OUTPUT VOLTAGE SET POINT

REF

OUT

REF

R5

V

R6 =

V

V

´

-

(4)

OUT

REF

R5

V

= V

+1

R6

é

ù

´ ê

ú

ë

û

(5)

INPUT CAPACITORS

(

)

OUT(MAX)

IN

OUT(MAX)

MAX

BULK

SW

I

0.25

V

=

+ I

ESR

C

f

´

D

´

´

(6)

2

OUT(MAX)

CIN

I

I

=

(7)

OUTPUT FILTER COMPONENTS

Inductor Selection

TPS54331

The output voltage of the TPS54331 is externally adjustable using a resistor divider network. In the application

circuit of Figure 13, this divider network is comprised of R5 and R6. The relationship of the output voltage to the

resistor divider is given by Equation 4 and Equation 5:

Choose R5 to be approximately 10.0 k

Ω. Slightly increasing or decreasing R5 can result in closer output voltage

matching when using standard value resistors. In this design, R4 = 10.2 k

Ω and R = 3.24 kΩ, resulting in a 3.31

V output voltage. The zero ohm resistor R4 is provided as a convenient place to break the control loop for

stability testing.

The TPS54331 requires an input decoupling capacitor and depending on the application, a bulk input capacitor.

The typical recommended value for the decoupling capacitor is 10

µF. A high-quality ceramic type X5R or X7R is

recommended. The voltage rating should be greater than the maximum input voltage. A smaller value may be

used as long as all other requirements are met; however 10

µF has been shown to work well in a wide variety of

circuits. Additionally, some bulk capacitance may be needed, especially if the TPS54331 circuit is not located

within about 2 inches from the input voltage source. The value for this capacitor is not critical but should be rated

to handle the maximum input voltage including ripple voltage, and should filter the output so that input ripple

voltage is acceptable. For this design two 4.7

µF capacitors are used for the input decoupling capacitor. They are

X7R dielectric rated for 50 V. The equivalent series resistance (ESR) is approximately 2m

Ω, and the current

rating is 3 A. Additionally, a small 0.01

µF capacitor is included for high frequency filtering.

This input ripple voltage can be approximated by Equation 6

The maximum RMS ripple current also needs to be checked. For worst case conditions, this can be

approximated by Equation 7

In this case, the input ripple voltage would be 143 mV and the RMS ripple current would be 1.5 A. It is also

important to note that the actual input voltage ripple will be greatly affected by parasitics associated with the

layout and the output impedance of the voltage source. The actual input voltage ripple for this circuit is shown in

Design Parameters and is larger than the calculated value. This measured value is still below the specified input

limit of 300 mV. The maximum voltage across the input capacitors would be VIN max plus

ΔVIN/2. The chosen

bulk and bypass capacitors are each rated for 50 V and the ripple current capacity is greater than 3 A, both

providing ample margin. It is very important that the maximum ratings for voltage and current are not exceeded

under any circumstance.

Two components need to be selected for the output filter, L1 and C2. Since the TPS54331 is an externally

compensated device, a wide range of filter component types and values can be supported.

To calculate the minimum value of the output inductor, use Equation 8

Copyright © 2008, Texas Instruments Incorporated

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