Datasheet

A
Hysteresis 17 V
R 8.5M
2 A 2 A
= = = W
m m
A
R 3 2.87M 8.61M= ´ W = W
A
B
IN _MIN
1.4 V R
1.4 V 8.61M
R 135.8k
V 0.75 2 1.4 V 85 V 0.75 2 1.4 V
´
´ W
= = = W
´ - ´ -
B
R 133k= W
( )
( )
2
IN _MIN
2
IN _MIN
OUT
MIN
OUT MAX
V 2
85 V 2
V 1
0.92 85 V 1
V
390 V
f 39.2kHz
P L1 300 W 390 H
æ ö
´
æ ö
´
ç ÷
h´ -
´ -
ç ÷
ç ÷ ç ÷
è ø è ø
= = =
´ ´ m
IN _MIN
T
MIN
V 2
85 V 2
133k 1
133k 1
Vout
390 V
R 121k
4 s 4 s
4.85 V f 4.85 V 39.2kHz
V V
æ ö
´
æ ö
´
ç ÷
W -
W -
ç ÷
ç ÷ ç ÷
è ø è ø
= = » W
m m
´ ´ ´ ´
MAX
T
133k 133k
f 550kHz
2 s R 2 s 121k
W W
= = »
m ´ m ´ W
UCC28063
www.ti.com SLUSAO7 –SEPTEMBER 2011
Brownout Protection
Resistor R
A
and R
B
are selected to activate brownout protection at ~75% of the specified minimum-operating
input voltage. Resistor R
A
programs the brownout hysteresis comparator, which is selected to provide 17 V (~12
V
RMS
) of hysteresis. Calculations for R
A
and R
B
are shown in Equation 44 through Equation 47.
(44)
To meet voltage requirements, three 2.87-MΩ resistors were used in series for R
A
.
(45)
(46)
Select a standard value for R
B
.
(47)
In this design example, brownout becomes active (shuts down PFC) when the input drops below 66 V
RMS
for
longer than 440 ms and deactivates (restarts with a full soft start) when the input reaches 78 V
RMS
.
Converter Timing
The maximum on-time T
ON
depends on f
MIN
as determined by Equation 48. To ensure proper operation, the
timing must be set based on the highest boost inductance (L1
MAX
) and output power (P
OUT
). In this design
example, the boost inductor could be as high as 390 µH. Calculate the timing resistor R
T
as shown in
Equation 49.
(48)
(49)
This result sets the maximum frequency clamp (f
MAX
), as shown in Equation 50, which improves efficiency at light
load.
(50)
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