SC4808B-2 Datasheet, PDF (14/28 Pages) Semtech Corporation – High Performance Dual Ended PWM Controller

SC4808B-2
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.
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