states Hodson. "With conventional converters, this aspect of dc-dc converter design is very critical, as it affects noise pickup and generation to degrade the end performance of the design." Nevertheless, TI recommends proper layout for the TPS5210 and provides guidelines for laying it out in a dc-dc converter design.
The on-chip, 2-A low- and high-side drivers are designed to drive low-on-resistance n-channel MOSFETs. Internally, they're connected to an 8-V gate-voltage regulator. The high-side driver can be configured either as a ground-referenced or floating bootstrap driver. Likewise, the output drivers incorporate overcurrent shutdown and crossover protection to eliminate destructive faults in the output FETs.
A softstart current source, made proportional to the reference voltage, minimizes variations in the softstart timing when changes are made to the output voltage. It controls the rate at which the output powers up. A built-in overvoltage circuit disables the output drivers if the output voltage rises above the 15% of the nominal value. The latch remains set until the Vcc goes below the undervoltage lockout value. A 3s deglitch timer is included for noise immunity. Likewise, the overcurrent protection circuit monitors the current through the high-side FET. As a result, it also protects the high-side FET from short circuits. The power-good circuit monitors for an undervoltage condition.When the output drops below 7% of the nominal output voltage, the power good pin is pulled low.
While the TTL-compatible inhibit pin enables the controller, it also can dominate power sequencing. To do this, it just locks out controller operation until the system logic supply exceeds the input threshold voltage of the inhibit circuit. Consequently, the inhibit and undervoltage lockout circuits ensure that the system supply voltages (12.0, 5.0, or 3.3 V) are within operating limits prior to controller operation.
Programmable Output
An internal, 5-bit digital-to-analog converter (DAC) supplies a programmable output voltage in accordance to Intel's 5-bit voltage-identification (VID) codes. The 5 VID pins are inputs to this network. Also, the TTL-compatible inputs are internally pulled upto 5 V. The programmable output-voltage range is 1.3 to 3.5 V. While the step increments between 1.3 and 2.1 V are 50 mV, the increments between 2.1 and 3.3 V are in steps of 100 mV. The output voltage is within ±1.0% of the nominal setting over the junction temperature range of 0 to +125 °C.
The input supply voltage range for the synchronous-buck-regulator controller is 11.4 to 13.0 V. The typical supply current for the controller unit is rated at 3 mA.
 
Price And Availability
The TPS5210 is available in 28-pin SOICs and TSSOPs from TI and its authorized distributors. In quantities of 10,000, the hysteretic power controller is priced at $2.25 each. Starting now, designers can also obtain a TPS5210 evaluation module.
Texas Instruments Inc., Semiconductor Group, SC98075, P.O. Box 172228, Denver, CO 80217; (800) 477-8924 ext. 4500; http://www.ti.com.

 


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