5
LT1585-1.5/LT1585A-1.5
APPLICATIONS INFORMATION
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stability and transient response of the LT1585-1.5/
LT1585A-1.5.
Large load current changes are exactly the situation pre-
sented by modern microprocessors and their peripheral
circuitry. 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 para-
sitic 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 instantaneous step in output volt-
age [∆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 proportional to the
time until the regulator can respond [∆V = ∆t(∆I/C)]. These
transient effects are illustrated in Figure 1.
range of input supply voltage. The lower current limit rating
and 7V maximum supply voltage rating for this device
permit this characteristic. Current limit oscillations are
typically nonexistent unless the input and output decou-
pling capacitors for the regulators are mounted several
inches from the terminals.
Protection Diodes
In normal operation, the LT1585-1.5/LT1585A-1.5 do not
require any protection diodes. Older 3-terminal regulators
require protection diodes between the output pin and the
input pin to prevent die overstress.
A protection diode between the input and output pins is
usually not needed. An internal diode between the input
and output pins on the LT1585-1.5/LT1585A-1.5 can
handle microsecond surge currents of 50A to 100A. Even
with large value output capacitors it is difficult to obtain
those values of surge currents in normal operation. Only
with large values of output capacitance, such as 1000µF to
5000µF, and with the input pin instantaneously shorted to
ground can damage occur. A crowbar circuit at the input of
the LT1585-1.5/LT1585A-1.5 can generate those levels of
current and a diode from output to input is then recom-
mended. This is shown in Figure 2. Usually, normal power
supply cycling or system “hot plugging and unplugging”
will not generate current large enough to do any damage.
Ripple Rejection
The typical curve for ripple rejection reflects values for the
LT1585-1.5/LT1585A-1.5 as a function of frequency. In
applications that require improved ripple rejection, use the
adjustable LT1585/LT1585A. A bypass capacitor from the
adjust pin to ground reduces the output ripple by the ratio
of V
OUT
/1.25V.
Figure 2
LT1585-1.5
D1
1N4002
(OPTIONAL)
GND
V
IN
V
OUT
IN OUT
C1
10µF
C2
10µF
1585-1.5 F02
+ +
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 microprocessor
applications. These requirements dictate a combination of
high quality surface mount tantalum capacitors and ce-
ramic capacitors. The location of the decoupling network is
critical to transient response performance. Place the
decoupling network as close as possible to the micropro-
cessor control circuitry because a trace run from the
decoupling capacitors to the actual circuitry is inductive. 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 LT1585-1.5/
LT1585A-1.5 essentially have a flat current limit over the
Figure 1
ESR
EFFECTS
1585-1.5 F01
ESL
EFFECTS
CAPACITANCE
EFFECTS
POINT AT WHICH REGULATOR
TAKES CONTROL
SLOPE, =
V
t
∆I
C
