LTC3856 datasheet(21/40 Pages) LINER

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LTC3856

3856f

applicaTions inForMaTion

Inductor Value Calculation and Output Ripple Current

The operating frequency and inductor selection are inter-

relatedinthathigheroperatingfrequenciesallowtheuseof

smaller inductor and capacitor values. A higher frequency

generally results in lower efficiency because of MOSFET

gate charge and transition losses. In addition to this basic

trade-off, the effect of inductor value on ripple current

and low current operation must also be considered. The

PolyPhase approach reduces both input and output ripple

currents while optimizing individual output stages to run

at a lower fundamental frequency, enhancing efficiency.

The inductor value has a direct effect on ripple current.

The inductor ripple current,

∆IL, per individual section

N, decreases with higher inductance or frequency and

increases with higher VIN or VOUT:

∆I

OUT

OSC

OUT









•

–

where fOSC is the individual output stage operating fre-

quency.

In a PolyPhase converter, the net ripple current seen by

the output capacitor is much smaller than the individual

inductor ripple currents due to the ripple cancellation. The

details on how to calculate the net output ripple current

can be found in Application Note 77.

Figure 8 shows the net ripple current seen by the output

capacitors for the different phase configurations. The

output ripple current is plotted for a fixed output voltage

as the duty factor is varied between 10% and 90% on the

x-axis. The output ripple current is normalized against the

inductor ripple current at zero duty factor. The graph can

be used in place of tedious calculations. The zero output

ripple current is obtained when:

where k

OUT

= , ,..., –

1 2

Power MOSFET and Schottky Diode

(Optional) Selection

At least two external power MOSFETs must be selected

for each power stage: One N-channel MOSFET for the top

(main) switch and one or more N-channel MOSFET(s) for

the bottom (synchronous) switch. The number, type and

on-resistance of all MOSFETs selected take into account

the voltage step-down ratio as well as the actual position

(main or synchronous) in which the MOSFET will be used.

AmuchsmallerandmuchlowerinputcapacitanceMOSFET

should be used for the top MOSFET in applications that

haveanoutputvoltagethatislessthanone-thirdoftheinput

voltage.InapplicationswhereVIN>>VOUT,thetopMOSFETs’

on-resistanceisnormallylessimportantforoverallefficiency

than its input capacitance at operating frequencies above

300kHz. MOSFET manufacturers have designed special

purpose devices that provide reasonably low on-resistance

with significantly reduced input capacitance for the main

switch application in switching regulators.

DUTY FACTOR (VOUT/VIN)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

3856 F08

6-PHASE

12-PHASE

4-PHASE

3-PHASE

2-PHASE

1-PHASE

Figure 8. Normalized Peak Output Current

vs Duty Factor [IRMS = 0.3(IOP-P)]

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