NCP1910
http://onsemi.com
22
Line Brown-Out Protection
EMI
Filter
Ac line
R
Q
S
L
reset
reset
reset
BO
Vdd
V
in
R
LBOU
R
LBOL
C
in
R
SENSE
C
LBO
PFC_BO
V
LBOT
I
LBOH
LBO comp.
V
LBOcomp
t
LBO(blank)
t
LBO(window)
V
LBO(clamp)
LBO
Figure 46. The Line Brown-Out Configuration
As shown in Figure 46, the Line Brown-Out pin
(represented LBO pin) as receives a portion of the input
voltage (V
in
). As V
in
is a rectified sinusoid, a capacitor must
integrate the ac line ripple so that a voltage proportional to
the average value of V
in
is applied to the brown-out pin.
The main function of the LBO block is to detect too low
input voltage conditions. A 7 mA current source lowers the
LBO pin voltage when a brown-out condition is detected.
This is for hysteresis purpose as required by this function.
In nominal operation, the voltage applied to LBO pin must
be above the internal reference voltage, V
LBOT
(1 V
typically). In this case, the output of the LBO comparator
V
LBOcomp
is low.
Conversely, if V
LBO
goes below 1 V, V
LBOcomp
turns high
and a 980 mV voltage source, V
LBO(clamp)
, is connected to
the LBO pin to maintain the pin level near 1 V. Then a 50 ms
blanking delay, t
LBO(blank)
, is activated during which no
fault is detected. The main goal of the 50 ms lag is to help
meet the hold-up requirements. In case of a short mains
interruption, no fault is detected and hence, both PFC and
LLC keep operating. In addition, LBO pin being kept at
980 mV, there is almost no extra delay between the line
recovery and the occurrence of a proper voltage applied to
LBO pin, that otherwise would exist because of the large
capacitor typically placed between LBO pin and ground to
filter the input voltage ripple. As a result, the NCP1910
effectively “blanks” any mains interruption that is shorter
than 25 ms (minimum guaranteed value of the 50 ms timer).
At the end of this blanking delay (t
LBO(blank)
), another
timer is activated that sets a 50 ms window during which a
fault can be detected. This is the role of the t
LBO(window)
in
Figure 46:
• If V
LBOcomp
is high during the second 50 ms delay
(t
LBO(window)
), a line brown-out condition is confirmed
and PFC_BO signal is asserted high.
• If V
LBOcomp
remains low for the duration of the
t
LBO(window)
, no fault is detected.
When the PFC_BO signal is high:
• The PFC driver is disabled, and the V
CTRL
pin is
grounded to recover operation with a soft-start when
the fault has gone.
• The V
LBO(clamp)
voltage source is removed from LBO
pin.
• The I
LBOH
current source (7 mA typically) is enabled
that lowers the LBO pin voltage for hysteresis purpose.
At startup, a pnp transistor ensures that the LBO pin
voltage remains below when: V
CC
< UVLO or ON/OFF pin
is released open or UVP or Thermal Shutdown. This is to
guarantee that the circuit starts operation in the right state,
which is “PFC_BO” high. When the NCP1910 is ready to
work, the pnp transistor turns off and the circuit enables the
I
LBOH
.
Also, I
LBOH
is enabled whenever the part is in off mode,
but at startup, I
LBOH
is disabled until V
CC
reaches V
CC(on)
.
Line Brown-Out Network Calculation
If the line brown-out network is connected to the voltage
after bridge diode, the monitored voltage can be very
different depending on the phase:
• Before operation, the PFC stage is off and the input
bridge acts as a peak detector. As a consequence, the
input voltage is approximately flat and nearly equates
