SLUS646J
–
NOVEMBER 2005
–
REVISED JULY 2011
APPLICATION INFORMATION
Functional Description
The UCC28600 is a multi-mode controller, as illustrated in
and
The mode of operation
depends upon line and load conditions. Under all modes of operation, the UCC28600 terminates the OUT = HI
signal based on the switch current. Thus, the UCC28600 always operates in current mode control so that the
power MOSFET current is always limited.
Under normal operating conditions, the FB pin commands the operating mode of the UCC28600 at the voltage
thresholds shown in
Soft-start and fault responses are the exception. Soft-start mode hard-switch
controls the converter at 40 kHz. The soft-start mode is latched-OFF when V
FB
becomes less than V
SS
for the
first time after UVLO
ON
. The soft-start state cannot be recovered until after passing UVLO
OFF
, and then, UVLO
ON
.
At normal rated operating loads (from 100% to approximately 30% full rated power) the UCC28600 controls the
converter in quasi-resonant mode (QRM) or discontinuous conduction mode (DCM), where DCM operation is at
the clamped maximum switching frequency (130 kHz). For loads that are between approximately 30% and 10%
full rated power, the converter operates in frequency foldback mode (FFM), where the peak switch current is
constant and the output voltage is regulated by modulating the switching frequency for a given and fixed V
IN
.
Effectively, operation in FFM results in the application of constant volt-seconds to the flyback transformer each
switching cycle. Voltage regulation in FFM is achieved by varying the switching frequency in the range from 130
kHz to 40 kHz. For extremely light loads (below approximately 10% full rated power), the converter is controlled
using bursts of 40-kHz pulses. Keep in mind that the aforementioned boundaries of steady-state operation are
approximate because they are subject to converter design parameters.
Refer to the typical applications block diagram for the electrical connections to implement the features.
V
FB
Control Range Limit
40kHz < f
S
< 130kHz
Green Mode-OFF,
FFM
GreenMode
GreenMode
Hysteresis
Burst
Hysteresis
QR Mode or DCM Mode
InternalReference
GreenMode-ON,
Burst-OFF
Burst-ON
V
FB
0V
0.5V
0.7V
1.4V
2.0V
4.0V
5.0V
Figure 2. Mode Control with FB Pin Voltage
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©
2005–2011, Texas Instruments Incorporated
11
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