LP3910 Datasheet, PDF (64/72 Pages) Texas Instruments – Power Management IC for Hard-Drive-Based Portable Media Players

LP3910
SNVS481M – NOVEMBER 2006 – REVISED DECEMBER 2015
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In particular, the output capacitor selection must take account of all the capacitor parameters, to ensure that the
specification is met within the application. The capacitance can vary with DC bias conditions as well as
temperature and frequency of operation. Capacitor values also show some decrease over time due to aging. The
capacitor parameters are also dependent on the particular case size, with smaller sizes giving poorer
performance figures in general. As an example, Figure 76 shows a typical graph comparing different capacitor
case sizes.
100%
0603, 10V, X5M
80%
60%
40%
0402, 6.3V, X5R
20%
0
1.0
2.0
3.0
4.0
5.0
DC BIAS (V)
Figure 76. Typical Variation in Capacitance vs DC Bias
As shown in Figure 76, increasing the DC Bias condition can result in the capacitance value that falls below the
minimum value given in the recommended capacitor specifications table. Note that Figure 76 shows the
capacitance out of spec for the 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 (for example, 0402) may not be suitable in the actual application.
Capacitance of a ceramic capacitor can vary with temperature. The capacitor type X7R, which operates over a
temperature range of −55°C to +125°C, only varies the capacitance to within ±15%. The capacitor type X5R has
a similar tolerance over a reduced temperature range of −55°C to +85°C. Many large value ceramic capacitors,
larger than 1 μF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance can drop by
more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over Z5U and
Y5V in applications where the ambient temperature changes significantly above or below 25°C.
Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more
expensive when comparing equivalent capacitance and voltage ratings in the 0.47-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. The ESR of a typical tantalum increases about 2:1 as the temperature goes
from +25°C down to −40°C, so some guard band must be allowed.
9.2.2.2.1.4 Noise Bypass Capacitors for VREFH Pin
Connecting respectively 100 nF and 1 nF grounded bypass capacitors to the V_REFH pin significantly reduces
noise on the LDO outputs. VREFH is a high-impedance node connected to a bandgap reference used for the
LDOs. Any significant loading on this node causes a change on the regulated output voltages. For this reason,
DC leakage current through these pins must be kept as low as possible for best output voltage accuracy. The
types of capacitors best suited for the noise bypass capacitors are ceramic and film capacitors. High-quality
ceramic capacitors with either NPI or COG dielectric typically have very low leakage. Polypropylene and
polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low
leakage current. Residual solder flux is another potential source of leakage, which mandates thorough cleaning
of the assembled PCBs.
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