AP72250

Document number: DS44135 Rev. 1 - 2

21 of 26

www.diodes.com

January 2022

© Diodes Incorporated

AP72250

Application Information (continued)

11

Setting the Output Voltage

The AP72250 has adjustable output voltages starting from 0.8V using an external resistive divider. The resistor values of the feedback network are

selected based on a design trade-off between efficiency and output voltage accuracy. There is less current consumption in the feedback network

for high resistor values, which improves efficiency at light loads. However, values too high cause the device to be more susceptible to noise affecting

its output voltage accuracy. R2 can be determined by the following equation:

������������ =

������. ������ ∙ ������������

������������������������ − ������. ������������

Eq. 3

Table 1 shows a list of recommended component selections for common AP72250 output voltages referencing Figure 1.

Table 1. Recommended Component Selections

AP72250

Output Voltage (V)

R1 (kΩ)

R2

(kΩ)

L (µH)

C1 (µF)

C2 (µF)

1.8

100.0

80.6

0.47

10

2 x 22

2.5

100.0

32.4

0.68

10

2 x 22

3.3

100.0

31.6

1.00

10

2 x 22

5.0

100.0

19.1

1.00

10

2 x 22

5.2

100.0

18.2

1.00

10

2 x 22

12

Inductor

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

the inductor value:

������ =

������������������ ∙ (������������������������ − ������������������)

������������������������ ∙ ∆������������ ∙ ������������������

Eq. 4

Where:



∆IL is the inductor current ripple



fSW is the boost converter switching frequency

For AP72250

, choose ∆IL to be 30% to 50% of the peak inductor current of 4.5A.

The inductor peak current is calculated by:

������������

������������������������ = ������������������������������ ∙ (

������������������������

������������������

) +

∆������������

������

Eq. 5

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 power MOSFETs. Therefore, choosing an inductor with the

appropriate saturation current rating is important. For most applications, it is recommended to select an inductor of approximately 1.0µH with a DC

current rating of at least 35% higher than the maximum peak current. For hi

ghest efficiency, the inductor’s DC resistance should be less than 50mΩ.

Use a larger inductance for improved efficiency under light load conditions

but beware of the “right-half-plane zero” frequency, F

RHPZ, which is:

������������������������������ =

������������������������

������ ∙ ������ ∙ ������������������������ ∙ ������������������������ ∙ ������

Eq. 6

The right-half-plane zero frequency can cause loop stability so it is ideal to have it be as high as possible. Therefore, for applications using a low

VIN and a high VOUT, the recommendation is to decrease the inductance, the output current, or both, to avoid any possible stability issues caused

by FRHPZ.