LP2985IM5-3.3 National Semiconductor, LP2985IM5-3.3 Datasheet - Page 13

• Manufacturer Part Number: LP2985IM5-3.3
• Description: Voltage Regulator IC
• Manufacturer: National Semiconductor

Specifications of LP2985IM5-3.3

• No. Of Pins: 5
• Output Current: 0.15A
• Mounting Type: Surface Mount
• Voltage Regulator Type: LDO Linear
• Peak Reflow Compatible (260 C): No
• Output Voltage: 3.3V
• Current Rating: 150mA
• Output Voltage Max: 3.3V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Hints
Important: The output capacitor must maintain its ESR
within the stable region over the full operating temperature
range of the application to assure stability.
The LP2985 requires a minimum of 2.2 µF on the output
(output capacitor size can be increased without limit).
It is important to remember that capacitor tolerance and
variation with temperature must be taken into consideration
when selecting an output capacitor so that the minimum re-
quired amount of output capacitance is provided over the full
operating temperature range. It should be noted that ceramic
capacitors can exhibit large changes in capacitance with
temperature (see next section, Capacitor Characteristics ).
The output capacitor must be located not more than 1 cm
from the output pin and returned to a clean analog ground.
Noise Bypass Capacitor:
Connecting a 10 nF capacitor to the Bypass pin significantly
reduces noise on the regulator output. It should be noted that
the capacitor is connected directly to a high-impedance cir-
cuit in the bandgap reference.
Because this circuit has only a few microamperes flowing in
it, any significant loading on this node will cause a change in
the regulated output voltage. For this reason, DC leakage
current through the noise bypass capacitor must never ex-
ceed 100 nA, and should be kept as low as possible for best
output voltage accuracy.
The types of capacitors best suited for the noise bypass ca-
pacitor are ceramic and film. High-quality ceramic capacitors
with either NPO or COG dielectric typically have very low
leakage. 10 nF polypropolene and polycarbonate film ca-
pacitors are available in small surface-mount packages and
typically have extremely low leakage current.
CAPACITOR CHARACTERISTICS
The LP2985 was designed to work with ceramic capacitors
on the output to take advantage of the benefits they offer: for
capacitance values in the 2.2 µF to 4.7 µF range, ceramics
are the least expensive and also have the lowest ESR val-
ues (which makes them best for eliminating high-frequency
noise). The ESR of a typical 2.2 µF ceramic capacitor is in
the range of 10 m
limits required for stability by the LP2985.
One disadvantage of ceramic capacitors is that their capaci-
tance can vary with temperature. Most large value ceramic
capacitors ( 2.2 µF) are manufactured with the Z5U or Y5V
temperature characteristic, which results in the capacitance
dropping by more than 50% as the temperature goes from
25˚C to 85˚C.
This could cause problems if a 2.2 µF capacitor were used
on the output since it will drop down to approximately 1 µF at
high ambient temperatures (which could cause the LM2985
to oscillate). If Z5U or Y5V capacitors are used on the output,
a minimum capacitance value of 4.7 µF must be observed.
A better choice for temperature coefficient in ceramic capaci-
tors is X7R, which holds the capacitance within
fortunately, the larger values of capacitance are not offered
by all manufacturers in the X7R dielectric.
Tantalum:
Tantalum capacitors are less desirable than ceramics for use
as output capacitors because they are more expensive when
comparing equivalent capacitance and voltage ratings in the
1 µF to 4.7 µF range.
to 20 m , which easily meets the ESR
(Continued)
±
15%. Un-
13
Another important consideration is that Tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a Tantalum ca-
pacitor with an ESR value within the stable range, it would
have to be larger in capacitance (which means bigger and
more costly) than a ceramic capacitor with the same ESR
value.
It should also be noted that the ESR of a typical Tantalum will
increase about 2:1 as the temperature goes from 25˚C down
to −40˚C, so some guard band must be allowed.
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
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
Electrical Characteristics section under V
mis-operation, the turn-on (and turn-off) voltage signals ap-
plied to the ON/OFF input must have a slew rate which is
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
listed under the electrical specification V
cal 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.
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
current to flow into V
damage the part.
In any application where the output may be pulled above the
input, an external Schottky diode must be connected from
V
verse voltage across the LP2985 to 0.3V (see Absolute
Maximum Ratings).
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 light-
ing used inside most buildings has very little effect on perfor-
IN
40 mV/µs.
to V
OUT
(cathode on V
IN
(and out the ground pin), which can
IN
, anode on V
IN
to keep the regulator
ON/OFF
OUT
ON/OFF
), to limit the re-
(see Electri-
. To prevent
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