LP2992_15 Datasheet, PDF (17/33 Pages) Texas Instruments – LP2992 Micropower 250-mA Low-Noise Ultralow-Dropout Regulator in SOT-23 and WSON Packages Designed for Use with Very Low-ESR Output Capacitors

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LP2992
SNVS171H – NOVEMBER 2001 – REVISED JANUARY 2015
The output capacitor must be located not more than 1 cm from the output pin and returned to a clean analog
ground.
8.2.2.1.3 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.
8.2.2.2 Capacitor Characteristics
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 because 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.
8.2.2.2.1 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 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.
8.2.2.3 Reverse Input-Output Voltage
The PNP power transistor used as the pass element in the LP2992 has an inherent diode connected between
the regulator 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_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 reverse voltage across the LP2992 to
0.3 V (see Absolute Maximum Ratings).
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