Datasheet

S Q
QR
REF
D Q
Q
CLR
SET
Power−Up Reset
Thermal
Shutdown
+
+
1.25 V
+
SS/DIS
RUN
3 CS
OVR_T
LINE_OVP
(QR Detect)
LOAD_OVP
(QR Detect)
UVLO
REF_OK
Burst
REF
(5 V)
FB
7
FB
CS
SS_OVR
STATUS8
UCC28600
BURST
Over−Current
Shutdown
20 kΩ
Fault Logic
0.5 V/0.7 V
0.5 V/1.4 V
UCC28600
SLUS646J –NOVEMBER 2005– REVISED JULY 2011 www.ti.com
Figure 8. Fault Logic Details
Quasi-Resonant / DCM Control
Quasi-resonant (QR) and DCM operation occur for feedback voltages V
FB
between 2.0 V and 4.0 V. In turn, the
peak CS voltage is commanded to be between 0.4 V and 0.8 V. During this control mode, the rising edge of OUT
always occurs at the valley of the resonant ring after demagnetization. Resonant valley switching is an integral
part of QR operation. Resonant valley switching is also imposed if the system operates at the maximum
switching frequency clamp. In other words, the frequency varies in DCM operation in order to have the switching
event occur on the first resonant valley that occurs after a 7.7-μs (130-kHz) interval. Notice that the CS pin has
an internal dependent current source, 1/2 I
LINE
. This current source is part of the cycle-by-cycle power limit
function that is discussed in the Protection Features section.
Frequency Foldback Mode Control
Frequency foldback mode uses elements of the FAULT LOGIC, shown in Figure 8 and the mode clamp circuit,
shown in Figure 6. At the minimum operating frequency, the internal oscillator sawtooth waveform has a peak of
4.0 V and a valley of 0.1 V. When the FB voltage is between 2.0 V and 1.4 V, the FB_CL signal in Figure 6
commands the oscillator in a voltage controlled oscillator (VCO) mode by clamping the peak oscillator voltage.
The additional clamps in the OSCILLATOR restrict VCO operation between 40 kHz and 130 kHz. The FB_CL
voltage is reflected to the modulator comparator effectively clamping the reflected CS command to 0.4 V.
Green-Mode Control
Green mode uses element of the fault logic, shown in Figure 8 and the mode clamps circuit, shown in Figure 6.
The OSC_CL signal clamps the Green-mode operating frequency at 40 kHz. Thus, when the FB voltage is
between 1.4 V and 0.5 V, the controller is commanding an excess of energy to be transferred to the load which
in turn, drives the error higher and FB lower. When FB reaches 0.5 V, OUT pulses are terminated and do not
resume until FB reaches 0.7 V. In this mode, the converter operates in hysteretic control with the OUT pulse
terminated at a fixed CS voltage level of 0.4 V. The power limit offset is turned OFF during Green mode and it
returns to ON when FB is above 1.4 V, as depicted in Figure 8. Green mode reduces the average switching
frequency in order to minimize switching losses and increase the efficiency at light load conditions.
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