UCC28600
SLUS646J –NOVEMBER 2005–REVISED JULY 2011
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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 VFB 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 RPL, as shown in the typical applications diagram.
REF
Oscillator
OSC Peak
Comparator
+
4.0V
SS_OVR
S
R
Q
Q
QR_DONE
OSC_CL
CLK
+
130 kHz OSC
Clamp
Comparator
0.1V
+
OSC Valley
Comparator
RUN
Figure 5. Oscillator Details
Mode Clamps
1.4 V
OSC_CL
+
+
450 kΩ
100 kΩ
100 kΩ
450 kΩ
FB
2.0 V
FB_CL
+
Figure 6. Mode Clamp Details
14
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