LP3913 Datasheet, PDF (59/70 Pages) Texas Instruments – Power Management IC for Flash Memory-Based Portable Media Players

LP3913
www.ti.com
SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013
The ceramic capacitor’s capacitance can vary with temperature. The capacitor type X7R, which operates over a
temperature range of −55°C to +125°C, will only vary 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 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 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.
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 loading 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
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.
Input Capacitor Selection for BUCK1, BUCK2 and BUCK3
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 recommended capacitor types are X7R or X5R. Y5V type capacitors should not be used. DC bias
characteristics of ceramic capacitors 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 input 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:
I_rms = I_outmax
V_in
V_in
r²
V_out ^{1 -} V_out ⁺ 12
(V_in - V_out) * V_out
where r =
L * f * I_outmax * V_in
The worse case is when V_IN = 2V_OUT
Output Capacitor Selection for BUCK1, BUCK2 and BUCK3
A 10 μF, 6.3V ceramic capacitor should be used on the output of the BUCK1 and BUCK2 magnetic DC/DC
converters. BUCK3 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 characteristics of ceramic
capacitors must be considered when selecting 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.
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