LP3910SQ-AN Datasheet, PDF (55/60 Pages) National Semiconductor (TI) – Power Management IC for Hard Drive Based Portable Media Players

different capacitor case sizes in a Capacitance vs. DC Bias
plot.
Graph Showing a Typical Variation
in Capacitance vs. DC Bias
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As shown in the graph, increasing the DC Bias condition can
result in the capacitance value that falls below the minimum
value given in the recommended capacitor specifications ta-
ble. Note that the graph 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 (e.g. 0402) may not
be suitable in the actual application.
The ceramic capacitor’s capacitance can vary with tempera-
ture. The capacitor type X7R, which operates over a temper-
ature range of −55°C to +125°C, will only vary the capacitance
to within ±15%. The capacitor type X5R has a similar toler-
ance 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 tem-
perature varies from 25°C to 85°C. Therefore X7R is recom-
mended over Z5U and Y5V in applications where the ambient
temperature will change 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 capac-
itor 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.
Noise Bypass Capacitors for V_REFH Pin
Connecting respectively 100 nF and 1 nF grounded bypass
capacitors to the V_REFH pin significantly reduces noise on the
LDO outputs. V_REFH is a high impedance nodes connected to
a bandgap reference used for the LDOs. Any significant load-
ing on this node will cause 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 polycar-
bonate 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.
Input Capacitor Selection for BUCK1, BUCK2 and the
BUCK-BOOST
A ceramic input capacitor of 10 μF, 6.3V is sufficient for the
magnetic DC/DC converters. Place the input capacitor as
close as possible to the input of the device. A large value may
be used for improved input voltage filtering. The recommend-
ed capacitor types are X7R or X5R. Y5V type capacitors
should not be used. DC bias characteristics of ceramic ca-
pacitors must be considered when selecting case sizes like
0805 and 0603. The input filter capacitor supplies current to
the PFET switch of the DC/DC converter in the first half of
each cycle and reduces voltage ripple imposed on the input
power source. A ceramic capacitor’s low ESR (Equivalent
Series Resistance) provides the best noise filtering of the in-
put voltage spikes due to fast current transients. A capacitor
with sufficient ripple current rating should be selected. The
Input current ripple can be calculated as:
The worse case is when V_IN = 2V_OUT
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Output Capacitor Selection for BUCK1, BUCK2 and the
BUCK-BOOST
A 10 μF, 6.3V ceramic capacitor should be used on the output
of the BUCK1 and BUCK2 magnetic DC/DC converters. The
Buck-Boost needs a 22 μF capacitor. The output capacitor
needs to be mounted as close as possible to the output of the
device. A large value may be used for improved input voltage
filtering. The recommended capacitor types are X7R or X5R.
Y5V type capacitors should not be used. DC bias character-
istics of ceramic capacitors must be considered when select-
ing case sizes like 0805 and 0603. DC bias characteristics
vary from manufacturer to manufacturer and DC bias curves
should be requested from them and analyzed as part of the
capacitor selection process.
The output filter capacitor of the magnetic DC/DC converter
smooths out current flow from the inductor to the load, helps
maintain a steady output voltage during transient load
changes and reduces output voltage ripple. These capacitors
must be selected with sufficient capacitance and sufficiently
low ESD to perform these functions.
The output voltage ripple is caused by the charging and the
discharging of the output capacitor and also due to its ESR
and can be calculated as follows:
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