LP3986

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SNVS142U – AUGUST 2001 – REVISED FEBRUARY 2013

NOISE BYPASS CAPACITOR

Connecting a 0.01µF capacitor between the CBYPASS pin and ground significantly reduces noise on the regulator

output. This cap is connected directly to a high impedance node in the band gap reference circuit. Any significant

loading on this node will cause a change on the regulated output voltage. For this reason, DC leakage current

through this pin must be kept as low as possible for best output voltage accuracy. The use of this 0.01µF bypass

capacitor is strongly recommended to prevent overshoot on the output during start up.

The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic

capacitors with either NPO or COG dielectric typically have very low leakage. Polypropolene and polycarbonate

film capacitors are available in small surface-mount packages and typically have extremely low leakage current.

Unlike many other LDOs, addition of a noise reduction capacitor does not effect the transient response of the

device.

ON/OFF INPUT OPERATION

The LP3986 is turned off by pulling the VEN pin low, and turned on by pulling it high. If this feature is not used,

the VEN pin should be tied to VIN to keep the regulator output on at all times. To assure proper operation, the

signal source used to drive the VEN input must be able to swing above and below the specified turn-on/off voltage

thresholds listed in the Electrical Characteristics section under VIL and VIH.

FAST ON-TIME

The LP3986 outputs are 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 capccitor. 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 TI's AN-1112 application report

(SNVA009), in particular the section Surface Mount Assembly Considerations.

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 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

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.

Copyright © 2001–2013, Texas Instruments Incorporated

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Product Folder Links: LP3986