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LT1584CT-3.3 Datasheet(PDF) 9 Page - Linear Technology |
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LT1584CT-3.3 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LT1584/LT1585/LT1587 158457a APPLICATIONS INFORMATION General The LT1584/LT1585/LT1587 family of three-terminal regulators is easy to use and has all the protection features expected in high performance linear regulators. The de- vices are short-circuit protected, safe-area protected, and provide thermal shutdown to turn off the regulators should the junction temperature exceed about 150 °C. The LT1584/LT1585/LT1587 family includes adjustable and fixed voltage versions. These ICs are pin compatible with the LT1083/LT1084/ LT1085 family of linear regulators but offer lower dropout voltage and faster transient response. The trade-off for this improved performance is a 7V maximum supply voltage. Similar to the LT1083/LT1084/LT1085 family, the LT1584/ LT1585/LT1587 regulators require 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 re- sponse as part of the specification. The LT1584/LT1585/ LT1587 family is specifically designed to meet the fast current load-step requirements of these microprocessors and saves total cost by needing less output capacitance in order to maintain regulation. Stability The circuit design in the LT1584/LT1585/LT1587 family 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 100µF aluminum electrolytic on the output ensures stability. Normally, the LT1584/LT1585/LT1587 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 LT1584/ LT1585/LT1587 frequency compensation optimizes fre- quency response with low ESR capacitors. In general, use capacitors with an ESR of less than 1 Ω. On the adjustable LT1584/LT1585/LT1587, bypassing the adjust terminal improves ripple rejection and transient response. Bypassing the adjust pin increases the required output capacitor value. The value of 22 µF tantalum or 100 µF aluminum 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 LT1584/LT1585/ LT1587 family. Large load current changes are exactly the situation pre- sented 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 instanta- neous 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 propor- tional to the time until the regulator can respond ( ∆V = ∆t × ∆I/C). These transient effects are illustrated in Figure 1. Figure 1 ESR EFFECTS LT1584 • F01 ESL EFFECTS CAPACITANCE EFFECTS POINT AT WHICH REGULATOR TAKES CONTROL SLOPE, = V t ∆I C |
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