AP65111A

Document number: DS39760 Rev. 1 - 2

9 of 13

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June 2017

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AP65111A

Setting the Output Voltage

The output voltage can be adjusted from 0.8V using an external resistor divider. Table 1 shows a list of resistor selection for common output

100pF used to boost the phase margin. Resistor R1 is selected based on a design tradeoff between efficiency and output voltage accuracy. For

high values of R1 there is less current consumption in the feedback network. R1 can be determined by the following equation:



















1

0.8

V

R

R

OUT

2

1

FB

R1

R2

CFF

Figure 3. Feedback Divider Network

Table 1. Recommended Component Selection

R1 (kΩ)

R2 (kΩ)

(kΩ)

L1 (µH)

1.05

10

32.4

150

2.2

1.2

15

30.1

130

2.2

1.8

40.2

32.4

100

3.3

2.5

40.2

19.1

59

4.7

3.3

40.2

13

59

6.5

5

40.2

7.68

59

6.5

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

SW

f

is the buck converter switching frequency.

Choose the inductor ripple current to be 30% to 40% 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, choose an inductor with appropriate

saturation current rating is important.

A 1µH to 10µH 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 20mΩ. Use a larger inductance for improved efficiency under light load

conditions.

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 hence 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 RMs rating that is greater than half of the maximum load current.

protected. Otherwise, capacitor failure could occur. For most applications, a 22µF ceramic capacitor is sufficient.

Output Capacitor

The output capacitor keeps the output voltage ripple small, ensures feedback loop stability and reduces the overshoot of the output voltage. The

output capacitor is a basic component for the fast response of the power supply. During load transient, the output capacitor supplies the current to

the load for the first few cycles. This caused the output voltage to drop and sets the duty cycle to maximum, but the current slope is limited by the

inductor value.