17

 2005 Semtech Corp.

www.semtech.com

SC4808A/B

POWER MANAGEMENT

D<50%

Instability in current mode operation

due to Duty cycle >50%

I

i

L

: Small Inductor current perturbation

I

L

i

L

Note: After afew c ycles the perturbation

disappears and stable operation returns.

D=50%

I

L

i

L

i

L

I

L

Note: After afew c ycles the perturbation

is still pres ent, although this w ill caus e

jitter, but there is no instability.

D>50%

i

L

Note: After afew c ycles the perturbation

becomes larger, and causes instability.

i

L

Time

Time

Time

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.

Application Information (Cont.)