400DAFBA2 datasheet(30/47 Pages) ONSEMI

30 / 47 page

NCP12400

www.onsemi.com

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 on−time duration control is kept during the

frequency foldback mode to provide the natural transformer

core anti−saturation 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 58. Frequency Foldback Mode Characteristic

Fsw

f OSC

Vskip(in)

VFB(foldS)

f OSC(min)

Voffset +KFB X VILIM

VFB(foldE)

Vskip(out)

Skip

VFB(freeze)

Fixed I peak

Figure 59. Current Setpoint Dependency on the Feedback Pin Voltage

VFB

Internal current setpoint

VILIM

Fixed I peak

VI(freeze)

Vskip(in)

VFB(foldS)

VFB(foldE)

Vskip(out)

VFB(freeze)

KFB X VILIM

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.

The NCP12400 device includes logic which allows going

into skip mode after the DRV cycle is finished by reaching

of the peak current value. This technique eliminates the last

short pulses in skip mode, which increases the system

efficiency at light loads and makes easier the application of

active secondary rectification circuitry.

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