ZL2005 datasheet(21/39 Pages) RENESAS

21 / 39 page

ZL2005

FN6848 Rev 2.00

Page 21 of 39

February 23, 2011

mance. A good starting point is to select the output

inductor ripple current (Iopp) equal to the expected load

transient step magnitude (Iostep):

Now the output inductance can be calculated using the

following equation:

where VINM is the maximum input voltage.

The average inductor current is equal to the maximum

output current. The peak inductor current (ILpk) is calcu-

lated using the following equation where IOUT is the

maximum output current:

Select an inductor rated for the average DC current with

a peak current rating above the peak current computed

above.

In over-current or short-circuit conditions, the inductor

may have currents greater than 2X the normal maximum

rated output current. It is desirable to use an inductor that

is not saturated at these conditions to protect the load

and the power supply MOSFETs from damaging cur-

rents.

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.

ILrms is given by:

where IOUT is the maximum output current. Next, calcu-

late the core loss of the selected inductor. Since this cal-

culation is specific to each inductor and manufacturer,

refer to the chosen inductor’s datasheet. Add the core

loss and the DCR loss and compare the total loss to the

maximum power dissipation recommendation in the

inductor datasheet.

Output Capacitor Selection

Several trade-offs also must be considered when select-

ing 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 combi-

nation 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 to

the new steady state output current value.

As a starting point, allocate one-half of the output volt-

age ripple to the capacitor ESR and the other half to its

capacitance, as shown in the following equations:

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:

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 out-

put ripple.

For more information on the performance of the power

supply in response to a transient load, refer to Applica-

tion Note AN11.

(3)

ostep

opp



(4)





opp

OUT

INM

OUT









(5)

opp

OUT





Lrms

LDCR

DCR





(6)

(7)

opp

OUT

Lrms





orip

opp

OUT





(8)

(9)

opp

orip

ESR





(10)

OUT

opp

orip

ESR









ZL2005 Datasheet(PDF) 21 Page - Renesas Technology Corp

ZL2005 Datasheet(HTML) 21 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Datasheet Download

Link URL