REF

OUT

REF

R5

V

R6 =

V

V

´

-

OUT

REF

R5

V

= V

+1

R6

é

ù

´ ê

ú

ë

û

(

)

OUT(MAX)

IN

OUT(MAX)

MAX

BULK

SW

I

0.25

V

=

+ I

ESR

C

f

´

D

´

´

2

OUT(MAX)

CIN

I

I

=

(

)

0.8

´

-

´

´

´

´

OUT(MAX)

IN(MAX)

OUT

MIN

IN(MAX)

IND

OUT

SW

V

V

V

L

=

V

K

I

F

TPS54332

www.ti.com

SLVS875B – JANUARY 2009 – REVISED FEBRUARY 2012

OUTPUT VOLTAGE SET POINT

The output voltage of the TPS54332 is externally adjustable using a resistor divider network. In the application

circuit of Figure 12, 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:

(4)

(5)

Choose R5 to be approximately 10 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 = 4.75 kΩ, resulting in a 2.5 V

output voltage.

INPUT CAPACITORS

The TPS54332 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 TPS54332 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 a single 10

μF capacitor is used for the input decoupling capacitor. It is

X5R dielectric rated for 25 V. The equivalent series resistance (ESR) is approximately 3 m

Ω, and the current

rating is 3 A.

This input ripple voltage can be approximated by Equation 6

(6)

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

approximated by Equation 7

(7)

In this case, the input ripple voltage would be 98 mV and the RMS ripple current would be 1.75 A. It is also

important to note that the actual input voltage ripple will be greatly affected by parasitic 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

200 mV. The maximum voltage across the input capacitors would be VIN max plus

ΔVIN/2. The chosen bypass

capacitor is rated for 25 V and the ripple current capacity is greater than 3 A, providing ample margin. It is

important that the maximum ratings for voltage and current are not exceeded under any circumstance.

OUTPUT FILTER COMPONENTS

Two components need to be selected for the output filter, the output inductor L1 and the output capacitance.

Since the TPS54332 is an externally compensated device, a wide range of filter component types and values can

be supported.

Inductor Selection

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

(8)

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