Datasheet LT1584, LT1585, LT1587 (Analog Devices) - 9

LT1584/LT1585/LT1587
U U W U APPLICATIONS INFORMATION General
LT1585/LT1587 frequency compensation optimizes fre- quency response with low ESR capacitors. In general, use The LT1584/LT1585/LT1587 family of three-terminal capacitors with an ESR of less than 1Ω. regulators is easy to use and has all the protection features expected in high performance linear regulators. The de- On the adjustable LT1584/LT1585/LT1587, bypassing the vices are short-circuit protected, safe-area protected, and adjust terminal improves ripple rejection and transient provide thermal shutdown to turn off the regulators response. Bypassing the adjust pin increases the required should the junction temperature exceed about 150°C. The output capacitor value. The value of 22µF tantalum or LT1584/LT1585/LT1587 family includes adjustable and 100µF aluminum covers all cases of bypassing the adjust fixed voltage versions. terminal. With no adjust pin bypassing, smaller values of capacitors provide equally good results. These ICs are pin compatible with the LT1083/LT1084/ LT1085 family of linear regulators but offer lower dropout Normally, capacitor values on the order of several hundred voltage and faster transient response. The trade-off for this microfarads are used on the output of the regulators to improved performance is a 7V maximum supply voltage. ensure good transient response with heavy load current Similar to the LT1083/LT1084/LT1085 family, the LT1584/ changes. Output capacitance can increase without limit LT1585/LT1587 regulators require an output capacitor for and larger values of output capacitance further improve the stability. However, the improved frequency compensation stability and transient response of the LT1584/LT1585/ permits the use of capacitors with much lower ESR while still LT1587 family. maintaining stability. This is critical in addressing the needs Large load current changes are exactly the situation pre- of modern, low voltage, high speed microprocessors. sented by modern microprocessors. The load current step Current generation microprocessors cycle load current contains higher order frequency components that the from almost zero to amps in tens of nanoseconds. Output output decoupling network must handle until the regulator voltage tolerances are tighter and include transient re- throttles to the load current level. Capacitors are not ideal sponse as part of the specification. The LT1584/LT1585/ elements and contain parasitic resistance and inductance. LT1587 family is specifically designed to meet the fast These parasitic elements dominate the change in output current load-step requirements of these microprocessors voltage at the beginning of a transient load step change. and saves total cost by needing less output capacitance in The ESR of the output capacitors produces an instanta- order to maintain regulation. neous step in output voltage (∆V = ∆I × ESR). The ESL of the output capacitors produces a droop proportional to the
Stability
rate of change of output current (V = L × ∆I/∆t). The output The circuit design in the LT1584/LT1585/LT1587 family capacitance produces a change in output voltage propor- requires the use of an output capacitor as part of the tional to the time until the regulator can respond (∆V = ∆t frequency compensation. For all operating conditions, the × ∆I/C). These transient effects are illustrated in Figure 1. addition of a 22µF solid tantalum or a 100µF aluminum electrolytic on the output ensures stability. Normally, the ESR LT1584/LT1585/LT1587 can use smaller value capacitors. EFFECTS Many different types of capacitors are available and have ESL CAPACITANCE EFFECTS widely varying characteristics. These capacitors differ in EFFECTS capacitor tolerance (sometimes ranging up to ±100%), LT1584 • F01 V ∆I SLOPE, = POINT AT WHICH REGULATOR equivalent series resistance, equivalent series inductance, t C TAKES CONTROL and capacitance temperature coefficient. The LT1584/
Figure 1
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