Datasheet

OSC_CL

QR_DONE

4.0V

0.1V

SS_OVR

REF

S Q

CLK

130 kHz OSC

Clamp

Comparator

OSC Valley

Comparator

OSC Peak

Comparator

RUN

Oscillator

FB_CL

OSC_CL

1.4 V

2.0 V

450 kΩ

100 kΩ

Mode Clamps

UCC28600

SLUS646J –NOVEMBER 2005– REVISED JULY 2011

www.ti.com

Details of the functional boxes in the Block Diagram/Typical Application drawing are shown in Figure 5, Figure 6,

Figure 7 and Figure 8. These figures conceptualize how the UCC28600 executes the command of the FB voltage

to have the responses that are shown in Figure 2, Figure 3 and Figure 4. The details of the functional boxes also

conceptualize the various fault detections and responses that are included in the UCC28600. During all modes of

operation, this controller operates in current mode control. This allows the UCC28600 to monitor the FB voltage

to determine and respond to the varying load levels such as heavy, light or ultra-light.

Quasi-resonant mode and DCM occurs for feedback voltages V

between 2.0 V and 4.0 V, respectively. In turn,

the CS voltage is commanded to be between 0.4 V and 0.8 V. A cycle-by-cycle power limit imposes a fixed 0.8-V

limit on the CS voltage. An overcurrent shutdown threshold in the fault logic gives added protection against

high-current, slew-rate shorted winding faults, shown in Figure 8. The power limit feature in the QR DETECT

circuit of Figure 7 adds an offset to the CS signal that is proportional to the line voltage. The power limit feature is

programmed with R

, as shown in the typical applications diagram.

Figure 5. Oscillator Details

Figure 6. Mode Clamp Details