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LTC3860

3860f

applicaTions inFormaTion

A multiphase power supply significantly reduces the

amount of ripple current in both the input and output ca-

pacitors. The RMS input ripple current is divided by, and

the effective ripple frequency is multiplied by, the number

of phases used (assuming that the input voltage is greater

than the number of phases used times the output volt-

age). The output ripple amplitude is also reduced by the

number of phases used. Figure 8 graphically illustrates

the principle.

The worst-case RMS ripple current for a single stage de-

sign peaks at an input voltage of twice the output voltage.

The worst case RMS ripple current for a two stage design

results in peak outputs of 1/4 and 3/4 of input voltage.

When the RMS current is calculated, higher effective duty

factor results and the peak current levels are divided as

long as the current in each stage is balanced. Refer to

Application Note 19 for a detailed description of how

to calculate RMS current for the single stage switching

regulator. Figures 9 and 10 illustrate how the input and

output currents are reduced by using an additional phase.

For a 2-phase converter, the input current peaks drop in

half and the frequency is doubled. The input capacitor

requirement is thus reduced theoretically by a factor of

four! Just imagine the possibility of capacitor savings with

even higher number of phases!

Output Current Sharing

When multiple LTC3860s are cascaded to drive a com-

mon load, accurate output current sharing is essential to

achieve optimal performance and efficiency. Otherwise,

if one stage is delivering more current than another, then

the temperature between the two stages will be different,

and that could translate into higher switch RDS(ON), lower

efficiency,andhigherRMSripple.WhentheCOMPandIAVG

pins of multiple LTC3860s are tied together, the amount

of output current delivered from each LTC3860 is actively

balanced by the IAVE loop. The SGND pins of the multiple

LTC3860s must be kelvined to the same point for optimal

current sharing.

3860 F08

SW1 V

ICIN

ICOUT

SINGLE PHASE

SW1 V

SW2 V

ICIN

IL2

IL1

ICOUT

DUAL PHASE

RIPPLE

Figure 8. Single and 2-Phase Current Waveforms

DUTY FACTOR (VOUT/VIN)

0.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.3

0.5 0.6

3860 F09

0.2

0.4

0.7 0.8 0.9

1 PHASE

2 PHASE

Figure 9. Normalized Output Ripple Current vs Duty Factor

[IRMS″ 0.3 (DIC(PP))]

0.1

0.2

0.3

0.4

3860 F10

0.5

0.6

DUTY FACTOR (VOUT/VIN)

0.1

0.3

0.5 0.6

0.2

0.4

0.7 0.8 0.9

1 PHASE

2 PHASE

Figure 10. Normalized RMS Input Ripple Current vs Duty

Factor for 1 and 2 Output Stages

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