Vinmax

Vout

Vout

L1

Iout Kind

Vinmax

sw

-

=

×

×

× f

f

-

=

×

×

Vinmax

Vout

Vout

Iripple

L1

Vinmax

sw

(

)

2

2

1

12

Vout

Vinmax

Vout

ILrms

Iout

Vinmax L1

sw

æ

ö

×

-

=

+

×ç

÷

ç

÷

×

×

è

ø

f

2

=

+

Iripple

ILpeak

Iout

2

f

× D

>

× D

Iout

Co

sw

Vout

TPS54521

www.ti.com

SLVS981 – JUNE 2010

Output Inductor Selection

To calculate the value of the output inductor Equation 18 is used. Kind is a coefficient that represents the amount

of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the

output capacitor. Therefore, choosing high inductor ripple currents impact the selection of the output capacitor

since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In

general, the inductor ripple value is at the discretion of the designer; however, Kind is normally from 0.3 to 0.4 for

the majority of low cost applications.

(18)

For this design example, using Kind = 0.35 the inductor value is calculated to be 3.2 µH. A low cost 3.3 µH

inductor from Coilcraft’s DR0608 series was chosen. For the output filter inductor, it is important that the RMS

current and saturation current ratings not be exceeded. The inductor ripple current, RMS current, and peak

inductor current can be found from Equation 19, Equation 20, and Equation 21.

(19)

(20)

(21)

For this design, the inductor ripple current is 1.68 A, the RMS inductor current is 5.02 A, and the peak inductor

current is 5.84 A. The chosen inductor has a RMS current rating of 7.5 A. Based on inductance vs. current data

from Coilcraft, this inductor has a saturation current greater than 6 A.

The current flowing through the inductor is the inductor ripple current plus the output current. During power up,

faults, or transient load conditions, the inductor current can increase above the calculated peak inductor current

level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of

the device. For this reason, the most conservative approach is to specify an inductor with a saturation current

rating equal to or greater than the switch current limit rather than the peak inductor current. However, this

approach was not used due to the low cost nature of this design.

Output Capacitor Selection

There are two primary considerations for selecting the output capacitor values: the minimum capacitance

required to meet the transient response specification and the maximum impedance at the switching frequency to

meet the output voltage ripple requirement. Any output capacitor type (ceramic, tantalum, polymer, electrolytic,

etc.) can be used with the TPS54521 to meet the design specifications. Considering low cost design, an

aluminum electrolytic output capacitor is used with a low value ceramic capacitor in parallel. The electrolytic

capacitor provides the bulk capacitance needed to react to a load step, while the ceramic capacitor absorbs the

majority of the current ripple in order to achieve low output voltage ripple.

The desired response to a large change in the load current is the first criterion. The output capacitor needs to

supply the load with current when the regulator cannot. This situation would occur if there are desired hold-up

times for the regulator where the output capacitor must hold the output voltage above a certain level for a

specified amount of time after the input power is removed. The regulator is also temporarily not able to supply

sufficient output current if there is a large, fast increase in the current needs of the load such as transitioning

from no load to a full load. The regulator usually needs two or more clock cycles for the control loop to see the

change in load current and output voltage and adjust the duty cycle to react to the change. The output capacitor

must be sized to supply the extra current to the load until the control loop responds to the load change. The

output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing

a tolerable amount of droop in the output voltage. Equation 22 shows the minimum output capacitance necessary

to accomplish this.

(22)

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