FAN9612 Fairchild Semiconductor, FAN9612 Datasheet - Page 16

• Manufacturer Part Number: FAN9612
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN9611 / FAN9612 • Rev. 1.1.4
10. Gate Drive Outputs (DRV1; DRV2)
High-current driver outputs DRV1 and DRV2 have the
capability to sink a minimum of 2A and source 1A. Due
to the low impedance of these drivers, the 1A source
current must be actively limited by an external gate
resistor. The minimum external gate resistance is:
To take advantage of the higher sink current capability
of the drivers, the gate resistor can be bypassed by a
small diode to facilitate faster turn-off of the power
MOSFETs. Traditional fast turn-off circuit using a PNP
transistor instead of a simple bypass diode can be
considered as well.
It is also imperative that the inductance of the gate drive
loop is minimized to avoid excessive ringing. If optimum
layout is not possible or the controller is placed on a
daughter card, it is recommended to use an external
driver circuit located near the gate and source terminals
of the boost MOSFET transistors. Small gate charge
power MOSFETs can be driven by a single 1A gate
driver, such as the FAN3111C; while higher gate charge
devices might require higher gate drive current capable
devices, such as the single-2A FAN3100C or the dual-
2A FAN3227C family of drivers.
11. MillerDrive™ Gate Drive Technology
FAN9611/12 output stage incorporates the MillerDrive™
architecture shown in Figure 25. It is a combination of
bipolar and MOS devices which are capable of providing
large currents over a wide range of supply voltage and
temperature variations. The bipolar devices carry the
bulk of the current as OUT swings between 1/3 to 2/3
V
or low rail.
AC input voltage
waveform sensed
at VIN pin
R
DD
GATE
and the MOS devices pull the output to the high

V
50Hz
1
DD
A
50Hz
50Hz
10ms
Figure 24. Input Voltage Sensing Waveforms
forced reset after 32ms
DC
(3)
16
The purpose of the MillerDrive™ architecture is to
speed switching by providing high current during the
Miller plateau region when the gate-drain capacitance of
the MOSFET is being charged or discharged as part of
the turn-on / turn-off process.
The output pin slew rate is determined by V
and the load on the output. It is not user adjustable, but
if a slower rise or fall time at the MOSFET gate is
needed, a series resistor can be added.
Figure 25. Current-Sense Protection Circuits
forced reset after 32ms
AC “RMS value” used
for feedforward and
protection functions
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DD
voltage