Datasheet

0.5V0.7V 1.4V 2.0V 4.0V 5.0V

GreenMode-ON,

Burst-OFF

Green Mode-OFF,

Burst-ON

Burst

Hysteresis

GreenMode

Hysteresis

GreenMode

FFM

40kHz < f

< 130kHz

Control Range Limit

InternalReference

QR Mode or DCM Mode

UCC28600

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SLUS646J –NOVEMBER 2005– REVISED JULY 2011

APPLICATION INFORMATION

Functional Description

The UCC28600 is a multi-mode controller, as illustrated in Figure 3 and Figure 4. 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 Figure 2. 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

becomes less than V

for the

first time after UVLO

. The soft-start state cannot be recovered until after passing UVLO

OFF

, and then, UVLO

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

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.

Figure 2. Mode Control with FB Pin Voltage