lp2985aim5-3.8 National Semiconductor Corporation, lp2985aim5-3.8 Datasheet - Page 15

• Manufacturer Part Number: lp2985aim5-3.8
• Description: Micropower 150 Ma Low-noise Ultra Low-dropout Regulator Designed For Use With Very Low Esr Output Capacitors
• Manufacturer: National Semiconductor Corporation

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 (out-
put capacitor size can be increased without limit).
It is important to remember that capacitor tolerance and vari-
ation with temperature must be taken into consideration when
selecting an output capacitor so that the minimum required
amount of output capacitance is provided over the full oper-
ating temperature range. It should be noted that ceramic
capacitors can exhibit large changes in capacitance with tem-
perature (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 circuit
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 capac-
itors are available in small surface-mount packages and typ-
ically 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 values
(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 mΩ to 20 mΩ, which easily meets the ESR 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 (
temperature characteristic, which results in the capacitance
dropping by more than 50% as the temperature goes from 25°
C to 85°C.
≥
2.2 µF) are manufactured with the Z5U or Y5V
10014007
15
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 os-
cillate). 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 capac-
itors is X7R, which holds the capacitance within ±15%. Un-
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.
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 Elec-
trical Characteristics section under V
operation, the turn-on (and turn-off) voltage signals applied 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 list-
ed under the electrical specification V
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 cur-
rent 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 Max-
imum 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
IN
40 mV/µs.
to V
OUT
(cathode on V
IN
(and out the ground pin), which can
IN
, anode on V
ON/OFF
IN
ON/OFF
to keep the regulator
OUT
. To prevent mis-
), to limit the re-
(see Electrical
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