Application Hints

External Capacitors

Like any low-dropout regulator, the LP3984 requires external

capacitors for regulator stability. The LP3984 is specifically

designed for portable applications requiring minimum board

space and smallest components. These capacitors must be

correctly selected for good performance.

Input Capacitor

An input capacitance of ) 1µF is required between the

LP3984 input pin and ground (the amount of the capacitance

may be increased without limit).

This capacitor must be located a distance of not more than

1cm from the input pin and returned to a clean analog

ground. Any good quality ceramic, tantalum, or film capacitor

may be used at the input.

Important: Tantalum capacitors can suffer catastrophic fail-

ures

due

to

surge

current

when

connected

to

a

low-impedance source of power (like a battery or a very

large capacitor). If a tantalum capacitor is used at the input,

it must be guaranteed by the manufacturer to have a surge

current rating sufficient for the application.

There are no requirements for the ESR on the input capaci-

tor, but tolerance and temperature coefficient must be con-

sidered when selecting the capacitor to ensure the capaci-

tance will be ) 1µF over the entire operating temperature

range.

Output Capacitor

The LP3984 is designed specifically to work with tantalum

output capacitors. A tantalum capacitor in 1 to 22 µF range

with 2

Ω to 10Ω ESR range is suitable in the LP3984 appli-

cation circuit.

It may also be possible to use film capacitors at the output,

but these are not as attractive for reasons of size and cost.

The output capacitor must meet the requirement for mini-

mum amount of capacitance and also have an ESR (Equiva-

lent Series Resistance) value which is within a stable range

(2

Ω to 10Ω).

No-Load Stability

The LP3984 will remain stable and in regulation with no

external load. This is specially important in CMOS RAM

keep-alive applications.

On/Off Input Operation

The LP3984 is turned off by pulling the V

turned on by pulling it high. If this feature is not used, the V

EN

pin should be tied to V

time. To assure proper operation, the signal source used to

drive the V

the specified turn-on/off voltage thresholds listed in the Elec-

trical Characteristics section under V

Fast On-Time

The LP3984 utilizes a speed up circuitry to ramp up the

internal V

output turn on time.

Micro SMD Mounting

The micro SMD package requires specific mounting tech-

niques which are detailed in National Semiconductor Appli-

cation Note (AN-1112). Referring to the section

Surface

Mount Technology (SMT) Assembly Considerations,it

should be noted that the pad style which must be used with

the 5 pin package is NSMD (non-solder mask defined) type.

For best results during assembly, alignment ordinals on the

PC board may be used to facilitate placement of the micro

SMD device.

Micro SMD Light Sensitivity

Exposing the micro SMD device to direct sunlight will cause

misoperation of the device. Light sources such as Halogen

lamps can effect electrical performance if brought near to the

device.

The wavelengths which have most detrimental effect are

reds and infra-reds, which means that the fluorescent light-

ing used inside most buildings has very little effect on per-

formance. A micro SMD test board was brought to within

1cm of a fluorescent desk lamp and the effect on the regu-

lated output voltage was negligible, showing a deviation of

less than 0.1% from nominal.

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