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ADP5053

Data Sheet

Rev. B | Page 24 of 36

SOFT START SETTING

The buck regulators in the ADP5053 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, 4 ms, or 8 ms, connect a resistor divider

from the SS12 or SS34 pin to the VREG pin and ground (see the

Soft Start section).

INDUCTOR SELECTION

The input voltage, output voltage, inductor ripple current, and

switching frequency determine the inductor value. Using a small

inductor value 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 is

required 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. Calculate

the inductor value 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 ADP5053 has internal slope compensation in the current

loop to prevent subharmonic oscillations when the duty cycle is

greater than 50%. Because the internal current sense signal is

required, the inductor value mustnotbe larger than10 µH for

Channel 1 and Channel 2 or 22µH for Channel 3 and Channel4.

Calculate the peak inductor current 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, to prevent the inductor from

becoming saturated by ensuring that the saturation current

rating of the inductor is higher than the current-limit threshold

of the buck regulator.

Calculate the rms current of the inductor using the following

equation:

OUT

RMS

∆

Shielded ferrite core materials are recommended for low core

loss and low EMI. Table 11 lists recommended inductors.

Table 11. Recommended Inductors

Vendor

Part No.

Value

(µH)

ISAT

(A)

IRMS

(A)

DCR

(mΩ)

Size

(mm)

Coilcraft XFL4020-102

1.0

5.4

10.8

4 × 4

XFL4020-222

2.2

3.7

8.0

21.35 4 × 4

XFL4020-332

3.3

2.9

5.2

34.8

4 × 4

XFL4020-472

4.7

2.7

5.0

52.2

4 × 4

XAL4030-682

6.8

3.6

3.9

67.4

4 × 4

XAL4040-103

3.0

3.1

4 × 4

XAL6030-102

1.0

5.62

6 × 6

XAL6030-222

2.2

15.9

12.7

6 × 6

XAL6030-332

3.3

12.2

8.0

19.92 6 × 6

XAL6060-472

4.7

10.5

14.4

6 × 6

XAL6060-682

6.8

9.2

9.0

18.9

6 × 6

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

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.

Calculate the output capacitance required to meet the

undershoot (voltage droop) requirement using the following

equation:

(

)

OUT

STEP

OUT

∆

−

∆

where:

KUV is a factor (typically set to 2).

ΔISTEP is the load step.

ΔVOUT_UV is the allowable undershoot on the output voltage.

Another example of the effect of the outputcapacitor 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.

Calculate the output capacitance required to meet the overshoot

requirement 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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