APPLICATION INFORMATION
Design Guide – Step-By-Step Design Procedure
Typical Application Schematic
Operating Frequency
Output Inductor Selection
TPS54620
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........................................................................................................................................................................................................ SLVS949 – MAY 2009
This example details the design of a high frequency switching regulator design using ceramic output capacitors.
A few parameters must be known in order to start the design process. These parameters are typically determined
at the system level. For this example, we start with the following known parameters:
Parameter Value
Output Voltage 3.3 V
Output Current 6 A
Transient Response 1A load step Δ Vout = 5 %
Input Voltage 12 V nominal, 8 V to 17 V
Output Voltage Ripple 33 mV p-p
Start Input Voltage (Rising Vin) 6.528 V
Stop Input Voltage (Falling Vin) 6.190 V
Switching Frequency 480 kHz
The application schematic of Figure 34 was developed to meet the requirements above. This circuit is available
as the TPS54620EVM-374 evaluation module. The design procedure is given in this section.
Figure 34. Typical Application Circuit
The first step is to decide on a switching frequency for the regulator. There is a trade off between higher and
lower switching frequencies. Higher switching frequencies may produce smaller a solution size using lower
valued inductors and smaller output capacitors compared to a power supply that switches at a lower frequency.
However, the higher switching frequency causes extra switching losses, which hurt the converter ’ s efficiency and
thermal performance. In this design, a moderate switching frequency of 480 kHz is selected to achieve both a
small solution size and a high efficiency operation.
To calculate the value of the output inductor, use Equation 17 . KIND is a coefficient that represents the amount
of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the
output capacitor. Therefore, choosing high inductor ripple currents impact the selection of the output capacitor
since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In
general, the inductor ripple value is at the discretion of the designer; however, KIND is normally from 0.1 to 0.3
for the majority of applications.
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