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VCC Under-Voltage Lockout (UVLO)
+
+
VCC
ON
10.5V
S Q
QR
C
DECOUPLE
VCC
Auxilary Supply
GND
VCC
OFF
9.5V
UVLO
Input Brown-Out Protection (IBOP)
Input Brown-Out Protection (IBOP)
+
+
VIN
ENABLE_th
1.5 V
S Q
QR
VIN
BROWNOUT_th
0.82 V
C
VINS
Rectified AC Line
R
VINS1
R
VINS2
VINS
5V
20k
IBOP
C
IN
UCC28019
SLUS755B – APRIL 2007 – REVISED DECEMBER 2007
During startup, UVLO keeps the device in the off state until VCC rises above the 10.5-V enable threshold,
VCC
ON
. With a typical 1 V of hysteresis on UVLO to eliminate noise, the device turns off when VCC drops to the
9.5-V disable threshold, VCC
OFF
.
Figure 22. UVLO
The VINS, (sensed input line voltage), input provides a means for the designer to set the desired mains RMS
voltage level at which the PFC pre-regulator should start-up, V
AC(turnon)
, as well as the desired mains RMS level
at which it should shut down, V
AC(turnoff)
. This prevents unwanted sustained system operation at or below a
“ brown-out ” voltage, where excessive line current could overheat components. In addition, because VCC bias is
not derived directly from the line voltage, IBOP protects the circuit from low line conditions that may not trigger
the VCC UVLO turn-off.
Figure 23. Input Brown-Out Protection (IBOP)
Input line voltage is sensed directly from the rectified ac mains voltage through a resistor divider filter network
providing a scaled and filtered value at the VINS input. IBOP puts the device in standby mode when VINS falls
(high-to-low) below 0.8 V, VINS
BROWNOUT_th
. The device comes out of standby when VINS rises (low-to-high)
above 1.5 V, VINS
ENABLE_th
. I
VINS_0 V
, bias current sourced from VINS, is less than 0.1 µ A. With a bias current
this low, there is little concern for any set-point error caused by this current flowing through the sensing network.
The highest reasonable value resistance for this network should be chosen to minimize power dissipation,
especially in applications requiring low standby power. Be aware that higher resistance values are more
susceptible to noise pickup, but low noise PCB layout techniques can help mitigate this. Also, depending on the
resistor type used and its voltage rating, R
VINS1
should be implemented with multiple resistors in series to reduce
voltage stresses.
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