UCC28056CDBVT datasheet(25/55 Pages) TI2

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BstOffb

BstRunbn +Ovp1Fltb

BstRunb & Ovp1Fltbn

Faultbn

Fstopb

IinitTOb

SSCntb = 4

SSCntb=1

SoftOFFb

SoftONb

Runb

Stopb

RDGRdb

UCC28056

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SLUSD37E – OCTOBER 2017 – REVISED NOVEMBER 2019

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8.4 Controller Functional Modes

8.4.1 Burst Mode Operation

The UCC28056 controller provides leading light-load efficiency and standby power by implementing Burst mode

of operation with the following key features:

1. Power during burst is controlled to be approximately 11% of maximum output power for UCC28056/C and

16% of maximum power for UCC28056A/B for all Line voltage levels.

2. During the Burst OFF period, the current consumption of UCC28056 drops to less than 132 μA.

3. The TON pulse width is ramped up over the first four cycles, and ramped down over the last four cycles of

each Burst-on period. This Soft-ON/OFF scheme ramps the Line current at the edge of each Burst ON

period to limit audible noise and disturbance of the EMI filter.

Two comparator thresholds applied to the COMP pin voltage provide Burst Mode Operation. Switching halts after

four soft-OFF cycles when the COMP pin voltage falls below the VBstFall threshold. Switching resumes with four

Soft-ON cycles, when the COMP pin voltage rises above the VBstRise threshold. The average voltage of these two

thresholds represents approximately 11% VCOMaxfor UCC28056/C and 16% VCOMax for UCC28056A/B. The

power delivered during Burst ON is approximately 11% of maximum input power for UCC28056/C.

Figure 25. Fault and Burst Mode State Diagram

8.4.2 Soft Start

During Stopb state operation, an internal resistor (RCODisch) is connected between the COMP and GND pins to

discharge the external compensation network. Start-up transitions through the BstOffb state and switching

commence only after the COMP pin voltage rises above the VBstRise threshold. Switching therefore always starts

with the power demand at 12.5% of its maximum value. The Soft-ON feature ensures that the on-time period

ramps up over the first four switching cycles to the demanded value. These features limit audible noise at start-

up.

UCC28056CDBVT Datasheet(PDF) 25 Page - Texas Instruments