UCC28704 datasheet(16/44 Pages) TI1

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IOCC

Output Current IO

VOCV

Fixed Cable Compensation

Vo at Cable-End

VOCV x (1+ 0.06 x IO/IOCC)

Vo (Board-End) =

VO_CCUV at

Board-End

VO_CCUV at

Cable-End

DMAG

OUT

tON

tDMAG

tSW

IPP

ISP x NS/NP

UCC28704

SLUSCA8A – FEBRUARY 2016 – REVISED FEBRUARY 2016

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Feature Description (continued)

7.3.4 Primary-Side Constant Current (CC) Regulation

Timing information at the VS pin and current information at the CS pin allow accurate regulation of the secondary

average current. The control law dictates that as power is increased in CV regulation and approaching CC

regulation the primary-peak current is at IPP(max). Referring to Figure 16 below, the primary peak current (IPP),

turns-ratio (NS/NP), secondary demagnetization time (tDMAG), and switching period (tSW) determine the secondary

average output current. Ignoring leakage inductance effects, the average output current is given by Equation 7.

By regulating the secondary rectifier conduction duty cycle, the output average current is constant for given

IPP and transformer turns-ratio. When the load increases, the secondary-side rectifier conduction duty cycle keep

increasing. Once it reaches preset value of 0.475, the converter switching frequency is then reduced to maintain

0.475 secondary-side duty cycle. Therefore, the output current is kept constant. Because the current is kept

constant, the increasing load results in lower output voltage. Converter can shut down in this condition if the

output voltage drops below CCUV protection level, or UCC28704 VDD drops below its UVLO turn-off threshold.

Figure 16. Transformer Currents

(7)

As shown in Figure 17 below, CV mode operation is from IO = 0 to IOCC; at IO = IOCC, the operation enters CC

mode and VO starts to drop as the load resistance becomes further lower while IO is maintained at IOCC until Vo

reaches the CCUV threshold. Details of the CCUV operation are given in Constant Current Under-Voltage

Protection. Figure 17 shows the output at board-end and at cable-end. The cable compensation nominally

compensates 300 mV for a 5V-output at the IOCC level.

Figure 17. Typical Target Output V-I Characteristic

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