Datasheet

AC _ OK AC _ BO BOHYS
BOHYS
A
BOHYS BODET
2(V V ) V
V
R 1
I V
æ ö
æ ö
ç ÷
ç ÷
ç ÷
è ø
è ø
- -
= +
A
B
AC _ BO LOSS
BODET
R
R
2V V
1
V
=
æ ö
-
ç ÷
-
ç ÷
è ø
BODET LOSS
A
AC _ BO
B
V V
R
V 1
R
2 2
æ ö
= + +
ç ÷
è ø
BOHYS
A BOHYS
AC _ OK AC _ BO
BOHYS
BODET
R I
V
V V
2
V
1
V
2
æ ö
ç ÷
+
ç ÷
è ø
= + +
UCC28063
SLUSAO7 – SEPTEMBER 2011 www.ti.com
Brownout Protection
As the power line RMS voltage decreases, RMS input current must increase to maintain a constant output
voltage for a specific load. Brownout protection helps prevent excess system thermal stress (due to the higher
RMS input current) from exceeding a safe operating level. Power-line voltage is sensed at VINAC. When the
VINAC fails to exceed the brownout threshold for the brownout filter time, a brownout condition is detected and
both gate drive outputs are turned off. During brownout, COMP is actively pulled low and a soft-start condition is
initiated. Hysteresis is built into the brownout detection circuit to avoid chatter around the threshold. When VINAC
rises above the brownout threshold, the power stage soft-starts as COMP rises with controlled current.
The brownout detection threshold and its hysteresis are set by the voltage-divider ratio and resistor values.
Brownout protection is based on VINAC peak voltage; the threshold and hysteresis are also based on the line
peak voltage. Major hysteresis is provided by a 2-μA current-sink (I
BOHYS
) enabled whenever VINAC falls below
the brownout detection threshold. Minor hysteresis is also present in the form of a 50-mV offset (V
BOHYS
)
between the VINAC detection and clear thresholds. The peak VINAC voltage can be easily translated into an
RMS value. Example resistor values for the voltage divider are 8.61 MΩ ±1% from the rectified input voltage to
VINAC and 133 kΩ ±1% from VINAC to ground. These resistors set the typical thresholds for RMS line voltages,
as shown in Table 1.
Table 1. Brownout Thresholds (for conditions stated in the text)
THRESHOLD AC-LINE VOLTAGE (RMS)
Falling 66 V
Rising 78 V
Equation 8 and Equation 9 can be used to calculate the VINAC divider-resistor values based on desired
brownout detection and brownout clear voltage levels. V
AC_OK
is the desired RMS turn-on voltage, V
AC_BO
is the
desired RMS turn-off brownout voltage, and V
LOSS
is total series voltage drop due to wiring, EMI-filter, and
bridge-rectifier impedances at V
AC_BO
. V
BODET
, V
BOHYS
and I
BOHYS
are found in the data-tables of this datasheet.
(8)
(9)
Once standard values for the VINAC divider-resistors R
A
and R
B
are selected, the actual turn-on and brownout
threshold RMS voltages for the ac-line can be back-calculated with Equation 10 and Equation 11:
(10)
(11)
An example of the timing for the brownout function is illustrated in Figure 31.
For a quick estimation of the turn-on and brownout voltages, simplify the foregoing equations by setting the V
LOSS
and V
BOHYS
terms to zero.
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