FAST_33_V_REGULATOR

Published:2009/6/23 3:45:00 Author:Jessie | From:SeekIC

New high-performance microprocessors require a fresh look at power-supply transient response.The LT1585 linear regulator features 1% initial accuracy, excellent temperature drift and load regulation, and virtually perfect line regulation. Complementing superb dc characteristics, the LT1585 exhibits extremely fast response to transients. Transient response is affected by more than the regulator itself. Stray inductances in the layout and bypass capacitors, as well as capacitor ESR dominate the response during the first 400 ns of transient.The ftgure shows a bypassing scheme developed to meet all the requirements for the Intel P44C-VR microprocessor. Input capacitors C1 and C2 function primarily to decouple load transients from the 5-V logic suppJy. The values used here are optimized for a typical 5-V desktop computer silver box powersupply input. C5 to C10 provide bulk capacitance at low ESR and ESL, and C11 to C20 keep the capacitance at low ESR and ESL low at high (>100 kHz) frequencies. C4 is a damper and it minimizes ringing during setting. Trace C is the load current step, which is essentially flat at 4 A with a 20-ns rise time.Trace A is the output settling response at 20 mV per division. Cursor trace B marks －46 mV relative to the initial output voltage. At the onset of load current, the microprocessor socket voltage dips to －38 ntV as a result of inductive effects in the board and capacitors, and the ESR of the capacitors. The inductive effects persist for approximately 400 ns. For the next 3 μs, the output droops as the load current drains the bypass capacitors. The trend then reverses as the LT1585 catches up with the load demand, and the output settles after approximately 50 μs. Running 4 A with a 1.7-V drop, the regulator dissipates 6.8 W.



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