O

O

S

O

2

I

C

ƒ

V

´ D

>

´ D

ripple

L peak

O

I

I

I

2

-

=

+

2

2

L RMS

O

ripple

1

I

I

I

12

-

=

+

(

)

O

I

O

ripple

I

o

S

V

V max V

I

V max

L

ƒ

´

-

=

´

´

24

TPS65321A-Q1

SLVSE55 – NOVEMBER 2017

www.ti.com

Product Folder Links: TPS65321A-Q1

Submit Documentation Feedback

Copyright © 2017, Texas Instruments Incorporated

design, select standard value which is 3.3 µH (see L1 in Figure 16). Use Equation 23 to calculate the inductor

(23)

(24)

(25)

For this design, the RMS inductor current is 3 A, the peak inductor current is 3.21 A, and the inductor ripple

current is 0.41 A. The selected inductor is a Coilcraft XAL4030-332ME, which has a saturation-current rating of

5.5 A and an RMS-current rating of 5 A. As the equation set demonstrates, lower ripple current reduces the

output ripple voltage of the buck regulator but requires a larger value of inductance. Selecting higher ripple

currents increases the output ripple voltage of the buck regulator but allows for a lower inductance value.

8.2.1.2.3

Output Capacitor Selection for the Buck Regulator

Consider three primary factors when selecting the value of the output capacitor. The output capacitor determines

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

current. Select the output capacitance based on the most stringent of these three criteria. The desired response

to a large change in the load current is the first criterion. The output capacitor must supply the load with current

when the regulator cannot. This situation occurs if the desired hold-up times are present for the buck regulator. In

this case, 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 unable to supply sufficient output current if a

large, fast increase occurs affecting the current requirements of the load, such as a transition from no load to full

load. The buck regulator usually requires two or more clock cycles for the control loop to notice the change in

load current and output voltage, and to adjust the duty cycle to react to the change. Size the output capacitor 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 two clock cycles while only allowing a tolerable

amount of droop in the output voltage. Use Equation 26 to calculate the minimum output capacitance required to

supply the difference in current.

where

•

•

•

(26)

minimum capacitance of 7.2 µF. This value does not consider the ESR of the output capacitor in the output

voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation. Aluminum

electrolytic and tantalum capacitors have higher ESR that must be take into consideration.

The catch diode of the regulator cannot sink current. Therefore any stored energy in the inductor produces an

output-voltage overshoot when the load current rapidly decreases. Also, size the output capacitor to absorb the

energy stored in the inductor when transitioning from a high load current to a lower load current. The excess

energy that is stored in the output capacitor increases the voltage on the capacitor. Size the capacitor to maintain

the desired output voltage during these transient periods.