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Design Procedure

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MAX

xESR

SW

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(2)

TPS5420

SLVS642A – APRIL 2006 – REVISED AUGUST 2006

APPLICATION INFORMATION (continued)

The following design procedure can be used to select component values for the TPS5420. Alternately, the

SWIFT™ Designer Software may be used to generate a complete design. The SWIFT™ Designer Software

uses an iterative design procedure and accesses a comprehensive database of components when generating a

design. This section presents a simplified discussion of the design process.

To begin the design process, a few parameters must be determined. The designer must know the following:

•

Input voltage range

•

Output voltage

•

Input ripple voltage

•

Output ripple voltage

•

Output current rating

•

Operating frequency

Design Parameters

For this design example, use the following as the input parameters:

EXAMPLE VALUE

Input voltage range

10 V to 36 V

Output voltage

5 V

Input ripple voltage

300 mV

Output ripple voltage

30 mV

Output current rating

2 A

Operating frequency

500 kHz

(1)

As an additional constraint, the design is set up to be small size and low component height.

Switching Frequency

The switching frequency for the TPS5420 is internally set to 500 kHz. It is not possible to adjust the switching

frequency.

Input Capacitors

The TPS5420 requires an input decoupling capacitor and, depending on the application, a bulk input capacitor.

The recommended value for the decoupling capacitor is 10

µF. A high quality ceramic type X5R or X7R is

required. For some applications, a smaller value decoupling capacitor may be used, if the input voltage and

current ripple ratings are not exceeded. The voltage rating must be greater than the maximum input voltage,

including ripple. For this design, two 4.7

µF capacitors, C1 and C4 are used to allow for smaller 1812 case size

to be used while maintaining a 50 V rating.

This input ripple voltage can be approximated by Equation 2 :

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

Equation 3:

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