LP3905 datasheet(12/15 Pages) NSC

12 / 15 page

Application Information (Continued)

Buck DC/DC Convertor Input Capacitor Selection

A ceramic input capacitor of 10µF, 6.3V is sufficient for most

applications. Place the input capacitor as close as possible

to the V

IN pin of the device. A larger value may be used for

improved input voltage filtering. 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. The input filter capacitor supplies current to the PFET

switch of the LP3905 in the first half of each cycle and

reduces voltage ripple imposed on the input power source. A

ceramic capacitor’s low ESR provides the best noise filtering

of the input voltage spikes due to this rapidly changing

current. Select a capacitor with sufficient ripple current rat-

ing. The input current ripple can be calculated as:

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 capaci-

tors 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 smoothes 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 func-

tions.

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 ex-

pressed as follows

Voltage peak-to-peak ripple due to ESR can be expressed

as follows

PP-ESR =(2*IRIPPLE)*RESR

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 ex-

pressed as follows

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

1µFceramic 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 inter-

pretation of the capacitor specification is required to ensure

correct device operation. The capacitor value can change

greatly dependant on the conditions of operation and capaci-

tor type.

In particular the output capacitor selection should take ac-

count 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 pa-

rameters 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 mini-

mum 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 applica-

tion.

20152906

FIGURE 11. Capacitor Performance (DC Bias)

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