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LTC3121

3121fa

For more information www.linear.com/LTC3121

applicaTions inForMaTion

PCB LAYOUT GUIDELINES

The high switching frequency of the LTC3121 demands

careful attention to board layout. A careless layout will

result in reduced performance. Maximizing the copper

area for ground will help to minimize die temperature rise.

A multilayer board with a separate ground plane is ideal,

but not absolutely necessary. See Figure 3 for an example

of a two-layer board layout.

COMPONENT SELECTION

Inductor Selection

TheLTC3121canutilizesmallsurfacemountinductorsdue

to its capability of setting a fast (up to 3MHz) switching

frequency. Larger values of inductance will allow slightly

greater output current capability by reducing the inductor

ripple current. To design a stable converter the range of

inductance values is bounded by the targeted magnitude

oftheinternalslopecompensationandisinverselypropor-

tional to the switching frequency. The inductor selection

for the LTC3121 has the following bounds:

µH > L >

µH

The inductor peak-to-peak ripple current is given by the

following equation:

RIPPLE(A) =

VIN •(VOUT – VIN)

f •L • VOUT

where:

L = Inductor Value in µH

f = Switching Frequency in MHz

The inductor ripple current is a maximum at the minimum

inductor value. Substituting 3/f for the inductor value in

the above equation yields the following:

RIPPLEMAX(A) =

VIN •(VOUT – VIN)

3 • VOUT

To realize greater output current capability at the guaran-

teed minimum (over temperature) 1.5A current limit, it is

recommendedthattheinductorripplecurrentbelimitedto

one-thirdofthisminimumvalue,ortoapproximately0.5A.

Choosing a minimum inductor value of 6/f μH (where f =

switching frequency in MHz) or greater typically results in

an inductor ripple current of 0.5A or less for the majority of

step-upratios.Highfrequencyferritecoreinductormateri-

als reduce frequency dependent power losses compared

to cheaper powdered iron types, improving efficiency.

Figure 3. Traces Carrying High Current Are Direct (PGND, SW, VIN

and VOUT). Trace Area at FB and VC Are Kept Low. Trace Length to

Input Supply Should Be Kept Short. VIN and VOUT Ceramic Capacitors

Should Be Placed as Close to the LTC3121 Pins as Possible

PGND

3121 F02

VIN

PGND

SGND

VCC

VOUT

PGND

CAP

SCHOTTKY DIODE

Although it is not required, adding a Schottky diode from

SW to VOUT can improve the converter efficiency by about

4%.Notethatthisdefeatstheoutputdisconnectandshort-

circuit protection features of the LTC3121.

LTC3121 Datasheet(PDF) 12 Page - Linear Technology

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