TPS65320-Q1

SLVSAY9A – DECEMBER 2012 – REVISED APRIL 2013

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APPLICATION INFORMATION

Design Guide – Step By Step Design Procedure

This example details the design of a high-frequency switching regulator and linear regulator using ceramic output

capacitors. A few parameters must be known in order to start the design process. Determination of these

parameters is typically t the system level. This example starts with the following known parameters (buck

converter first):

Input voltage, VIN1

9 V to 16 V, typical 12 V

Output voltage, VREG1 (buck regulator)

5 V ± 2%

3 A

0.01 A

Transient response 0.01 A to 0.8 A

3%

Output ripple voltage

1%

2.2 MHz

Output voltage, VREG2 (LDO regulator)

3.3 V ± 2%

Overvoltage threshold

106% of output voltage

Undervoltage threshold

91% of output voltage

Selecting the Switching Frequency

The first step is to decide on a switching frequency for the regulator. Typically, the user chooses the highest

switching frequency possible because this produces 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 selectable switching frequency is limited by the minimum on-time of the internal power

switch, the input voltage, the output voltage, and the frequency-shift limitation.

Consider minimum on-time and frequency-shift protection as described in Equation 3 and Equation 4. To find the

maximum switching frequency for the regulator, choose the lower value of the two results. Switching frequencies

higher than these values result in pulse skipping or the lack of overcurrent protection during a short circuit. The

voltage is 5 V and the maximum input voltage is 16 V, use a switching frequency of 2200 kHz. To determine the

timing resistance for a given switching frequency, use Equation 2. The R1 resistor in Figure 23 sets the switching

frequency. A 2.2-MHz switching frequency requires a 47-k

Ω resistor.

20

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