ADP2165ACPZ-1.2-R7 datasheet(15/23 Pages) AD

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Data Sheet

ADP2165/ADP2166

Rev. B | Page 15 of 23

INDUCTOR SELECTION

The inductor value is determined by the operating frequency,

input voltage, output voltage, and inductor ripple current. Using

a small inductor leads to a faster transient response; however, it

degrades efficiency due to a larger inductor ripple current.

Conversely, using a large inductor value leads to a smaller ripple

current and better efficiency; however, it results in a slower

transient response.

As a guideline, the inductor ripple current, ΔIL, is typically set

to one-third of the maximum load current. The inductor value

is calculated using the following equation:

L =

OUT

PVIN

∆

−

)

(

where:

VPVIN is the input voltage.

VOUT is the output voltage.

ΔIL is the inductor ripple current.

fSW is the switching frequency.

D is the duty cycle, D = VOUT/VPVIN.

The ADP2165/ADP2166 use adaptive slope compensation in

the current loop to prevent subharmonic oscillations when the

duty cycle is larger than 50%. The internal slope compensation

limits the minimum inductor value.

For a duty cycle that is larger than 50%, the minimum inductor

value is determined by using the following equation:

L (Minimum) =

OUT

−

)

(

The peak inductor current is calculated by using the following

equation:

IPEAK = IOUT +

∆

The saturation current of the inductor must be larger than the peak

inductor current. For ferrite core inductors with a quick saturation

characteristic, the saturation current rating of the inductor must

be higher than the current limit threshold of the switch. This

prevents the inductor from reaching saturation.

The rms current of the inductor is calculated from the following

equation:

IRMS =

OUT

∆

Shielded ferrite core materials are recommended for low core

loss and low EMI. Table 6 lists some recommended inductors.

Table 6. Recommended Inductors

Vendor

Part No.

(µH)

ISAT

(A)

IRMS

(A)

DCR

(mΩ)

Würth

Elektronik

744311022

0.22

1.10

744314047

0.47

1.35

744314076

0.76

15.5

2.25

744311100

1.0

4.6

744311150

1.5

6.6

7443340220

2.2

12.5

16.5

4.4

7443340330

3.3

8.5

6.5

Coilcraft

XAL7020-271ME

0.27

2.9

XAL7020-331ME

0.33

4.0

XAL7020-471ME

0.47

24.3

4.75

XAL7020-681ME

0.68

22.3

7.9

XAL7020-102ME

1.0

16.4

9.8

XAL7030-152ME

1.5

23.5

7.6

XAL7030-222ME

2.2

12.9

13.7

OUTPUT CAPACITOR SELECTION

The output capacitor selection affects the output ripple voltage

load step transient and the loop stability of the regulator.

For example, during a load step transient where the load is

suddenly increased, the output capacitor supplies the load until

the control loop can ramp up the inductor current. The delay

caused by the control loop causes the output to undershoot. The

output capacitance that is required to satisfy the voltage droop

requirement can be calculated by using the following equation:

COUT_UV =

OUT

PVIN

STEP

)

(

∆

−

∆

where:

KUV is a factor, with a typical setting of KUV = 2.

ΔISTEP is the load step.

ΔVOUT_UV is the allowable undershoot on the output voltage.

Another example occurs when a load is suddenly removed from

the output, and the energy stored in the inductor rushes into

the output capacitor, causing the output to overshoot.

The output capacitance that is required to meet the overshoot

requirement can be calculated using the following equation:

COUT_OV =

)

(

OUT

STEP

−

∆

where:

KOV is a factor, with a typical setting of KOV = 2.

ΔVOUT_OV is the allowable overshoot on the output voltage.

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