UCC28070-Q1
SLUSA71A –JULY 2010–REVISED JUNE 2011
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This IIN(pk) value represents the combined average current through the boost inductors at the peak of the line
voltage. Each inductor current is detected and scaled by a current-sense transformer (CT). Assuming equal
currents through each interleaved phase, the signal voltage at each current sense input pin (CSA and CSB) is
developed across a sense resistor selected to generate ~3 V based on (1/2) x IIN(pk) x RS/NCT, where RS is the
current sense resistor and NCT is the CT turns-ratio.
IIMO is then calculated at that same lowest maximum-power point, as
0.76V 5V -1V
)(
0.398
(
)
IIMO(max) =17mA´
=130mA
(16)
(17)
RIMO is selected such that:
1
æ ö
ç ÷
RS
RIMO ´ IIMO(max)
=
´ IIN( pk ) ´
2
NCT
è ø
Therefore:
æ 1
ç
ö
÷
ø
æ ö
ç ÷
´ IIN( pk ) ´ RS
2
è ø
è
RIMO
=
N ´ I
(
)
CT
IMO(max)
(18)
At the increasing side of the level-1 to level-2 threshold, it should be noted that the IMO current would allow
higher input currents at low-line:
1.0V 5V -1V
( )(
0.398
)
IIMO( L1-L2 ) =17mA´
=171mA
(19)
However, this current may easily be limited by the programmable peak current limiting (PKLMT) feature of the
UCC28070 if required by the power stage design.
The same procedure can be used to find the lowest and highest input power limits at each of the QVFF level
transition thresholds. At higher line voltages, where the average current with inductor ripple is traditionally below
the PKLMT threshold, the full variation of maximum input power will be seen, but the input currents will inherently
be below the maximum acceptable current levels of the power stage.
The performance of the multiplier in the UCC28070 has been significantly enhanced when compared to previous
generation PFC controllers, with high linearity and accuracy over most of the input ranges. The accuracy is at its
worst as VVAO approaches 1 V because the error of the (VVAO-1) subtraction increases and begins to distort the
IMO reference current to a greater degree.
24
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