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TPS65321A-Q1

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SLVSE55 – NOVEMBER 2017

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Typical Application (continued)

8.2.1.1 Design Requirements

A few parameters must be known to begin the design process. The determination of these parameters is typically

at the system level. This example begins with the parameters listed in Table 3.

Table 3. Design Requirements

DESIGN PARAMETER

EXAMPLE VALUE

Input voltage, VIN1

6 V to 36 V, nominal 12 V

Output voltage, VREG1 (buck regulator)

3.3 V ± 2%

3 A

0.01 A

Transient response, 0.01 A to 0.8 A

3%

Output ripple voltage

1%

2 MHz

Input voltage, VIN_LDO

6 V to 36 V, nominal 12 V

Output voltage, VREG2 (LDO regulator)

5 V ± 2%

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Switching Frequency Selection for the Buck Regulator

The first step is to decide on a switching frequency for the regulator. Typically, the user selects 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 calculated with Equation 4 and Equation 5. To find

the maximum switching frequency for the regulator, select 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

the output voltage is 3.3 V and the maximum input voltage is 36 V, use a switching frequency of 2200 kHz. Use

Equation 3 to calculate the timing resistance for a given switching frequency. The R4 resistor sets the switching

frequency. A 2-MHz switching frequency requires a 47-kΩ resistor (see R4 in Figure 16).

8.2.1.2.2

Output Inductor Selection for the Buck Regulator

Use Equation 22 to calculate the minimum value of the output inductor. The output capacitor filters the inductor

ripple current. Therefore, selecting high inductor-ripple currents impacts the selection of the output capacitor

because 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, the following guidelines

to the maximum output current.

(22)

inductor ripple current on the larger side is best because it allows the inductor to still have a measurable ripple

current with the input voltage at a minimum.