Datasheet

2
2
1 2
0 9
dischrg
VINS
BROWNOUT _th
VINS
VINS
AC min
VINS VINS
t
C
VINS
R ln
R
. V
R R
-
=
æ ö
ç ÷
ç ÷
ç ÷
ç ÷
+
è ø
(3)
1
2
dischrg
LINE
t N
f
=
(4)
Output Over-Voltage Protection (OVP)
Open Loop Protection/Standby (OLP/Standby)
ISENSE Open-Pin Protection (ISOP)
UCC28019A
SLUS828B – DECEMBER 2008 – REVISED APRIL 2009 .................................................................................................................................................. www.ti.com
The filter capacitor, C
VINS
, has two functions. First, to attenuate the voltage ripple to levels between the enable
and brown-out threshold to prevent ripple on VINS from falsely triggering IBOP when the converter is operating
at low line. Second, C
VINS
delays the brown-out protection operation for a desired number of line-half-cycle
periods while still having a good response to an actual brown-out event.
The capacitor is chosen so that it will discharge to the VINS
BROWNOUT_th
level after a delay of N number of line
-cycles to accommodate ac-line dropout ride-through requirements.
Where,
and V
ACmin
is the lowest normal operating rms input voltage.
V
OUT(OVP)
is the output voltage exceeding 5% of the rated value, causing VSENSE to exceed a 5.25-V threshold
(5-V reference voltage + 5%), V
OVP
. The normal control loop is bypassed and the GATE output is disabled until
VSENSE falls below 5.25 V. V
OUT(OVP)
is 420 V in a system with a 400-V rated output, for example.
If the output voltage feedback components were to fail and disconnect (open loop) the signal from the VSENSE
input, then it is likely that the voltage error amp would increase the GATE output to maximum duty cycle. To
prevent this, an internal pull-down forces VSENSE low. If the output voltage falls below 16% of its rated voltage,
causing VSENSE to fall below 0.8 V, the device is put in standby, a state where the PWM switching is halted and
the device is still on but draws standby current below 2.9 mA. This shutdown feature also gives the designer the
option of pulling VSENSE low with an external switch.
If the current feedback components were to fail and disconnect (open loop) the signal to the ISENSE input, then
it is likely that the PWM stage would increase the GATE output to maximum duty cycle. To prevent this, an
internal pull-up source drives ISENSE above 0.1 V so that a detector forces a state where the PWM switching is
halted and the device is still on but draws standby current below 2.9 mA. This shutdown feature avoids continual
operation in OVP and severely distorted input current.
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