V
IN
I
OUT(MAX)
0.25
C
BULK
ƒ
sw
I
OUT(MAX)
ESR
MAX
(2)
(3)
TPS5430
SLVS632 – JANUARY 2006
APPLICATION INFORMATION (continued)
• 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 10.8 V to 19.8 V
Output voltage 5.0 V
Input ripple voltage 300 mV
Output ripple voltage 30 mV
Output current rating 3 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 TPS5430 is internally set to 500 kHz. It is not possible to adjust the switching
frequency.
Input Capacitors
The TPS5430 requires an input decoupling capacitor and, depending on the application, a bulk input capacitor.
The recommended value for the decoupling capacitor, C1, 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, so long as the input voltage
and current ripple ratings are not exceeded. The voltage rating must be greater than the maximum input voltage,
including ripple.
This input ripple voltage can be approximated by Equation 2 :
Where I
OUT(MAX)
is the maximum load current, f
SW
is the switching frequency, C
IN
is the input capacitor value and
ESR
MAX
is the maximum series resistance of the input capacitor.
The maximum RMS ripple current also needs to be checked. For worst case conditions, this can be
approximated by Equation 3 :
In this case the input ripple voltage would be 156 mV and the RMS ripple current would be 1.5 A. The maximum
voltage across the input capacitors would be VIN max plus delta VIN/2. The chosen input decoupling capacitor is
rated for 25 V and the ripple current capacity is greater than 3 A, providing ample margin. It is very important that
the maximum ratings for voltage and current are not exceeded under any circumstance.
Additionally some bulk capacitance may be needed, especially if the TPS5430 circuit is not located within about
2 inches from the input voltage source. The value for this capacitor is not critical but it also should be rated to
handle the maximum input voltage including ripple voltage and should filter the output so that input ripple voltage
is acceptable.
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