ADP1877ACPZ-R7 datasheet(20/32 Pages) AD

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ADP1877

Rev. 0 | Page 20 of 32

VIN

ADP1877

VIN

AGND

2Ω TO 5Ω

1µF

Figure 33. Input Filter Configuration

BOOST CAPACITOR SELECTION

To lower system component count and cost, the ADP1877 has a

built-in rectifier (equivalent to the boost diode) between VCCO

and BSTx. Choose a boost ceramic capacitor with values

between 0.1 μF and 0.22 μF, which provides the current for the

high-side driver during switching.

INDUCTOR SELECTION

The output LC filter smoothes the switched voltage at SWx.

Choose an inductor value such that the inductor ripple current

is approximately 1⁄3 of the maximum dc output load current.

Using a larger value inductor results in a physical size larger

than required, and using a smaller value results in increased

losses in the inductor and/or MOSFET switches and larger

voltage ripples at the output.

Choose the inductor value by the following equation:

OUT

−

where:

L is the inductor value.

fSW is the switching frequency.

VOUT is the output voltage.

VIN is the input voltage.

ΔI

is the inductor ripple current, typically 1⁄3 of the maximum

dc load current.

OUTPUT CAPACITOR SELECTION

Choose the output bulk capacitor to set the desired output voltage

ripple. The impedance of the output capacitor at the switching

frequency multiplied by the ripple current gives the output

voltage ripple. The impedance is made up of the capacitive

impedance plus the nonideal parasitic characteristics, the

equivalent series resistance (ESR), and the equivalent series

inductance (ESL). The output voltage ripple can be

approximated with

⎟

⎠

⎞

⎜

⎝

⎛

≅

ESL

OUT

ESR

OUT

where:

ΔV

OUT

is the output ripple voltage.

ΔI

is the inductor ripple current.

RESR is the equivalent series resistance of the output capacitor (or

the parallel combination of ESR of all output capacitors).

LESL is the equivalent series inductance of the output capacitor

(or the parallel combination of ESL of all capacitors).

Solving COUT in the previous equation yields

ESL

ESR

OUT

−

≅

Usually, the impedance is dominated by ESR, such as in

electrolytic or polymer capacitors, at the switching frequency, as

stated in the maximum ESR rating on the capacitor data sheet;

therefore, output ripple reduces to

ESR

OUT

≅

Electrolytic capacitors also have significant ESL, on the order of

5 nH to 20 nH, depending on type, size, and geometry. PCB

traces contribute some ESR and ESL, as well. However, using

the maximum ESR rating from the capacitor data sheet usually

provides some margin such that measuring the ESL is not

usually required.

In the case of output capacitors where the impedance of the ESR

and ESL are small at the switching frequency, for instance,

where the output cap is a bank of parallel MLCC capacitors, the

capacitive impedance dominates and the output capacitance

equation reduces to

OUT

≅

Make sure that the ripple current rating of the output capacitors

is greater than the maximum inductor ripple current.

During a load step transient on the output, for instance, when

the load is suddenly increased, the output capacitor supplies the

load until the control loop has a chance to ramp the inductor

current. This initial output voltage deviation results in a voltage

droop or undershoot. The output capacitance, assuming 0 ESR,

required to satisfy the voltage droop requirement can be

approximated by

DROOP

STEP

OUT

≅

where:

ΔISTEP is the step load.

ΔVDROOP is the voltage droop at the output.

When a load is suddenly removed from the output, the energy

stored in the inductor rushes into the capacitor, causing the

output to overshoot. The output capacitance required to satisfy

the output overshoot requirement can be approximated by

)

(

OUT

OVERSHOOT

OUT

STEP

OUT

−

≅

where:

ΔVOVERSHOOT is the overshoot voltage during the step load.

Select the largest output capacitance given by any of the

previous three equations.

ADP1877ACPZ-R7 Datasheet(PDF) 20 Page - Analog Devices

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