Datasheet

Brownout Protection
R =
A
» W3M
Hysteresis
7 Am
=
21V
7 Am
(32)
R =
B
» W47k
1.4V R´
A
V 0.75 1.4V´ Ö -
IN_MIN
2
=
1.4V ´ W3M
85V 0.75 1.4V´ Ö -2
(33)
Converter Timing
f =
MIN
=39.2kHz
P L1
OUT MAX
´
=
h (V )´
IN_MIN
2
V Ö
IN_MIN
´ 2
V
OUT
1 -
300W 390 Hm´
0.92 (85V)´
2
85V Ö´ 2
390V
1 -
(34)
R =
TSET
» W121k
4.85V 4 s fm´ ´
MIN
=
133kW
V Ö
IN_MIN
´
2
V
OUT
1 -
4.85V 4 s 39.2kHzm´ ´
133kW
85V Ö´
2
390V
1 -
(35)
f =
MAX
» 550kHz
2 s Rm ´
T
=
133kW
2 s 121km W´
133kW
(36)
Programming V
OUT
R =3MW
C
(37)
V =6V
REF
(38)
R =
D
» W47k
(V V )-
OUT REF
=
V R
REF C
´
(390V 6V)-
6V 3M´ W
(39)
V =6.45V
OVP
=418V
R
D
=6.45V
R +R
C D
47kW
3M +47kW W
(40)
UCC28061
SLUS837A – JUNE 2008 – REVISED JULY 2009 .............................................................................................................................................................. www.ti.com
Resistor R
A
and R
B
are selected to activate brownout protection at 75% of the specified minimum operated input
voltage. Resistor R
A
programs the brownout hysteresis comparator, which was selected to provide 21 V of
hysteresis. R
A
and R
B
are shown in Equation 32 and Equation 33 .
In this design example, brownout becomes active when the input drops below 64 V
RMS
and deactivates when the
input reaches 79 V
RMS
.
Select the timing resistor, R
TSET
, for the correct on-time (T
ON
) based on K
TL
, as shown in Equation 34 . To ensure
proper operation, the timing must be set based on the highest boost inductance (L1
MAX
). In this design example,
the boost inductor could be as high as 390 µ H, based on line and load conditions, as shown in Equation 35 .
This result sets the maximum frequency clamp (f
MAX
), as shown in Equation 36 , which improves efficiency at light
load.
Resistor R
C
is selected to minimize error because of VSENSE input bias current and minimize loading on the
power line when the PFC is disabled. Construct resistor R
C
from two or more resistors in series to meet
high-voltage requirements. R
C
was also selected to be of a similar value of R
A
and R
E
to simplify the bill of
materials and reduce design costs.
Based on the resistor values shown in Equation 37 to Equation 39 , the primary output over-voltage protection
threshold should be as shown in Equation 40 :
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