LP2985LV Datasheet, PDF (10/15 Pages) National Semiconductor (TI) – Micropower 150 mA Low-Noise Low-Dropout Regulator in SOT-23 and micro SMD packages for Applications

LP2985LV
150ĆmA LOWĆNOISE, LOWĆDROPOUT REGULATOR
WITH SHUTDOWN FOR OUTPUT VOLTAGES <2.3 V
SLVS552 − OCTOBER 2004
APPLICATION INFORMATION
output capacitor (C_OUT) (continued)
It is critical that both the minimum capacitance and ESR requirement be met over the entire operating
temperature range. Depending on the type of capacitors used, both these parameters can vary significantly with
temperature (see Capacitor Characteristics section).
noise bypass capacitor (C_BYPASS)
The LP2985LV allows for low noise performance with the use of a bypass capacitor that is connected to the
internal band-gap reference via the BYPASS pin. This high-impedance band-gap circuitry is biased in the
microampere range and, thus, cannot be loaded significantly, else its output − and correspondingly the output
of the regulator − will change. Thus, for best output accuracy, dc leakage current through C_BYPASS should be
minimized as much as possible and should never exceed 100 nA.
A 10-nF capacitor is recommended for C_BYPASS; ceramic and film capacitors are well suited for this purpose.
capacitor characteristics
ceramic
Ceramic capacitors are ideal choices for use on the output of the LP2985LV for several reasons. For
capacitances in the range of 2.2 µF to 4.7 µF, ceramic capacitors have the lowest cost and the lowest ESR,
making them choice candidates for filtering high-frequency noise. For instance, a typical 2.2-µF ceramic
capacitor has an ESR in the range of 10 mΩ to 20 mΩ and satisfies minimum ESR requirements of the regulator.
Ceramic capacitors have one glaring disadvantage that must be taken into account − a poor temperature
coefficient, where the capacitance can vary significantly with temperature. For instance, a large-value ceramic
capacitor (≥2.2 µF) can lose more than half of its capacitance as temperature rises from 25°C to 85°C. Thus,
a 2.2 µF at 25°C will drop well below the minimum C_OUT required for stability as ambient temperature rises. For
this reason, select an output capacitor that maintains the minimum 2.2 µF required for stability over the entire
operating temperature range. Note that there are some ceramic capacitors that can maintain a ±15%
capacitance tolerance over temperature.
tantalum
Tantalum capacitors can be used at the output of the LP2985LV, but there are significant disadvantages that
could prohibit their use.
• In the 1-µF to 4.7-µF range, tantalum capacitors are more expensive than ceramics of the equivalent
capacitance and voltage ratings.
• Tantalum capacitors have higher ESRs than their equivalent-sized ceramic counterparts. Thus, to meet
the ESR requirements, a higher-capacitance tantalum may be required, at the expense of larger size
and higher cost.
• The ESR of a tantalum capacitor increases as temperature drops, as much as double from 25°C to
−40°C. Thus, ESR margins must be maintained over the temperature range in order to prevent
regulator instability.
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