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| SCY99194 Datasheet, PDF (27/40 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller | |||
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SCY99194
Low Load Operation Modes: Frequency Foldback
Mode (FFM) and Skip Mode
In order to improve the efficiency in light load conditions,
the frequency of the internal oscillator is linearly reduced
from its nominal value down to fOSC(min). This frequency
foldback starts when the voltage on FB pin goes below
VFB(foldS), and is complete when VFB reaches VFB(foldE).
The maximum onâtime duration control is kept during the
frequency foldback mode to provide the natural transformer
core antiâsaturation protection. The frequency jittering is
still active while the oscillator frequency decreases as well.
The current setpoint is fixed to 300 mV in the frequency
foldback mode if the feedback voltage decreases below the
VFB(freeze) level. This feature increases efficiency under the
light loads conditions as well.
Figure 53. Frequency Foldback Mode Characteristic
Figure 54. Current Setpoint Dependency on the Feedback Pin Voltage
When the FB voltage reaches Vskip(in) while decreasing,
skip mode is activated: the driver stops, and the internal
consumption of the controller is decreased. While VFB is
below Vskip(out), the controller remains in this state; but as
soon as VFB crosses the skip out threshold, the DRV pin
starts to pulse again.
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