FAN3121T Fairchild Semiconductor, FAN3121T Datasheet - Page 14

• Manufacturer Part Number: FAN3121T
• Description: The FAN3121 and FAN3122 MOSFET drivers are designed to drive N-channel enhancement MOSFETs in low-side switching applications by providing high peak current pulses
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN3121 / FAN3122 • Rev. 1.0.0
Applications Information
The FAN3121 and FAN3122 family offers versions in
either TTL or CMOS input configuration. In the
FAN3121T and FAN3122T, the input thresholds meet
industry-standard TTL-logic thresholds independent of
the V
approximately 0.7V. These levels permit the inputs to
be driven from a range of input logic signal levels for
which a voltage over 2V is considered logic HIGH. The
driving signal for the TTL inputs should have fast rising
and falling edges with a slew rate of 6V/µs or faster, so
the rise time from 0 to 3.3V should be 550ns or less.
The FAN3121 and FAN3122 output can be enabled or
disabled using the EN pin with a very rapid response
time. If EN is not externally connected, an internal pull-
up resistor enables the driver by default. The EN pin
has logic thresholds for parts with either TTL or CMOS
IN thresholds.
In the FAN3121C and FAN3122C, the logic input
thresholds are dependent on the V
of 12V, the logic rising edge threshold is approximately
55% of V
approximately
configuration
approximately 17% of V
used with relatively slow edges (approaching DC) if
good
incorporated in the system design to prevent noise from
violating the input voltage hysteresis window. This
allows setting precise timing intervals by fitting an R-C
circuit between the controlling signal and the IN pin of
the driver. The slow rising edge at the IN pin of the
driver introduces a delay between the controlling signal
and the OUT pin of the driver.
Static Supply Current
In the I
the curves are produced with all inputs / enables
floating (OUT is LOW) and indicates the lowest static
I
additional current flows through the 100kΩ resistors on
the inputs and outputs, as shown in the block diagram
(see Figure 7). In these cases, the actual static I
current is the value obtained from the curves, plus this
additional current.
MillerDrive™ Gate-Drive Technology
FAN312x gate drivers incorporate the MillerDrive™
architecture shown in Figure 46. For the output stage, a
combination of bipolar and MOS devices provide 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
LOW rail.
The purpose of the Miller Drive™ architecture is to
speed up 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.
DD
DD
current for the tested configuration. For other states,
and the MOS devices pull the output to the HIGH or
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decoupling
voltage, and there is a hysteresis voltage of
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(static) Typical Performance Characteristics,
and the input falling edge threshold is
offers
38%
and
of
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a
. The CMOS inputs can be
V
bypass
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hysteresis
.
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The
level and, with V
techniques
CMOS
voltage
input
are
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DD
of
For applications with zero voltage switching during the
MOSFET turn-on or turn-off interval, the driver supplies
high peak current for fast switching, even though the
Miller plateau is not present. This situation often occurs
in synchronous rectifier applications because the body
diode is generally conducting before the MOSFET is
switched on.
The output pin slew rate is determined by V
and the load on the output. It is not user adjustable, but
a series resistor can be added if a slower rise or fall
time at the MOSFET gate is needed.
Under-Voltage Lockout (UVLO)
The FAN312x startup logic is optimized to drive ground-
referenced N-channel MOSFETs with an under-voltage
lockout (UVLO) function to ensure that the IC starts in
an orderly fashion. When V
4.0V operational level, this circuit holds the output low,
regardless of the status of the input pins. After the part
is active, the supply voltage must drop 0.25V before the
part shuts down. This hysteresis helps prevent chatter
when low V
power switching. This configuration is not suitable for
driving high-side P-channel MOSFETs because the low
output voltage of the driver would turn the P-channel
MOSFET on with V
V
The FAN3121 and FAN3122 are available in either
8-lead SOIC or MLP packages. In either package, the
V
connected together on the PCB.
In typical FAN312x gate-driver applications, high-
current pulses are needed to charge and discharge the
gate of a power MOSFET in time intervals of 50ns or
less. A bypass capacitor with low ESR and ESL should
be connected directly between the V
provide these large current pulses without causing
unacceptable ripple on the V
requirements in a small size, a ceramic capacitor of 1µF
or larger is typically used, with a dielectric material such
as X7R, to limit the change in capacitance over the
temperature and / or voltage application ranges.
14
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Figure 46. Miller Drive™ Output Architecture
pins 1 and 8 and the GND pins 4 and 5 should be
Bypassing and Layout Considerations
DD
supply voltages have noise from the
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below 4.0V.
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is rising, yet below the
supply. To meet these
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and GND pins to
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voltage