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LTC3829

3829f

applicaTions inForMaTion

Slope Compensation and Inductor Peak Current

Slopecompensationprovidesstabilityinconstantfrequen-

cycurrentmodearchitecturesbypreventingsub-harmonic

oscillationathighdutycycles.Itisaccomplishedinternally

by adding a compensating ramp to the inductor current

signal at duty cycles in excess of 40%. Normally, this

results in a reduction of maximum inductor peak current

for duty cycles greater than 40%. However, the LTC3829

uses a scheme that counteracts this compensating ramp,

whichallowsthemaximuminductorpeakcurrenttoremain

unaffected throughout all duty cycles.

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

f L

OUT









•

–

where f is the individual output stage operating frequency.

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

Figure 8. Normalized Peak Output Current

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

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

3829 F08

6-PHASE

4-PHASE

12-PHASE

3-PHASE

2-PHASE

1-PHASE

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