ADP5140WACCZ-R7 datasheet(56/128 Pages) AD

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ADP5140

Data Sheet

Rev. 0 | Page 56 of 128

selected inductor is large enough to avoid the current limit

throughout the entire input voltage range. Use the following

equation to calculate the peak inductor current:

OUT BOOST

PP BOOST

PEAK BOOST

BOOST





−



where

IPEAK_BOOST is the peak inductor current of the boost.

IPP_BOOST is the peak-to-peak inductor ripple current.

IN BOOST

BOOST

PP BOOST

BOOST

where LBOOST is the inductor value of the boost.

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

boost regulator to prevent the inductor from reaching saturation.

OUTPUT CAPACITOR SELECTION

The output capacitor selection affects both the output voltage

ripple and the loop dynamics of the regulators in the ADP5140.

The ADP5140 functions with small ceramic capacitors that

have low equivalent series resistance (ESR) and low equivalent

series inductance (ESL) and can, therefore, easily meet the

output voltage ripple specifications. Ceramic capacitors are

manufactured with a variety of dielectrics, each with different

behavior over temperature and applied voltage. X5R or X7R

dielectrics are recommended for best performance because of

the low ESR and small temperature coefficients.

Output Capacitor Selection for the Buck Regulators

During a load step transient where the load suddenly increases,

the output capacitor supplies the load until the control loop can

ramp up the inductor current. The delay caused by the control

loop causes output to undershoot. To calculate the output

capacitance that is required to satisfy the voltage drop

requirement, use the following equation:

2 (

)

STEP

BUCK

OUT_UV

IN BUCK

OUT BUCK

OUT UV

×∆

−

×∆

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 of

buck regulator.

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.

To calculate the output capacitance that is required to meet the

overshoot requirement, use the following equation:

()

STEP

BUCK

OUT_OV

OUT BUCK

OUT OV

OUT BUCK

×∆

+∆

−

where:

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

LBUCK is the inductor current ripple

ΔVOUT_OV is the allowable overshoot on the output voltage of

buck regulator. When the buck regulator operates in

continuous conduction mode, the overall output voltage ripple

is the sum of the voltage spike caused by the output capacitor

ESR plus the voltage ripple caused by the charging and

discharging of the output capacitor, as follows:

RIPPLE BUCK

L BUCK

BUCK

OUT RIPPLE BUCK

ESR

∆=



∆×



××



where:

ΔVRIPPLE_BUCK is the allowable output ripple voltage of the buck

regulator.

COUT_RIPPLE_BUCK is the output capacitance to meet the voltage

ripple requirement.

ESRBUCK is the equivalent series resistance of the output

capacitor of buck regulator in ohms (Ω).

Capacitors with lower ESR are preferable to guarantee low

output voltage ripple, as shown in the following equation:

RIPPLE BUCK

BUCK

L BUCK

ESR

∆

≤

∆

Select the largest output capacitance given by COUT_UV, COUT_OV,

and COUT_RIPPLE_BUCK to meet both load transient and output

ripple performance.

Output Capacitor Selection for the Boost Regulator

In the boost regulator, the output capacitor supplies the load

current while the inductor current ramps up. Therefore, the

output ripple usually determines the output capacitance

required, as follows:

(

)

RIPPLE BOOST

OUT BOOST

BOOST

PEAK BOOST

OUT RIPPLE BOOST

ESR

∆=

+×

where:

ΔVRIPPLE_BOOST is the allowable output ripple voltage of the boost

regulator.

ESRBOOST is the equivalent series resistance of the output

capacitor of boost regulator in Ω.

The output voltage of the boost regulator in the ADP5140 is fixed.

A ceramic capacitor with a typical value of 10 μF or greater and

low ESR is recommended. Larger capacitance values lower the

output ripple and improve transient response and loop stability.

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