ADP5054ACPZ-R7 datasheet(21/31 Pages) AD

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Data Sheet

ADP5054

Rev. B | Page 21 of 31

SOFT START SETTING

The buck regulators in the ADP5054 include soft start circuitry

that ramps the output voltage in a controlled manner during

startup, thereby limiting the inrush current. To set the soft start

time to a value of 2 ms or 16 ms, connect a resistor divider from

the CFG12 pin or the CFG34 pin to the VREG pin and ground

(see the Soft Start section).

INDUCTOR SELECTION

The inductor value is determined by the operating frequency,

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

a small inductor yields faster transient response but degrades

efficiency due to the larger inductor ripple current. Using a

large inductor value yields a smaller ripple current and better

efficiency but results in slower transient response. Thus, a

trade-off must be made between transient response and

efficiency. As a guideline, the inductor ripple current, ΔIL, is

typically set to a value from 30% to 40% of the maximum load

current. The inductor value can be calculated using the

following equation:

L = ((VIN − VOUT) × D)/(ΔIL × fSW)

where:

VIN is the input voltage.

VOUT is the output voltage.

D is the duty cycle (D = VOUT/VIN).

ΔIL is the inductor ripple current.

fSW is the switching frequency.

The ADP5054 has internal slope compensation in the current

loop to prevent subharmonic oscillations when the duty cycle

is greater than 50%.

The inductor peak current is calculated using the following

equation:

IPEAK = IOUT + (ΔIL/2)

The saturation current of the inductor must be larger than the

peak inductor current. For ferrite core inductors with a fast

saturation characteristic, the saturation current rating of the

inductor must be higher than the current-limit threshold of the

buck regulator to prevent the inductor from becoming saturated.

The rms current of the inductor can be calculated using the

following equation:

OUT

RMS

∆

Shielded ferrite core materials are recommended for low core

loss and low electromagnetic interference (EMI). Table 11 lists

the recommended inductors.

Table 11. Recommended Inductors

Vendor

Part No.

Value

(µH)

ISAT

(A)

IRMS

(A)

DCR

(mΩ)

Size

(mm)

Coilcraft

XFL4030-332

3.3

5.5

6.6

4 × 4

XFL4030-472

4.7

4.5

5.1

40.1

4 × 4

XFL4030-682

6.8

3.6

3.9

67.4

4 × 4

XFL5030-801

0.8

18.5

5.14

5 × 5

XAL5030-122

1.2

12.5

11.1

8.5

5 ×5

XAL5030-222

2.2

9.2

9.7

13.2

5× 5

XAL5030-332

3.3

8.7

8.1

21.2

5 × 5

XAL5030-472

4.7

6.7

5.9

5 × 5

TOKO

FDV0530-1R0

1.0

11.2

9.1

9.4

6.2 × 5.8

FDV0530-2R2

2.2

7.1

7.0

17.3

6.2 × 5.8

FDV0530-3R3

3.3

5.5

5.3

29.6

6.2 × 5.8

FDV0530-4R7

4.7

4.6

4.2

46.6

6.2 × 5.8

WE-HCI

744314076

0.76

15.5

2.25

7 × 7

744314110

1.1

3.15

7 × 7

744314200

2.0

11.5

5.85

7 × 7

744311330

3.3

9.0

7 × 7

OUTPUT CAPACITOR SELECTION

The selected output capacitor affects both the output voltage

ripple and the loop dynamics of the regulator. For example,

during load step transients on the output, when the load is

suddenly increased, the output capacitor supplies the load until

the control loop can ramp up the inductor current, causing an

undershoot of the output voltage.

The output capacitance required to meet the voltage drop.

requirement can be calculated using the following equation:

(

)

OUT

STEP

OUT

∆

−

∆

where:

KUV is a factor (typically set to 2).

ΔISTEP is the load step.

L is the output inductor.

ΔVOUT_UV is the allowable undershoot on the output voltage.

Another example of the effect of the output capacitor on the

loop dynamics of the regulator is when the load is suddenly

removed from the output and the energy stored in the inductor

rushes into the output capacitor, causing an overshoot of the

output voltage.

The output capacitance required to meet the overshoot

requirement can be calculated using the following equation:

(

)

OUT

OUT_OV

OUT

STEP

OUT

−

∆

where:

KOV is a factor (typically set to 2).

ΔISTEP is the load step.

ΔVOUT_OV is the allowable overshoot on the output voltage.

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