ADP5056ACCZ-R7 datasheet(22/31 Pages) AD

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ADP5056

Data Sheet

Rev. 0 | Page 22 of 31

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

buck regulator to prevent the inductor from becoming saturated.

To avoid overheating and poor efficiency, an inductor must be

chosen with an rms current rating that is greater than the

calculated maximum rms current. Calculate the rms current of

the inductor by using the following equation:

RMS

OUT





Shielded ferrite core materials are recommended for low core

loss and low electromagnetic interference (EMI). Table 8 lists

recommended inductors.

Table 8. Recommended Inductors

Vendor

Series1

Coilcraft

XAL5030, XEL5030

Toko

FDUE0650

Wurth

WE-HCI, WE-XHMI

1 Visit the Coilcraft, Toko, and Wurth manufacturer websites for more

information about the recommended series inductors.

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.

To calculate the output capacitance required to meet the

undershoot (voltage droop) requirement, use the following

equation:



STEP

OUT UV

OUT

OUT UV









 

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 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.

To calculate the output capacitance required to meet the

overshoot requirement, use the following equation:



STEP

OUT OV

OUT

OUT_OV

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.

The effective series resistance (ESR) of the output capacitor and

the capacitance value of the output capacitor determine the

output voltage ripple. Use the following equations to select

output capacitors that can meet the output ripple requirements:

OUT RIPPLE





 

OUT RIPPLE

ESR







where:

ΔIL is the inductor ripple current.

ΔVOUT_RIPPLE is the allowable output voltage ripple.

RESR is the equivalent series resistance of the output capacitor.

Select the largest output capacitance given by COUT_UV, COUT_OV,

and COUT_RIPPLE to meet both load transient and output ripple

requirements.

Ceramic capacitors have very low ESR and provide optimal

ripple performance. For recommended starting values, see the

Typical Application Circuits section. Use X5R or X7R type

capacitors to achieve low output ripple and small output

deviation during transient response. Transient performance can

be improved with a higher value output capacitor and the

addition of a feedforward capacitor placed between VOUTx

and FBx. Increasing the output capacitance also decreases the

output voltage ripple. A lower value output capacitor can be

used to save space and cost, but transient performance suffers

and may cause loop instability. See the Typical Application

Circuits section for suggested capacitor values.

When choosing a capacitor, calculate the effective capacitance

under the relevant operating conditions of voltage bias and

temperature. A physically larger capacitor or a capacitor with a

higher voltage rating may be required.

INPUT CAPACITOR SELECTION

The input decoupling capacitor attenuates high frequency noise on

the input and acts as an energy reservoir. Use a ceramic capacitor

and place it near to the PVINx pin. The loop composed of the

input capacitor, the high-side MOSFET, and the low-side

MOSFET must be kept as small as possible. The voltage rating of

the input capacitor must be greater than the maximum input

voltage. Ensure that the rms current rating of the input

capacitor is larger than the following equation:



Crms

OUT



 

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