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Rev. B, Sep 2005
Operation (3)
Low Noise Fixed Frequency Operation
Burst Mode Operation (2)
Figure 4
Inductor Discharge Cycle During Burst Mode Operation
Burst Mode Operation to Fixed Frequency
Transient Response
When transitioning from Burst Mode operation to fixed
frequency, the system exhibits a transient since the modes
of operation have changed. For most systems this transient
is acceptable, but the application may have stringent input
current and/or output voltage requirements that dictate a
broad-band voltage loop to minimize the transient.
Lowering the DC gain of the loop will facilitate the task (10M
FB to Vc) at the expense of DC load regulation.
SOFT-START
The soft-start function is combined with shutdown. When
the SHDN/SS pin is brought above typically 1V, the IC is
enabled but the EA duty cycle is clamped from the Vc pin.
A detailed diagram of this function is shown in Figure 5.
The components Rss and Css provide a slow ramping
voltage on the SHDN/SS pin to provide a soft-start
function.
Figure 5 Soft-Start Circuitry