Datasheet LT1585A, LT1585A-3.3 (Analog Devices) - 6

LT1585A/LT1585A-3.3
U U W U APPLICATIONS INFORMATION
tor tolerance (sometimes ranging up to ±100%), equiva- ESR lent series resistance, equivalent series inductance and EFFECTS capacitance temperature coefficient. The LT1585A/ ESL LT1585A-3.3 frequency compensation optimizes fre- CAPACITANCE EFFECTS EFFECTS quency response with low ESR capacitors. In general, use LT1585A • F01 capacitors with an ESR of less than 1Ω. V SLOPE, = ∆I POINT AT WHICH REGULATOR t C TAKES CONTROL On the adjustable LT1585A, bypassing the adjust terminal
Figure 1
improves ripple rejection and transient response. Bypass- ing the adjust pin increases the required output capacitor cessors. These requirements dictate a combination of value. The value of 100µF tantalum or aluminum covers all high quality, surface mount tantalum capacitors and cases of bypassing the adjust terminal. With no adjust pin ceramic capacitors. The location of the decoupling net- bypassing, smaller values of capacitors provide equally work is critical to transient response performance. Place good results. the decoupling network as close as possible to the pro- Normally, capacitor values on the order of several hun- cessor pins because trace runs from the decoupling dred microfarads are used on the output of the regulators capacitors to the processor pins are inductive. The ideal to ensure good transient response with heavy load current location for the decoupling network is actually inside the changes. Output capacitance can increase without limit microprocessor socket cavity. In addition, use large power and larger values of output capacitance further improve and ground plane areas to minimize distribution drops. the stability and transient response of the LT1585A/ A possible stability problem that occurs in monolithic LT1585A-3.3. linear regulators is current limit oscillations. The LT1585A/ Large load current changes are exactly the situation LT1585A-3.3 essentially have a flat current limit over the presented by modern microprocessors. The load current range of input supply voltage. The lower current limit step contains higher order frequency components that rating and 7V maximum supply voltage rating for these the output decoupling network must handle until the devices permit this characteristic. Current limit oscilla- regulator throttles to the load current level. Capacitors are tions are typically nonexistent, unless the input and out- not ideal elements and contain parasitic resistance and put decoupling capacitors for the regulators are mounted inductance. These parasitic elements dominate the change several inches from the terminals. in output voltage at the beginning of a transient load step
Protection Diodes
change. The ESR of the output capacitors produces an instantaneous step in output voltage (∆V = ∆I • ESR). The In normal operation, the LT1585A/LT1585A-3.3 do not ESL of the output capacitors produces a droop propor- require any protection diodes. Older 3-terminal regulators tional to the rate of change of output current (V = L • require protection diodes between the output pin and the ∆I/∆t). The output capacitance produces a change in input pin or between the adjust pin and the output pin to output voltage proportional to the time until the regulator prevent die overstress. can respond (∆V = ∆t • ∆I/C). These transient effects are On the adjustable LT1585A, internal resistors limit inter- illustrated in Figure 1. nal current paths on the adjust pin. Therefore, even with The use of capacitors with low ESR, low ESL and good bypass capacitors on the adjust pin, no protection diode high frequency characteristics is critical in meeting the is needed to ensure device safety under short-circuit output voltage tolerances of these high speed micropro- conditions. 1585afa 6