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Design Procedure
DV
IN
=
()
I
OUT(MAX)
x0.25
+I
OUT(MAX) MAX
xESR
C
BULK SW
x ƒ
(2)
TPS5420-EP
SLVS717 – DECEMBER 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:
DESIGN PARAMETER
(1)
EXAMPLE VALUE
Input voltage range 10 V to 35 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 µ F. 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- µ F 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 :
Where:
I
OUT(MAX)
= Maximum load current
f
SW
= Switching frequency
C
I
= Input capacitor value
ESR
MAX
= Maximum series resistance of the input capacitor
11
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