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FAN6755 Fiches technique(PDF) 6 Page - Fairchild Semiconductor

No de pièce FAN6755
Description  Design Guideline to Replace FAN6753 with FAN6755
Download  9 Pages
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Fabricant  FAIRCHILD [Fairchild Semiconductor]
Site Internet  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN6755 Fiches technique(HTML) 6 Page - Fairchild Semiconductor

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AN-6755
APPLICATION NOTE
© 2009 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.2 • 1/14/11
6
Output Driver / Soft Driving
The output stage is a fast totem-pole gate driver capable of
directly driving external MOSFETs. An internal Zener
diode clamps the driver voltage under 18V to protect the
MOSFET gate from over voltage. Due to integrated circuits
that control the switching speed, the external resistor RG
(Figure 13) may not be necessary to reduce switching noise.
Figure 13.
Gate Driver
High / Low Line Compensation in VIN Pin
The conventional pulse-by-pulse current-limiting scheme
has a constant threshold for current limit comparator, which
results in higher power limit for high line voltage.
FAN6755 has a current-limit threshold that decreases as
line voltage increases to make the actual power limit level
almost constant over different line voltages of universal
input range, as shown in Figure 14. In the FAN6755, the
peak-current-limiting threshold is adjusted by the peak
voltage of the VIN pin. When the circuit senses the bulk
capacitor input voltage (VBulk) at the VIN pin and for R1,
R2, and C1 set to 20MΩ (10MΩ +10MΩ), 160KΩ, and
2.2µF; then when VBulk is around 126V and the threshold
voltage for current limit is around 0.83V as measured from
the reference board design.
Figure 14.
VLimit Level vs. VIN
VIN
GND
FAN6755
V
Bulk
R1
R2
C1
Figure 15.
Input Voltage Compensation for
Constant Output Power Limit
Brownout Protection in VIN Pin
Since the VIN pin is connected through a resistive divider
to the bulk capacitor voltage, it can also be used for
brownout protection. If the VIN voltage is less than 0.7V,
the PWM output is shut off. As the VIN voltage reaches
0.9V, the PWM output is enabled again. The hysteresis
window for ON/OFF is around 0.2V. The recommended
values for R1, R2, and C1 are 20MΩ (10MΩ +10MΩ),
160KΩ, and 2.2µF. Using these values in the test board, the
power supply is turned off at 62V (maximum load) and
recovered at 80V.
The VIN-ON and VIN-OFF are calculated by:
2
/
)
R
R
R
9
.
0
(
)
RMS
(
V
2
2
1
ON
-
IN
(8)
2
/
)
R
R
R
7
.
0
(
)
RMS
(
V
2
2
1
OFF
-
IN
(9)
Recovery Function in VIN Pin
The recovery function using the VIN pin is available in the
FAN6755. When VIN is higher than 5.3V, FAN6755 stops
operation, then restarts. Figure 16 shows the external circuit
for secondary-side output OVP. If output voltage (VO) is
higher than the Zener diode voltage (VZ), the VIN pin is
pulled HIGH and the FAN6755 is in recovery.
Figure 16.
External Circuit for Second OVP


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