AN69 describes considerations for linear regulators based on the LT1575 UltraFastTM Linear Regulator controller. The LT1575 drives an external MOSFET pass transistor as an overdriven source follower. This configuration results in an extremely high speed regulation loop which minimizes the need for output capacitors. Target applications are state of the art microprocessor core supplies which exhibit large, high speed load transients while having very tight supply voltage tolerance requirements. The application note shows numerous examples of circuit implementations and offers detailed discussions of design considerations.
The current generation of microprocessors places stringent demands on the power supply that powers the processor core. Allowable supply voltage variations are as low as ± 100 mV and transient currents are in the range of 5 A with 20 ns rise and fall times. These requirements demand very accurate, very high speed regulators. The linear solutions employed to date rely on monolithic regulators or PNP transistors driven by low cost control circuits. Because, under the best of circumstances, the response times of these circuits to transient loads are measured in microseconds, these solutions depend on the presence of several hundred microfarads of low ESR decoupling capacitance surrounding the CPU. The cost of these capacitors is a significant percentage of the total power supply cost.
The LT® 1575 is an UltraFast™ linear controller that drives a low cost, N-channel, power MOSFET pass transistor and does so with sufficient bandwidth that the bulk decoupling can be completely eliminated. As an example, a 200 MHz Pentium® processor can operate with only the twenty-four 1 µF ceramic capacitors that Intel already requires for the microprocessor. The design uses no tantalum or aluminum electrolytic capacitors on the output. By selecting a sufficiently low on-resistance FET, the dropout voltage of the regulator can be made arbitrarily low. In general, the best performance is obtained with the highest on-resistance (hence, lowest cost) FET that will meet the dropout requirement. Overall system cost and parts count are significantly reduced compared to competing solutions. Reference accuracy is specified as ±0.6% at room temperature and ±1% over the full operating temperature range. Line regulation is typically unmeasurable and load regulation is approximately 1 mV for a 5 A load change. The LT1575 permits the addition of no-cost current limiting, provides on/off control and has the ability to incorporate thermal limiting for the cost of an external NTC thermistor. The part is available with adjustable outputs or fixed output voltages, in singles and duals (LT1577).
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