LP3921 Datasheet, PDF (26/41 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

LP3921
SNVS580A – AUGUST 2008 – REVISED MAY 2013
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OUTPUT CAPACITOR
Correct selection of the output capacitor is critical to ensure stable operation in the intended application. The
output capacitor must meet all the requirements specified in the recommended capacitor table over all conditions
in the application. These conditions include DC-bias, frequency and temperature. Unstable operation will result if
the capacitance drops below the minimum specified value.
The LP3921 is designed specifically to work with very small ceramic output capacitors. The LDO's on the LP3921
are specifically designed to be used with X7R and X5R type capacitors. With these capacitors selection of the
capacitor for the application is dependant on the range of operating conditions and temperature range for that
application. (See CAPACITOR CHARACTERISTICS).
It is also recommended that the output capacitor be placed within 1 cm from the output pin and returned to a
clean ground line.
CAPACITOR CHARACTERISTICS
The LDO's on the LP3921 are designed to work with ceramic capacitors on the input and output to take
advantage of the benefits they offer. For capacitance values around 1 µF, ceramic capacitors give the circuit
designer the best design options in terms of low cost and minimal area.
Generally speaking, input and output capacitors require careful understanding of the capacitor specification to
ensure stable and correct device operation. Capacitance value can vary with DC bias conditions as well as
temperature and frequency of operation.
Capacitor values will also show some decrease over time due to aging. The capacitor parameters are also
dependant on the particular case size with smaller sizes giving poorer performance figures in general.
100%
0603, 10V, X5R
80%
60%
40%
0402, 6.3V, X5R
20%
0
1.0 2.0 3.0 4.0 5.0
DC BIAS (V)
Figure 9. DC Bias (V)
As an example, Figure 9 shows a typical graph showing a comparison of capacitor case sizes in a Capacitance
vs DC Bias plot. As shown in the graph, as a result of DC Bias condition the capacitance value may drop below
minimum capacitance value given in the recommended capacitor table (0.7 µF in this case). Note that the graph
shows the capacitance out of spec for 0402 case size capacitor at higher bias voltages. It is therefore
recommended that the capacitor manufacturers specifications for the nominal value capacitor are consulted for
all conditions as some capacitor sizes (e.g., 0402) may not be suitable in the actual application. Ceramic
capacitors have the lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of
a typical 1 µF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, and also meets the ESR requirements for
stability. The temperature performance of ceramic capacitors varies by type. Capacitor type X7R is specified with
a tolerance of ±15% over temperature range -55ºC to +125ºC. The X5R has similar tolerance over the reduced
temperature range -55ºC to +85ºC. Most large value ceramic capacitors (<2.2 µF) are manufactured with Z5U or
Y5V temperature characteristics, which results in the capacitance dropping by more than 50% as the
temperature goes from 25ºC to 85ºC. Therefore X7R is recommended over these other capacitor types in
applications where the temperature will change significantly above or below 25ºC.
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