FAN6756 Fairchild Semiconductor, FAN6756 Datasheet - Page 11

• Manufacturer Part Number: FAN6756
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN6756 • Rev. 1.0.4
(2.0V) and V
ZDCR2
switching loss more by increasing the energy delivered
to the load per switching operation, which eventually
reduces the total switching for a given load condition.
FAN6756 exits Standby Mode after more than 104
consecutive switching pulses in deep Burst Mode. Once
FAN6756 exits Standby Mode, the feedback impedance
is modulated to 8.5k to keep original loop response.
FAN6756 also exits Standby Mode when opto-coupler
transistor current is virtually zero and V
0.75V while switching is suspended in deep burst mode.
Figure 27. Deep Burst Operation in Standby Mode
Sensed Current
Figure 26. Feedback Impedance Modulation
Comparator
Signal
(2.6V) in deep Burst Mode. This reduces the
PWM
Figure 25. Entering Standby Mode
+
-
FB-ZDCR1
FAN6756
V
1R
DD
3R
(2.1V) to V
Impedance
Modulator
FB-ZDC2
5.4V
Z
(2.55V) and V
FB
FB
2
FB
C
rises above
FB
V
FB
FB-
11
V
V
High-Voltage Startup and Line Sensing
The HV pin is typically connected to the AC line input
through two external diodes and one resistor (R
shown in Figure 28. When the AC line voltage is
applied, the V
voltage through the diodes and resistor. After V
voltage reaches the turn-on threshold voltage (V
the startup circuit charging V
and V
transformer.
operating until V
time with a given AC line input voltage is given as:
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 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 power consumption in
light-load condition.
Based on the detected line voltage, brown-in and
brownout thresholds are determined as:
Since the internal resistor (R
divider is much smaller than R
simply given as s function of R
Note:
8.
t
STARTUP
BROWN
BROWNOUT
V
the sensed line voltage satisfies Equation (2), as
shown in Figure 5.
DD
DD
-
must be larger than V
IN

is supplied by the auxiliary winding of the
R
Figure 28. Startup Circuit
(RMS)
(RMS)
HV
DD
Once
DD

hold-up capacitor is charged by the line
C


DD
drops below 6.5V (V
R
200k
R
200k
HV
HV

FAN6756
ln
LS

V
V
), as shown in Figure 28. The

AC IN
V
AC
AC

DD
V

2
HV
DD-AC
ON

LS
AC IN
2
OFF
capacitor is switched off

=1.6kΩ) of the voltage
.
HV
2 2

starts,

, the thresholds are
to start, even though

2 2


V
UVLO
DD ON
www.fairchildsemi.com
it

). IC startup
continues
HV
DD-ON
(1)
), as
(2)
(3)
HV
DD
),
)