LP6480-01

Apr.-2017

www.lowpowersemi.com

Page 8 of 10

Preliminary Datasheet

LP6480

Output Capacitor Selection

The function of output capacitance is to store energy to

attempt to maintain a constant voltage. The energy is stored

in the capacitor’s electric field due to the voltage applied. The

value of output capacitance is generally selected to limit

output voltage ripple to the level required by the specification.

Since the ripple current in the output inductor is usually

capacitor primarily determines the out-put voltage ripple. The

three elements of the capacitor that contribute to its

impedance (and output voltage ripple) are equivalent series

resistance (ESR), equivalent series inductance (ESL), and

capacitance (C). The output voltage droop due to a load

transient is dominated by the capacitance of the ceramic

output capacitor. During a step increase in load current, the

ceramic output capacitor alone supplies the load current until

the loop responds. Within three switching cycles, the loop

responds and the inductor current increases to match the load

current demand.

The relationship of the output voltage

droop during the three switching cycles to the output

capacitance can be estimated by:

In many practical designs, to get the required ESR, a

capacitor with much more capacitance than is needed must

be selected. For continuous or discontinuous inductor current

Ripple current flowing through a capacitor’s ESR causes

power dissipation in the capacitor. This power dissipation

causes a temperature increase internal to the capacitor.

Excessive temperature can seriously shorten the expected life

of a capacitor. Capacitors have ripple current ratings that are

dependent on ambient temperature and should not be

exceeded. The output capacitor ripple cur-rent is the inductor

Inductor Selection

For most designs, the LP6480 operates with inductor values

of 2.2μH to 10μH. Low inductance values are physically

smaller but require faster switching, which results in some

efficiency loss. The inductor value can be derived from the

following equation:

lower ripple current and small value inductors result in high

ripple currents. Choose inductor ripple current approximately

60% of the maximum load current 2A, or

Manufacturer’s specifications list both the inductor DC current

rating, which is a thermal limitation, and the peak current

rating, which is determined by the saturation characteristics.

The inductor should not show any appreciable saturation

under normal load conditions. Some inductors may meet the

peak and average current ratings yet result in excessive

losses due to a high DCR.

Always consider the losses associated with the DCR and its

effect on the total converter efficiency when selecting an

inductor. For optimum voltage-positioning load transients,

100mΩ range. For higher efficiency at heavy loads (above

200mA), or minimal load regulation (but some transient

overshoot), the resistance should be kept below 100mΩ.

The DC current rating of the inductor should be at least equal

to the maximum load current plus half the ripple current to

prevent core saturation (2A + 600mA).