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Part # LP2992IM5-1.8
Description MICROPOWER 250 MA LOW-NOISE ULTRA LOW-DR - Tape and Reel
Category IC
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Manufacturer Available Qty
National Semiconductor Corp
Date Code: 0506
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Technical Document

DISCLAIMER: The information provided herein is solely for informational purposes. Customers must be aware of the suitability of this product for their application, and consider that variable factors such as Manufacturer, Product Category, Date Codes, Pictures and Descriptions may differ from available inventory.

Typical Performance Characteristics (continued)
Unless otherwise specified: C
= 1µF, C
= 4.7µF, V
= V
(NOM) +1, T
= 25°C, ON/OFF pin is tied to V
Turn-On Time Turn-On Time
Figure 33. Figure 34.
Turn-On Time
Figure 35.
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Like any low-dropout regulator, the LP2992 requires external capacitors for regulator stability. These capacitors
must be correctly selected for good performance.
Input Capacitor
An input capacitor whose capacitance is 1 µF is required between the LP2992 input and ground (the amount of
capacitance may be increased without limit).
This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean
analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.
Important: Tantalum capacitors can suffer catastrophic failure due to surge current when connected to a low-
impedance source of power (like a battery or very large capacitor). If a Tantalum capacitor is used at the input, it
must be specified by the manufacturer to have a surge current rating sufficient for the application.
There are no requirements for ESR on the input capacitor, but tolerance and temperature coefficient must be
considered when selecting the capacitor to ensure the capacitance will be 1 µF over the entire operating
temperature range.
Output Capacitor:
The LP2992 is designed specifically to work with ceramic output capacitors, utilizing circuitry which allows the
regulator to be stable across the entire range of output current with an output capacitor whose ESR is as low as
5 mΩ. It may also be possible to use Tantalum or film capacitors at the output, but these are not as attractive for
reasons of size and cost (see next section, CAPACITOR CHARACTERISTICS).
The output capacitor must meet the requirement for minimum amount of capacitance and also have an ESR
(equivalent series resistance) value which is within the stable range. Curves are provided which show the stable
ESR range as a function of load current (see ESR graph below).
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 LP2992 requires a minimum of 4.7 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 required 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
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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 exceed 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 capacitor 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 capacitors are available in small surface-mount packages and typically have extremely low
leakage current.
The LP2992 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 10 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 4.7 µF
ceramic capacitor is in the range of 5 mΩ to 10 mΩ, which easily meets the ESR limits required for stability by
the LP2992.
One disadvantage of ceramic capacitors is that their capacitance 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 4.7 µF capacitor were used on the output since it will drop down to approximately
2.3 µF at high ambient temperatures (which could cause the LP2992 to oscillate). If Z5U or Y5V capacitors are
used on the output, a minimum capacitance value of 10 µF must be observed.
A better choice for temperature coefficient in ceramic capacitors is X7R, which holds the capacitance within
±15%. Unfortunately, the larger values of capacitance are not offered by all manufacturers in the X7R dielectric.
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 capacitor 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.
The LP2992 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
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 Electrical Characteristics section under
. 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.
The regulator output voltage can not be ensured 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
(see Electrical Characteristics).
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