in the band gap reference circuit. Any significant loading on

this node will cause a change on the regulated output volt-

age. For this reason, DC leakage current through this pin

must be kept as low as possible for best output voltage

accuracy.

The types of capacitors best suited for the noise bypass

capacitor are ceramic and film. High-quality ceramic capaci-

tors with either NPO or COG dielectric typically have very

low leakage. Polypropolene and polycarbonate film capaci-

tors are available in small surface-mount packages and

typically have extremely low leakage current.

Unlike many other LDO’s, addition of a noise reduction

capacitor does not effect the load transient response of the

device.

ON/OFF INPUT OPERATION

The LP3985 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 LP3985 output is turned on after Vref voltage reaches its

final value (1.23V nomial). To speed up this process, the

noise reduction capacitor at the bypass pin is charged with

an internal 70uA current source. The current source is turned

off when the bandgap voltage reaches approximately 95% of

its final value. The turn on time is determined by the time

constant of the bypass capacitor. The smaller the capacitor

value, the shorter the turn on time, but less noise gets

reduced. As a result, turn on time and noise reduction need

to be taken into design consideration when choosing the

value of the bypass capacitor.

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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