FAN6749 Fairchild Semiconductor, FAN6749 Datasheet - Page 12
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FAN6749
Manufacturer Part Number
FAN6749
Description
The FAN6749 highly integrated PWM controller enhances the performance of flyback converters
Manufacturer
Fairchild Semiconductor
Datasheet
1.FAN6749.pdf
(16 pages)
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Part Number:
FAN6749MLM
Manufacturer:
FAIRCHILD/仙童
Quantity:
20 000
© 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
),
;