ADP1876ACPZ-R7 datasheet(19/24 Pages) AD

19 / 24 page

Data Sheet

ADP1876

Rev. A | Page 19 of 24

Select an input bulk capacitor based on its ripple current rating.

First, determine the duty cycle of the output.

OUT

D =

The input capacitor rms ripple current is given by

)

(

RMS

−

where:

IO is the output current.

D is the duty cycle.

The minimum input capacitance required for a particular load is

ESR

MIN

)

(

)

(

−

where:

VPP is the desired input ripple voltage.

RESR is the equivalent series resistance of the capacitor.

If an MLCC capacitor is used, the ESR is near 0, then the

equation is simplified to

MIN

−

)

(

The capacitance of MLCC is voltage dependent. The actual

capacitance of the selected capacitor must be derated according to

the manufacturer’s specification. In addition, add more bulk

capacitance, such as by using electrolytic or polymer capacitors,

as necessary for large step load transients. Make sure the

current ripple rating of the bulk capacitor exceeds the

maximum input current ripple of a particular design.

INPUT FILTER

Normally a 0.1 µF (or greater value) bypass capacitor from the

input pin (VIN) to AGND is sufficient for filtering any unwanted

switching noise. However, depending on the printed circuit

board (PCB) layout, some switching noise can enter the ADP1876

internal circuitry; therefore, it is recommended to have a low-

pass filter at the VIN pin. Connecting a resistor, between 2 Ω

and 5 Ω, in series with VIN and a 1 µF ceramic capacitor

between VIN and AGND creates a low-pass filter that effectively

filters out any unwanted glitches caused by the switching regulator.

Note that the input current can be larger than 100 mA when

driving large MOSFETs. A 100 mA current across a 5 Ω resistor

creates a 0.5 V drop, which is the same voltage drop in VCCO.

In this case, a lower resistor value is desirable.

Figure 30. Input Filter Configuration

BOOST CAPACITOR SELECTION

To lower system component count and cost, the ADP1876 has

an integrated rectifier (equivalent to the boost diode) between

VCCO and BSTx. Choose a boost ceramic capacitor with a

value between 0.1 µF and 0.22 µF; this capacitor provides the

current for the high-side driver during switching.

INDUCTOR SELECTION

The output LC filter smoothes the switched voltage at SWx.

For most applications, choose an inductor value such that

the inductor ripple current is between 20% and 40% of the

maximum dc output load current. Generally, a larger inductor

current ripple generates more power loss in the inductor and

larger voltage ripples at the output. Check the inductor data

sheet to make sure that the saturation current of the inductor

is well above the peak inductor current of a particular design.

Choose the inductor value by using 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.

∆I

L is the inductor ripple 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 comprises the capacitive impedance plus

the nonideal parasitic characteristics, the equivalent series resis-

tance (ESR), and the equivalent series inductance (ESL). The

output voltage ripple can be approximated by













∆

≅

∆

ESL

OUT

ESR

OUT

where:

∆V

OUT is the output ripple voltage.

∆I

L 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

∆

−

∆

−

∆

≅

ADP1876

VIN

AGND

2Ω TO 5Ω

1µF

ADP1876ACPZ-R7 Datenblatt(PDF) 19 Page - Analog Devices

ADP1876ACPZ-R7 Datenblatt(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datenblatt Download

Link URL