Optimal Transient Response for Processor Based Systems

Jan 1, 2011 12:00 PM
Chris Glaser, Applications Engineer, Texas Instruments

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Processor-based systems place strenuous voltage regulation requirements on their power supply. In many instances, this voltage regulation must be as tight as ±5 percent. This tolerance includes DC set point accuracy, accuracy over temperature and input voltage variations, and transient response.

For processors, a well-regulated power supply is critical for proper operation and reliability. If the voltage on the processor is too high, then the processor could be permanently damaged. Another possible consequence of a power supply voltage that is too high is excessive power consumption, which leads to elevated operating temperatures, reduced reliability, and possibly system thermal shutdown.

If the voltage from the power supply is too low, however, then the processor may go into a brown out or under voltage lockout (UVLO) condition and need to be reset, resulting in lost or corrupted data.

Even a processor voltage that is only temporarily too high or too low can cause problems with the end equipment's operation. Under worst-case conditions, variation in voltage due to load, line, temperature, or simply part-to-part variation and/or tolerance in the power supply IC can all sum together to exceed the processor's requirements. For any end equipment that uses a processor, it is critical to have a power supply that always regulates its output voltage within the processorís input voltage requirements.

As an example of a processor's power supply requirements, Fig. 1 shows the basic power requirements for TI's DaVinci family of processors using the TMS320DM643 as an example. The most difficult voltage to regulate is the Vcore supply, as it has the largest load variation (up to 1.48 A) and smallest allowed voltage deviation (±5% = ±60 mV).

DC ACCURACY

The first step in meeting such rigorous voltage requirements is superb DC accuracy. This is the accuracy of the output voltage in a steady state condition. As such, DC accuracy incorporates the steady state variation due to changes in line, load, temperature, and from part to part.

An IC that can provide the required output voltage and current for the processor is the TPS62060 from TI, a 1.6 A integrated switch buck converter that provides very good efficiency at a very reasonable cost and size. The datasheet for this converter specifies the accuracy of the output voltage feedback pin over temperature, input voltage, and from part to part at ± 1.5 % maximum. The accuracy across load is specified to be +/-0.5% / A. For the processor's full load current of 1.48 A, this equates to a +/-0.74 % (+/-0.5% / A * 1.48 A) load regulation accuracy. Total DC regulation for the step-down converter is ±1.5% (±18 mV) at no load, and +1.5% / +/-2.24% (+18 / +/-27 mV) at the full load of 1.48 A. The external feedback resistors also affect the output voltage accuracy. For the purposes of this article, 0.1% resistors are assumed to be used; therefore, their effect is negligible.

TPS62060 PERFORMANCE

The TPS62060 step down converter operates with typically 3MHz fixed frequency PWM at moderate to heavy load currents. At light load currents the converter can automatically enter Power Save Mode and then operates in Pulse Frequency Mode (PFM) mode. During PWM operation the converter uses a fast response voltage mode controller with input voltage feed-forward to achieve good line and load regulation.

At the beginning of each clock cycle the high side MOSFET switch turns on. Now, current flows from the input capacitor via the high side MOSFET switch through the inductor to the output capacitor and load. During this phase, the current ramps up until the PWM comparator trips and the control logic turns off the switch. The current limit comparator will also turn off the switch if the current limit of the high side MOSFET switch is exceeded. After a dead time (preventing shoot through current) the low side MOSFET rectifier turns on and the inductor current ramps down. Then, current flows from the inductor to the output capacitor and load. It returns back to the inductor through the low side MOSFET rectifier.

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