Datasheet

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OUT REF
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R6 =
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TPS54233
www.ti.com SLUS859B –OCTOBER 2008– REVISED FEBRUARY 2011
STEP BY STEP DESIGN PROCEDURE
The following design procedure can be used to select component values for the TPS54233. Alternately, the
SwitcherPro™Software may be used to generate a complete design. The SwitcherPro™ 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 decided upon. The designer needs to know the following:
• Input voltage range
• Output voltage
• Input ripple voltage
• Output ripple voltage
• Output current rating
• Operating frequency
For this design example, use the following as the input parameters
Table 3. Design Parameters
DESIGN PARAMETER EXAMPLE VALUE
Input voltage range 8 V to 18 V
Output voltage 3.3 V
Input ripple voltage 300 mV
Output ripple voltage 100 mV
Output current rating 2 A
Operating Frequency 300 kHz
SWITCHING FREQUENCY
The switching frequency for the TPS54233 is fixed at 300 kHz.
OUTPUT VOLTAGE SET POINT
The output voltage of the TPS54233 is externally adjustable using a resistor divider network. In the application
circuit of Figure 12, this divider network is comprised of R5 and R6. The relationship of the output voltage to the
resistor divider is given by Equation 4 and Equation 5:
(4)
(5)
Choose R5 to be approximately 10 kΩ. Slightly increasing or decreasing R5 can result in closer output voltage
matching when using standard value resistors. In this design, R4 = 10.2 kΩ and R = 3.24 kΩ, resulting in a 3.31
V output voltage. The 0 Ω resistor R4 is provided as a convenient place to break the control loop for stability
testing.
INPUT CAPACITORS
The TPS54233 requires an input decoupling capacitor and depending on the application, a bulk input capacitor.
The typical recommended value for the decoupling capacitor is 10 μF. A high-quality ceramic type X5R or X7R is
recommended. The voltage rating should be greater than the maximum input voltage. A smaller value may be
used as long as all other requirements are met; however 10 μF has been shown to work well in a wide variety of
circuits. Additionally, some bulk capacitance may be needed, especially if the TPS54233 circuit is not located
within about 2 inches from the input voltage source. The value for this capacitor is not critical but should be rated
to handle the maximum input voltage including ripple voltage, and should filter the output so that input ripple
voltage is acceptable. For this design two 4.7 μF capacitors are used for the input decoupling capacitor. They are
X7R dielectric rated for 50 V. The equivalent series resistance (ESR) is approximately 2 mΩ, and the current
rating is 3 A. Additionally, a small 0.01 μF capacitor is included for high frequency filtering.
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Product Folder Link(s): TPS54233