f

-

=

×

×

×

Vinm ax

Vout

Vout

L1

Io Kind

Vinm ax

sw

f

-

=

×

×

Vinmax

Vout

Vout

Iripple

L1

Vinmax

sw

(

)

2

2

1

12

f

æ

ö

×

-

=

+

× ç

÷

ç

÷

×

×

è

ø

o

V

Vinmax Vo

ILrms

Io

Vinmax L1

sw

2

=

+

Iripple

ILpeak

Iout

2

f

× D

>

× D

Iout

Co

sw

Vout

1

1

8 f

>

×

×

Co

Voripple

sw

Iripple

TPS54620

SLVS949C

– MAY 2009 – REVISED MAY 2011

www.ti.com

(18)

For this design example, use KIND = 0.3 and the inductor value is calculated to be 3.08

µH. For this design, a

nearest standard value was chosen: 3.3

µH. For the output filter inductor, it is important that the RMS current

and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from

Equation 20 and Equation 21.

(19)

(20)

(21)

For this design, the RMS inductor current is 6.02 A and the peak inductor current is 6.84 A. The chosen inductor

is a Coilcraft MSS1048 series 3.3

µH. It has a saturation current rating of 7.38 A and a RMS current rating of

7.22 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.

Output Capacitor Selection

There are three primary considerations for selecting the value of the output capacitor. The output capacitor

determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in

load current. The output capacitance needs to be selected based on the more stringent of these three criteria

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

supply the load with current when the regulator can not. 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 a transition from

no load to 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)

Where

ΔIout is the change in output current, Fsw is the regulators switching frequency and ΔVout is the

allowable change in the output voltage. For this example, the transient load response is specified as a 5%

change in Vout for a load step of 1A. For this example,

ΔIout = 1.0 A and ΔVout = 0.05 x 3.3 = 0.165 V. Using

these numbers gives a minimum capacitance of 25

μF. This value does not take the ESR of the output capacitor

into account in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in

this calculation.

Equation 23 calculates the minimum output capacitance needed to meet the output voltage ripple specification.

Where fsw is the switching frequency, Vripple is the maximum allowable output voltage ripple, and Iripple is the

inductor ripple current. In this case, the maximum output voltage ripple is 33mV. Under this requirement,

Equation 23 yields 13.2

µF.

(23)

22

Copyright

© 2009–2011, Texas Instruments Incorporated