TPS54240_15 Datasheet, PDF (31/56 Pages) Texas Instruments – TPS54240 3.5-V to 42-V Step-Down DC - DC Converter With Eco-Mode™

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8.2 Typical Application
8.2.1 TPS54240 3.3-V Output Design
TPS54240
SLVSAA6B – APRIL 2010 – REVISED NOVEMBER 2014
TPS54240DGQ
Figure 49. 3.3-V Output TPS54240 Design Example
8.2.1.1 Design Requirements
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 will start with the following known parameters:
DESIGN PARAMETERS
Output Voltage
Transient Response 0 to 1.5-A load step
Maximum Output Current
Input Voltage
Output Voltage Ripple
Start Input Voltage (rising VIN)
Stop Input Voltage (falling VIN)
VALUE
3.3 V
ΔVout = 3 %
2.5 A
12 V nom. 10.8 V to 13.2 V
1% of Vout
6.25 V
5.9 V
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Selecting the Switching Frequency
The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the
highest switching frequency possible since this will produce the smallest solution size. The high-switching
frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that
switches at a lower frequency. The switching frequency that can be selected is limited by the minimum on-time of
the internal power switch, the input voltage and the output voltage and the frequency shift limitation.
Equation 12 and Equation 13 must be used to find the maximum switching frequency for the regulator, choose
the lower value of the two equations. Switching frequencies higher than these values will result in pulse skipping
or the lack of overcurrent protection during a short circuit.
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