AP65352

Document number: DS38482 Rev. 2 - 3

11 of 15

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December 2017

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AP65352

Application Information (cont.)

Inductor

Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to calculate

the inductor value:

SW

L

IN

OUT

IN

OUT

f

ΔI

V

)

V

(V

V

L











Where

L

ΔI is the inductor ripple current and

SW

f

is the buck converter switching frequency.

Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from:

2

ΔI

I

I

L

LOAD

L(MAX)





Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the inductor decreases the

converter efficiency while increasing the temperatures of the inductor and the internal MOSFETs. Hence choosing an inductor with appropriate

saturation current rating is important.

A 1µH to 3.3µH inductor with a DC current rating of at least 25% higher than the maximum load current is recommended for most applications. For

highest efficiency, the inductor’s DC resistance should be less than 100mΩ. Use a larger inductance for improved efficiency under light load

conditions.

The phase boost can be achieved by adding an additional feed forward capacitor (C7) in parallel with R1.

Output Voltage (V)

C7(pF)

L1(µH)

C8+C9(µF)

1

—

1.0-1.5

22-68

1.05

—

1.0-1.5

22-68

1.2

—

1.0-1.5

22-68

1.5

—

1.5

22-68

1.8

5-22

1.5

22-68

2.5

5-22

2.2

22-68

3.3

5-22

2.2

22-68

5

5-22

3.3

22-68

Table 2 Recommended Component Selection

Input Capacitor

The input capacitor reduces the surge current drawn from the input supply and the switching noise from the device. The input capacitor has to

sustain the ripple current produced during the on time on the upper MOSFET. It must have a low ESR to minimize the losses.

The RMS current rating of the input capacitor is a critical parameter that must be higher than the RMS input current. As a rule of thumb, select an

input capacitor which has an RMs rating greater than half of the maximum load current.

Due to large dI/dt through the input capacitors, electrolytic or ceramics should be used. If a tantalum must be used it must be surge protected,

otherwise, capacitor failure could occur. For most applications greater than 10µF, ceramic capacitor is sufficient.

Figure 5 Feedback Divider Network

Output Voltage (V)

R1 (kΩ)

R2 (kΩ)

1

6.81

22.1

1.05

8.25

22.1

1.2

12.7

22.1

1.5

21.5

22.1

1.8

30.1

22.1

2.5

49.9

22.1

3.3

73.2

22.1

5

124

22.1

Table 1 Resistor Selection for Common Output

Voltages