f

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(

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2

2

1

12

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ç

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×

è

ø

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TPS54821

www.ti.com

SLVSB14

– OCTOBER 2011

lower switching frequencies. Higher switching frequencies may produce smaller a solution size using lower

valued inductors and smaller output capacitors compared to a power supply that switches at a lower frequency.

However, the higher switching frequency causes extra switching losses, which hurt the converter

’s efficiency and

thermal performance. In this design, a moderate switching frequency of 480 kHz is selected to achieve both a

small solution size and a high efficiency operation.

Output Inductor Selection

To calculate the value of the output inductor, use Equation 18. 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 0.1 to 0.3

for the majority of applications.

(18)

For this design example, use KIND = 0.3 and the minimum inductor value is calculated to be 2.31

µH. For this

design, a larger 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 8.015 A and the peak inductor current is 8.839 A. The chosen

inductor is a Vishay IHLP4040DZER3R3M1series 3.3

µH. It has a saturation current rating of 18.6 A (-20%

inductance) and a RMS current rating of 10 A (40

° C temperature rise).

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)

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© 2011, Texas Instruments Incorporated

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