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 2005 Semtech Corp.

www.semtech.com

SC4808A/B

POWER MANAGEMENT

Application Information (Cont.)

Below the benefits from the slope compensation become apparent. The top wave form shows the stable operation

before the perturbation. The second wave form shows the perturbation and the instability caused from it if no slope

compensation is added to the current information. The last wave form shows the slope compensation and the effect of

it. The increase in the slope of the current information results in an early termination of the inductor current, hence a

reduction in the amount of error. As the cycle is repeated, the perturbation is reduced and finally eliminated.

Stable current mode operation with Slope

Compensation

I

L

∆

: Inductor current

: Small Inductor current perturbation

I

L

∆

D>50%

i

L

δ

: Sensed Mosfet current

I

Sense

I

Sense

i

L

δ

i

L

δ

Stable operation

( no perturbation)

Instable operation

( with perturbation)

Stable operation

(Slope Compensation Added)

I

L

∆

I

Sense

I

L

∆

I

CS

Error Signal from Error amplifier

Error Signal from Error amplifier

I

CS

: Summation of Isense and slope compensation, at the CS pin of the SC4808.

Error Signal from Error amplifier

Note: After a few cycles the perturbation

disappears and stable operation returns.

Note: After a few cycles the perturbation

becomes larger, and causes instability.