LT1584/LT1585/LT1587
U
W U U
APPLICATIONS INFORMATION
100µF aluminum covers all cases of bypassing the adjust
terminal. With no adjust pin bypassing, smaller values of
capacitors provide equally good results.
network is actually inside the microprocessor socket cav-
ity. In addition, use large power and ground plane areas to
minimize distribution drops.
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
andlargervaluesofoutputcapacitancefurtherimprovethe
stability and transient response of the LT1584/LT1585/
LT1587 family.
Apossiblestabilityproblemthatoccursinmonolithiclinear
regulatorsiscurrentlimitoscillations.TheLT1585/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
valueofthenegativeresistanceisafunctionofhowfastthe
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
seriesresistances,theoverallsystemmayendupunstable.
However, the oscillation causes no problem and the IC
remainsprotected. Ingeneral, ifthisproblemoccursandis
unacceptable,increasingtheamountofoutputcapacitance
helps dampen the system.
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.
ESR
EFFECTS
ESL
CAPACITANCE
EFFECTS
EFFECTS
Protection Diodes
LT1584 • F01
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.
V
t
∆I
C
SLOPE,
=
POINT AT WHICH REGULATOR
TAKES CONTROL
Figure 1
TheuseofcapacitorswithlowESR,lowESL,andgoodhigh
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
networkascloseaspossibletotheprocessorpinsbecause
traceruns from the decouplingcapacitorsto theprocessor
pins are inductive. The ideal location for the decoupling
On the adjustable LT1584/LT1585/LT1587, internal resis-
torslimitinternalcurrentpathsontheadjustpin.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
usuallynotneeded.Aninternaldiodebetweentheinputand
output pins on the LT1584/LT1585/LT1587 family can
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Linear [ Linear ]
