5

LT1585-1.5/LT1585A-1.5

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 1000

µF to

5000

µF, 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

Figure 2

LT1585-1.5

D1

1N4002

(OPTIONAL)

GND

IN

OUT

C1

10

µF

C2

10

µF

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