LT8686SJVPBF datasheet(18/26 Pages) AD

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LT8686S

Rev. 0

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The inductor value should be chosen to give a peak-to-

peak ripple current ΔIL of between 35% and 45% of the

rated channel output current at the nominal input volt-

age. Note, the rated channel output current is 2A for

each individual channel and is 4A for combined chan-

nels. Rearranging Equation 10, select the inductor value

according to Equation 11.

L =

VOUT

fSW •∆IL

• 1–

VOUT

VIN

(11)

To avoid overheating and poor efficiency, an induc-

tor must be chosen with an RMS current rating that is

greater than the maximum expected output load of the

application. In addition, for best efficiency the inductor

series resistance should be as small as possible, and

the core material should be intended for the application

switching frequency.

The saturation current rating of the inductor must be

higher than the load plus half the ripple current. This peak

inductor current can be computed with Equation 12.

IL(PEAK) =IOUT(MAX) +

ΔIL

(12)

where IOUT(MAX) is the maximum output current for a

given application.

The optimum inductor for a given application may differ

from the one indicated by this design guide. Careful eval-

uation of the application circuit should be completed with

the chosen inductor to ensure adequate design margin.

INPUT CAPACITOR SELECTION

Buck, or step-down, converters draw current from the

input supply in pulses with very fast rise and fall times. An

input capacitor is required to reduce the resultant voltage

ripple at the input and minimize EMI. For this function, a

ceramic X7R or X5R bypass capacitor should be placed

between each buck regulator’s VIN pin and ground. To be

most effective, the input capacitor must have low imped-

ance at the switching frequency and an adequate ripple

current rating.

The worst-case ripple current occurs when VOUT is one-

half VIN. Under this condition, the ripple current is given

by Equation 13.

ICIN(RMS) =

IOUT

(13)

Reasonable starting values for the input capacitance are

2.2μF for channels 1 and 2 and 1μF for channels 3 and 4.

When combining channels, the shared channel VIN pins

must be connected together, and the input capacitor

should be chosen based on the total current supplied by

the combined channels.

OUTPUT CAPACITOR SELECTION

The output capacitor performs two functions. First, it

filters the inductor current to generate an output with

low voltage ripple. Second, it stores energy to minimize

droop and overshoot during transient loads. Because the

LT8686S buck converters are able to operate at a high

frequency, minimal output capacitance is necessary. The

internally compensated current mode control loops are

stable without requiring a minimum series resistance

(ESR) in the output capacitor. Therefore, ceramic capaci-

tors may be used and will result in very low output ripple.

An estimate for the output ripple is given by Equation 14

and Equation 15 for a given capacitor type.

VRIPPLE =

8 • fSW •COUT

(14)

for ceramic capacitors, and

VRIPPLE = ΔIL •ESR

(15)

for aluminum or tantalum capacitors. VRIPPLE is the peak-

to-peak output ripple, fSW is the switching frequency in

MHz, ΔIL is the peak-to-peak ripple current in the inductor,

COUT is the output capacitor value in µF and ESR is the

output capacitor effective series resistance.

The low ESR and small size of ceramic capacitors make

them the preferred type for LT8686S applications.

APPLICATIONS INFORMATION

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