ZL2004 datasheet(20/42 Pages) RENESAS

20 / 42 page

ZL2004

FN6846 Rev. 3.00

Page 20 of 42

February 15, 2011

Once an inductor is selected, the DCR and core losses

in the inductor are calculated. Use the DCR specified

in the inductor manufacturer’s datasheet.

Lrms

LDCR

DCR





ILrms is given by

 

opp

OUT

Lrms





where IOUT is the maximum output current. Next,

calculate the core loss of the selected inductor. Since

this calculation is specific to each inductor and

manufacturer, refer to the chosen inductor datasheet.

Add the core loss and the ESR loss and compare the

total

loss

the

maximum

power

dissipation

recommendation in the inductor datasheet.

5.7.4 Output Capacitor Selection

Several trade-offs must also be considered when

selecting an output capacitor. Low ESR values are

needed to have a small output deviation during

transient load steps (Vosag) and low output voltage

ripple (Vorip). However, capacitors with low ESR, such

as semi-stable (X5R and X7R) dielectric ceramic

capacitors, also have relatively low capacitance values.

Many designs can use a combination of high

capacitance devices and low ESR devices in parallel.

For high ripple currents, a low capacitance value can

cause a significant amount of output voltage ripple.

Likewise, in high transient load steps, a relatively large

amount of capacitance is needed to minimize the

output voltage deviation while the inductor current

ramps up or down to the new steady state output

current value.

As a starting point, apportion one-half of the output

ripple voltage to the capacitor ESR and the other half

to capacitance, as shown in the following equations:

orip

opp

OUT





opp

orip

ESR





Use these values to make an initial capacitor selection,

using a single capacitor or several capacitors in

parallel.

After a capacitor has been selected, the resulting output

voltage ripple can be calculated using the following

equation:

OUT

opp

orip

ESR









Because each part of this equation was made to be less

than or equal to half of the allowed output ripple

voltage, the Vorip should be less than the desired

maximum output ripple.

5.7.5 Input Capacitor

It is highly

recommended that dedicated input

capacitors be used in any point-of-load design, even

when the supply is powered from a heavily filtered 5 or

12 V “bulk” supply from an off-line power supply.

This is because of the high RMS ripple current that is

drawn by the buck converter topology. This ripple

(ICINrms) can be determined from the following

equation:

)

(

OUT

CINrms







Without capacitive filtering near the power supply

circuit, this current would flow through the supply bus

and return planes, coupling noise into other system

circuitry. The input capacitors should be rated at 1.2X

the

ripple

current

calculated

above

avoid

overheating of the capacitors due to the high ripple

current, which can cause premature failure. Ceramic

capacitors with X7R or X5R dielectric with low ESR

and 1.1X the maximum expected input voltage are

recommended.

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