SC4808C 데이터 시트보기 (PDF) - Semtech Corporation

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SC4808C

High Performance Dual Ended PWM Controller

Semtech Corporation 

SC4808C

POWER MANAGEMENT

Application Information (Cont.)

SLOPE COMPENSATION (Current or Voltage mode of operation)

In applications where a current mode control is used for regulation, the peak inductor current information is used to

produce the average output current. If a small perturbation due to changes in supply voltage or noise pick up is gener-

ated, instability may occur if the duty cycle is >50%.

This phenomenon is graphically shown below. The inductor current and disturbed inductor current are shown for three

different duty cycles conditions.

The top wave form shows the applications where the duty cycle D is less than 50%. As shown, even if an error is

introduced, after only a few cycles the error converges to zero.

The second wave form shows the case where D = 50%. Under this condition, even though the error does not completely

disappear, it stays constant and is not getting larger. This will be seen as jitter at the inductor voltage.

The bottom wave form shows D>50%. As shown, a very small error results in a much larger error only after a few cycles.

This will cause instability in the converter and the average output inductor current. The output load will not be able to be

kept in regulation.

iL : Small Inductor current perturbation

IL : Inductor current

Instability in current mode operation

due to Duty cycle >50%

D<50%

IL

iL

Tim e

I D=50%

L

iL

Tim e

I D>50%

L

iL

Note: After afew c ycles the perturbation

disappears and stable operation returns.

iL

Note: After afew c ycles the perturbation

is still pres ent, although this w ill caus e

jitter, but there is no instability.

iL

Tim e

 2006 Semtech Corp.

Note: After afew c ycles the perturbation

becomes larger, and causes instability.

10

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