LP3220S-00

Jun.-2017

www.lowpowersemi.com

Page 8 of 10

Preliminary Datasheet

LP3220S

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 several switching cycles, the loop

responds and the inductor current increases to match the load

current demand.

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 to ∆VO:

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 rippled current ratings that are

dependent on ambient temperature and should not be

exceeded. The output capacitor ripple current is the inductor

current, IL, minus the output current, Io. The RMS value of the

ripple current flowing in the output capacitance (continuous

inductor current mode operation) is given by:

ESL can be a problem by causing ringing in the low

megahertz region but can be controlled by choosing low ESL

capacitors, limiting lead length (PCB and capacitor), and

replacing one large device with several smaller ones

connected in parallel. In conclusion, in order to meet the

requirement of out-put voltage ripple small and regulation loop

stability, ceramic capacitors with X5R or X7R dielectrics are

recommended due to their low ESR and high ripple current

ratings. The output ripple VOUT is determined by:

A 22μF ceramic capacitor can satisfy most applications.

Thermal Calculations

There are three types of losses associated with the LP3220S

step-down converter: switching losses, conduction losses,

and quiescent current losses. Conduction losses are

switching devices. Switching losses are dominated by the

gate charge of the power output switching devices.

At full load, assuming continuous conduction mode (CCM), a

simplified form of the losses is given by:

is used to estimate the full load step-down converter switching

losses.

For the condition where the step-down converter is in dropout

at 100% duty cycle, the total device dissipation reduces to:

with input voltage, the total losses should be investigated over

the complete input voltage range. Given the total losses, the

the SOT23-5 package which is 250℃/W.