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LT1587-1.5

Fixed 1.5V, 3A Low Dropout Fast Response GTL+ Regulator

Linear Technology 

LT1587-1.5

APPLICATIONS INFORMATION

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.

ESR

EFFECTS

ESL

EFFECTS

CAPACITANCE

EFFECTS

SLOPE,

V

t

=

∆I

C

POINT AT WHICH REGULATOR

TAKES CONTROL

Figure 1

LT1587 • F01

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 LT1587-1.5

essentially has a flat current limit over the range of input

supply voltage. The lower current limit rating and 7V

maximum supply voltage rating for this device permits this

characteristic. Current limit oscillations are typically non-

existent unless the input and output decoupling capacitors

for the regulators are mounted several inches from the

terminals.

Protection Diodes

In normal operation, the LT1587-1.5 does 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 LT1587-1.5 can handle microsec-

ond 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 dam-

age occur. A crowbar circuit at the input of the LT1587-1.5

can generate those levels of current, and a diode from

output to input is then recommended. 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.

D1

1N4002

(OPTIONAL)

LT1587-1.5

VIN

+ C1

IN

OUT

GND

10µF

+

VOUT

C2

10µF

LT1587 • F02

Figure 2

Ripple Rejection

The typical curve for ripple rejection reflects values for the

LT1587-1.5 as a function of frequency. In applications that

require improved ripple rejection, use the adjustable

LT1587. A bypass capacitor from the adjust pin to ground

reduces the output ripple by the ratio of VOUT/1.25V.

Load Regulation

It is not possible to provide true remote load sensing

because the LT1587-1.5 is a 3-terminal device. Load

5

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