LP3905 Datasheet, PDF (15/24 Pages) National Semiconductor (TI) – Power Management Unit For Low Power Handheld Applications

LP3905
www.ti.com
SNVS374D – JUNE 2006 – REVISED MAY 2013
DC/DC CONVERTOR OUTPUT CAPACITOR SELECTION
Use a 10µF, 6.3V ceramic capacitor. Use X7R or X5R types, do not use Y5V. 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 as part of the capacitor
selection process.
The output filter capacitor 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 ESR to perform these functions.
The output voltage ripple is caused by the charging and discharging of the output capacitor and also due to its
R_ESR and can be calculated as:
Voltage peak-to-peak ripple due to capacitance can be expressed as follows:
I_RIPPLE
V_PP-C = _4*f*C
(7)
Voltage peak-to-peak ripple due to ESR can be expressed as follows:
V_PP-ESR = (2 * I_RIPPLE) * R_ESR
Because these two components are out of phase the rms value can be used to get an approximate value of
peak-to-peak ripple.
Voltage peak-to-peak ripple, root mean squared can be expressed as follows:
V_PP-RMS = V_PP-C2 + V_PP-ESR2
(8)
Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series
resistance of the output capacitor (R_ESR).
The R_ESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations
is at the switching frequency of the part.
LINEAR REGULATORS
Capacitor Selection
The LP3955 is designed to work with ceramic capacitors on the output to take advantage of the benefits they
offer: for capacitance values in the range of 0.47µF to 10µF range, ceramic capacitors are the smallest, least
expensive and have the lowest ESR values (which makes them best for eliminating high frequency noise). The
ESR of a typical 1µF ceramic capacitor is in the range of 20mW to 40mW, which easily meets the ESR
requirement for stability by the LP3955. For both input and output capacitors careful interpretation of the
capacitor specification is required to ensure correct device operation. The capacitor value can change greatly
dependant on the conditions of operation and capacitor type.
In particular the output capacitor selection should take account of all the capacitor parameters to ensure that the
specification is met within the application. 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. As an example Figure 11 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 the DC Bias
condition the capacitance value may drop below the minimum capacitance value given in the recommended
capacitor table (0.7µF in this case). 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.
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