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TPS54521

SLVS981 – JUNE 2010

www.ti.com

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APPLICATION INFORMATION

Design Guide – Step-By-Step Design Procedure

This example details the design of a low cost, high frequency switching regulator using an aluminum

electrolytic output capacitor and Type III frequency compensation. 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:

Table 1.

Parameter

Value

Output Voltage

3.3 V

Output Current

5 A

Transient Response 5A load step

ΔVout = 3 %

Input Voltage

12 V nominal, 8 V to 17 V

Output Voltage Ripple

2% (66 mV p-p)

Start Input Voltage (Rising Vin)

6.806 V

Stop Input Voltage (Falling Vin)

4.824 V

Switching Frequency

480 kHz

Typical Application Schematic

The application schematic of Figure 34 was developed to meet the requirements above. The design procedure is

given in this section. For more information about Type II and Type III frequency compensation circuits, see

Designing Type III Compensation for Current Mode Step-Down Converters (SLVA352) and

Design Calculator (SLVC219).

Figure 34. Typical Application Circuit

Operating Frequency

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 a smaller 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 additional 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. This frequency is set using the resistor at the RT/CLK pin

(R3). Using Equation 4, the resistance required for a switching frequency of 480 kHz is 103 k

Ω. A 100 kΩ

resistor is used for this design.

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