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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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LT1585A 데이터시트(HTML) 4 Page - ON Semiconductor |
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LT1585A 데이터시트(HTML) 4 Page - ON Semiconductor |
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LT1585A 4 MOTOROLA ANALOG IC DEVICE DATA Figure 7. LT1585A Adjust Pin Current vs Temperature Figure 8. Ripple Rejection vs Frequency 10 100,000 0 TEMPERATURE ( °C) 10,000 10 20 30 40 50 60 80 70 90 100 1000 –40 –60 20 160 0 TEMPERATURE ( °C) 40 100 10 20 40 30 50 60 70 90 80 100 0 –20 60 80 140 120 OPERATING DESCRIPTION APPLICATIONS INFORMATION General The LT1585A 3–terminal adjustable positive voltage regulator is easy to use and has all the protection features expected in high performance linear regulators. The device is short–circuit protected, safe–area protected and provides thermal shutdown to turn off the regulator should the junction temperature exceed about 150 °C. The LT1585A voltage regulator requires an output capacitor for stability. However, the improved frequency compensation permits the use of capacitors with much lower ESR while still maintaining stability. This is critical in addressing the needs of modern, low voltage, high speed microprocessors. Current generation microprocessors cycle load current from almost zero to amps in tens of nanoseconds. Output voltage tolerances are tighter and include transient response as part of the specification. The LT1585A is specifically designed to meet the fast current load–step requirements of these microprocessors and save total cost by needing less output capacitance in order to maintain regulation. Stability The circuit design in the LT1585A requires the use of an output capacitor as part of the frequency compensation. For all operating conditions, the addition of a 22 µF solid tantalum or a 100 µF aluminium electrolytic on the output ensures stability. Normally, the LT1585A can use smaller value capacitors. Many different types of capacitors are available and have widely varying characteristics. These capacitors differ in capacitor tolerance (sometimes ranging up to ±100%), equivalent series resistance, equivalent series inductance and capacitance temperature coefficient. The LT1585A frequency compensation optimizes frequency response with low ESR capacitors. In general, use capacitors with an ESR of less than 1 Ω. On the LT1585A, bypassing the adjust pin improves ripple rejection and transient response. Bypassing the adjust pin increases the required output capacitor value. The value of 22 µF tantalum or 100µF aluminium covers all cases of bypassing the adjust terminal. With no adjust pin bypassing, smaller values of capacitors provide equally good results. Normally, capacitor values on the order of several hundred microfarads are used on the output of the regulators to ensure good transient response with heavy load current changes. Output capacitance can increase without limit and larger values of output capacitance further improve the stability and transient response of the LT1585A. Large load current changes are exactly the situation presented by modern microprocessors. The load current step contains higher order frequency components that the output decoupling network must handle until the regulator throttles to the load current level. Capacitors are not ideal elements and contain parasitic resistance and inductance. These parasitic elements dominate the change in output voltage at the beginning of a transient load step change. The ESR of the output capacitors produces an instantaneous step in output voltage ( ∆V = ∆I • ESR). The ESL of the output capacitors produces a droop proportional to the rate of change of output current (V = L • ∆I/∆t). The output capacitance produces a change in output voltage proportional to the time until the regulator can respond ( ∆V = ∆t • ∆l/C). These transient effects are illustrated in Figure 9. Figure 9. ESR Effects ESL Effects Capacitance Effects Slope, V t + DI C Point at which Regulator Takes Control |
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