8

LT1584/LT1585/LT1587

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.

network is actually inside the microprocessor socket cav-

ity. In addition, use large power and ground plane areas to

minimize distribution drops.

A possible stability problem that occurs in monolithic linear

regulators is current limit oscillations. The LT1585/LT1587

essentially have a flat current limit over the range of input

supply voltage. The lower current limit rating and 7V

maximum supply voltage rating for these devices permit

this characteristic. Current limit oscillations are typically

nonexistent, unless the input and output decoupling ca-

pacitors for the regulators are mounted several inches

from the terminals. The LT1584 differs from the LT1585/

LT1587 and provides current limit foldback as input-to-

output differential voltage increases. This safe-area char-

acteristic exhibits a negative impedance because increas-

ing voltage causes output current to decrease. Negative

resistance during current limit is not unique to the LT1584

devices and is present on many power IC regulators. The

value of the negative resistance is a function of how fast the

current limit is folded back as input-to-output voltage

increases. This negative resistance can react with capaci-

tors and inductors on the input and output to cause

oscillation during current limit. Depending on the values of

series resistances, the overall system may end up unstable.

However, the oscillation causes no problem and the IC

remains protected. In general, if this problem occurs and is

unacceptable, increasing the amount of output capacitance

helps dampen the system.

Protection Diodes

In normal operation, the LT1584/LT1585/LT1587 family

does not require any protection diodes. Older three-termi-

nal regulators require protection diodes between the out-

put pin and the input pin or between the adjust pin and the

output pin to prevent die overstress.

On the adjustable LT1584/LT1585/LT1587, internal resis-

tors limit internal current paths on the adjust pin. Therefore,

even with bypass capacitors on the adjust pin, no protec-

tion diode is needed to ensure device safety under short-

circuit conditions.

A protection diode between the input and output pins is

usually not needed. An internal diode between the input and

output pins on the LT1584/LT1585/LT1587 family can

Figure 1

ESR

EFFECTS

LT1584 • F01

ESL

EFFECTS

CAPACITANCE

EFFECTS

POINT AT WHICH REGULATOR

TAKES CONTROL

SLOPE,

=

V

t

∆I

C

The use of capacitors with low ESR, low ESL, and good high

frequency characteristics is critical in meeting the output

voltage tolerances of these high speed microprocessors.

These requirements dictate a combination of high quality,

surface mount tantalum capacitors and ceramic capaci-

tors. The location of the decoupling network is critical to

transient response performance. Place the decoupling

network as close as possible to the processor pins because

trace runs from the decoupling capacitors to the processor

pins are inductive. The ideal location for the decoupling