LP3985ITLX-3.2 National Semiconductor, LP3985ITLX-3.2 Datasheet - Page 14

• Manufacturer Part Number: LP3985ITLX-3.2
• Description: Micropower/ 150mA Low-Noise Ultra Low-Dropout CMOS Voltage Regulator
• Manufacturer: National Semiconductor

www.national.com
Application Hints
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
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
EN
input must be able to swing above and below
IN
to keep the regulator output on at all
(Continued)
IL
and V
EN
IH
.
pin low, and
EN
14
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.