FAN6749 Fairchild Semiconductor, FAN6749 Datasheet - Page 12

• Manufacturer Part Number: FAN6749
• Description: The FAN6749 highly integrated PWM controller enhances the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2012 Fairchild Semiconductor Corporation
FAN6749 • Rev. 1.0.0
The HV pin detects the AC line voltage using a switched
voltage divider that consists of external resistor (R
and internal resistor (R
internal line-sensing circuit detects line voltage using a
sampling circuit and a peak-detection circuit. Since the
voltage divider causes power consumption when it is
switched on, the switching is driven by a signal with a
very narrow pulse width to minimize power loss. The
sampling frequency is adaptively changed according to
the load condition to minimize the power consumption in
the light-load condition.
Based on the detected line voltage, brown-in and brown-
out thresholds are determined as:
Since the internal resistor (R
divider is much smaller than R
given as s function of R
Note that V
even though sensed line voltage satisfies Equation 2.
High/Low Line Compensation for Constant
Power Limit
FAN6749 has cycle-by-cycle current limit, as shown in
Figure 29, which limits the maximum input power with a
given input voltage. If the output consumes beyond this
maximum power, the dropping output voltage triggers
the overload protection.
As shown in Figure 29, the high/low line compensation
block adjusts the current limit level, V
line voltage. Figure 30 shows how the cycle-by-cycle
current-limit level changes with the line voltage for
different R
power limit regardless of line voltage, the cycle-by-cycle
current limit level, V
increases. The current-limit level is also proportional to
the R
using different R
V
V
BROWN
BROWN
HV
resistor value and power limit level can be tuned
-
-
OUT
IN
HV
Figure 28. Startup Circuit
DD
(RMS)
(RMS)
resistors. To maintain the constant output
must be larger than V
HV
resistors.


200k
200k
R
R
LIMIT
HV
LS
HV
HV
), as shown in Figure 28. The
.
, decreases as line voltage


V
V
AC
AC
LS

=1.62kΩ) of the voltage
2

2
OFF
ON
HV
, the thresholds are
LIMIT
DD-AC
, based on the
to start up,
(2)
(3)
HV
)
12
Figure 29.
Two-Level Over-Current Protection (OCP)
Other than cycle-by-cycle current limiting, FAN6749
applies another threshold voltage, V
sense. As shown in Figure 29, when peak of V
exceeds V
over-current
protection is designed for applications with surge-current
outputs. If a peak load is present for less than t
controller operates as usual. If the peak load continues
longer than t
converter from overload condition.
Like V
compensation block to maintain a constant over-current
protection level, regardless of line voltage. Figure 32
shows how V
different R
When OCP is triggered, it is recommended to have
V
switching frequency is fixed at 65kHz. If OCP is
triggered in the frequency-reduction region (V
output current is reduced due to lower switching
frequency. It causes the difference of OCP-triggering
point between high and low AC input voltages.
FB
>V
FB-N
Figure 30. Current Limit vs. Line Voltage
LIMIT
HV
OCP
for whole AC input range. V
, the V
D-OCP
resistors.
Cycle-by-Cycle Current-Limit Circuit
protection
OCP
at each pulse for a period of time, t
, GATE output is stopped to protect the
changes with the line voltage with
OCP
is adjusted by high/low line
(OCP)
is
FB
OCP
triggered.
>V
, for current
FB-N
www.fairchildsemi.com
FB
D-OCP
ensures
<V
D-OCP
SENSE
FB-N
, the
This
),
;