© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

400mA SmartLDO

MOSFET

SC1532

January 3, 2000

7

TYPICAL CHARACTERISTICS (Cont.)

Transient Load Response

(1)

Trace 1: VO, offset 3.3V, 50mV/div.

Trace 2: I

NOTES:

(1) In Application Circuit on page 2.

APPLICATIONS INFORMATION

Introduction

The SC1532 is intended for applications such as power

managed PCI and network interface cards (NICs),

where operation from a 3.3V VAUX supply may be re-

quired when the 5V supply has been shut down. It pro-

vides a simple, low cost solution that uses very little

pcb real estate. During regular operation, 3.3V power

for the PCI card is provided by the SC1532’s on-board

low dropout regulator, generated from the 5V supply.

When the 5V supply is removed and 3.3V VAUX is

available, the SC1532 connects this supply directly to

its output using an internal NMOS pass device.

Component Selection

Output capacitors - Semtech recommends a 4.7µF or

greater ceramic capacitor at the output for the best

combination of performance and cost effectiveness.

Increasing the capacitance value improves transient

response and glitch performance. The SC1532 is very

tolerant of output capacitor value and ESR variations,

in fact any combination of capacitors with C

≥ 1µF and

ESR < 1

Ω is sufficient for stability. This target is easily

met using surface mount ceramic or tantalum capaci-

tors.

Input capacitors - Semtech recommends the use of a

4.7µF ceramic capacitor at both inputs. This allows for

the device being some distance from any bulk capaci-

tance on the rail. Additionally, input droop due to load

transients is reduced, improving load transient re-

sponse and aiding smooth supply transitions. Tanta-

lum capacitors should not be used.

Charge pump capacitor - Semtech recommends the

use of a 1nF ceramic capacitor between CP and GND.

Thermal Considerations

When operating from the 5V supply, the power dissipa-

tion in the SC1532 is approximately equal to the prod-

uct of the output current and the input to output voltage

differential:

The absolute worst-case dissipation is given by:

Note that the VIN2

VIN2 is not supplied.

()

O

D

I

VO

1

VIN

P

•

−

≈

()

)

MAX

(

2

Q

)

MAX

(

)

MAX

(

1

Q

)

MAX

(

)

MAX

(

O

)

MIN

(

)

MAX

(

)

MAX

(

D

I

2

VIN

I

1

VIN

I

VO

1

VIN

P

•

+

•

+

•

−

=