LP2985 Datasheet, PDF (16/22 Pages) National Semiconductor (TI) – Micropower 150 mA Low-Noise Ultra Low-Dropout Regulator Designed for Use with Very Low ESR Output Capacitors

On/Off Input Operation
The LP2985 is shut off by driving the ON/OFF input low, and
turned on by pulling it high. If this feature is not to be used,
the ON/OFF input should be tied to V_IN to keep the regulator
output on at all times.
To assure proper operation, the signal source used to drive
the ON/OFF input must be able to swing above and below the
specified turn-on/turn-off voltage thresholds listed in the Elec-
trical Characteristics section under V_ON/OFF. To prevent mis-
operation, the turn-on (and turn-off) voltage signals applied to
the ON/OFF input must have a slew rate which is
≥ 40 mV/µs.
Caution: the regulator output voltage can not be guaranteed
if a slow-moving AC (or DC) signal is applied that is in the
range between the specified turn-on and turn-off voltages list-
ed under the electrical specification V_ON/OFF (see Electrical
Characteristics).
REVERSE INPUT-OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP2985 has an inherent diode connected between the regu-
lator output and input. During normal operation (where the
input voltage is higher than the output) this diode is reverse-
biased.
10014054
FIGURE 1. Reverse Current Path
However, if the output is pulled above the input, this diode will
turn ON and current will flow into the regulator output. In such
cases, a parasitic SCR can latch which will allow a high cur-
rent to flow into V_IN (and out the ground pin), which can
damage the part.
In any application where the output may be pulled above the
input, an external Schottky diode must be connected from
V_IN to V_OUT (cathode on V_IN, anode on V_OUT), to limit the re-
verse voltage across the LP2985 to 0.3V (see Absolute Max-
imum Ratings).
10014055
FIGURE 2. Reverse Current Protection
MICRO SMD MOUNTING
The micro SMD package requires specific mounting tech-
niques which are detailed in National Semiconductor Appli-
cation Note # 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 the 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 also affect electrical performance if brought near
to the device.
The wavelenghts which have the most detrimental effect are
reds and infra-reds, which means that the fluorescent lighting
used inside most buildings has very little effect on perfor-
mance. A micro SMD test board was brought to within 1 cm
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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