LP3985

SNVS087AC – OCTOBER 2000 – REVISED MAY 2013

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ON/OFF INPUT OPERATION

FAST ON-TIME

The LP3985 output is turned on after Vref voltage reaches its final value (1.23V nominal). To speed up this

process, the noise reduction capacitor at the bypass pin is charged with an internal 70µA 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.

DSBGA MOUNTING

The DSBGA package requires specific mounting techniques which are detailed in Texas Instruments Application

Note AN-1112(SNOA401). 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-bump 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

DSBGA device.

DSBGA LIGHT SENSITIVITY

Exposing the DSBGA device to direct sunlight will cause mis-operation 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

lighting used inside most buildings has very little effect on performance. A DSBGA test board was brought to

within 1cm of a fluorescent desk lamp and the effect on the regulated output voltage was negligible, showing a

deviation of less than 0.1% from nominal.

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