AN4354
Application note
L648x devices: gate drivers setup
Enrico Poli
Introduction
This document describes the operation of the gate driving circuitry which is integrated in the
L648x devices and provides guidance on how to set parameters according to the application
requirements.
March 2015
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Contents
AN4354
Contents
1
Gate driving circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
1.3
Advantages of constant current driving of the MOSFET gates . . . . . . . . . 5
Turn-on sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Turn-off sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2
Gate drivers setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1
2.2
2.3
2.4
Gate current (IGATE), controlled current time (TCC) and turn-off boost
time (TBOOST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Deadtime (TDT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Blanking time (TBLANK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Wrong setup issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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Gate driving circuitry
1
Gate driving circuitry
The L648x devices integrate a programmable gate driving circuitry which allows to drive
a wide range of external N-channel MOSFETs.
The parameters which can be set are listed
Table 1.
Table 1. Gate driving circuit parameters
Parameter
Gate source/sink current
Controlled current time (charging
time)
Turn-off current boost time
Deadtime
Description
The current used to charge/discharge the gate of the
MOSFETs.
The charging/discharging time of the gate of the MOSFETs.
The part of the discharging time during which the gate
current is forced to the maximum value.
The time between the turn-off of the MOSFETs and the turn-
on of the opposite one.
The time after the MOSFETs commutation where the
sensing circuitry is disabled in order to avoid spurious
triggering.
Blanking time
Figure 1. Gate driving circuit diagram
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Gate driving circuitry
AN4354
Both the high-side and low-side gate drivers are programmable current generators which
allow to charge and discharge the MOSFETs gate with a constant current. The gate drivers
also integrate a Miller clamp MOSFET which avoids induced turn-on effect.
The low-side gate driver is supplied by the VCC pin which can be connected both to an
external voltage source and to the integrated linear regulator which generates the supply
voltage (7.5 V or 15 V according to the configuration) from the VSREG supply pin.
The high-side gate driver is supplied by a charge pump circuitry. This way on time of the
high-side MOSFET is not limited as in the case of a bootstrap capacitor. A clamping diode
limits the gate-source voltage of the high-side MOSFET to 17 V avoiding the breaking of the
gate in case of the power stage output is short-circuited to ground. In fact in this case, if the
voltage limiter was not present, the final gate-source voltage of the high-side MOSFET was
equal to V
BOOT
(see
Figure 2).
Figure 2. High-side gate driver operation when output is shorted to ground
Warning:
The clamping diode protects the MOSFET from the V
gs
breakdown, but not by the high current which can be
generated by the short-circuit condition.
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Gate driving circuitry
1.1
Advantages of constant current driving of the MOSFET gates
The L648x gate drivers, thanks to the possibility to drive the MOSFET gates using
a programmable constant current, have two main advantages:
1.
2.
The slew rate of the power stage is controlled allowing a precise EMI management.
No gate resistors are required reducing the number of components in the bill of
material.
The output slew rate depends on how fast the MOSFET gate is charged in the Miller plateau
region. In a classic gate driver the charging/discharging current is adjusted adding a resistor
between the gate driver output and the MOSFET gate (see
Figure 3).
This way the output
slew rate can be limited, but its value depends on the load current (the plateau voltage
changes with the drain current).
Using a true current generator, the L648x gate driver forces a constant gate current during
the entire plateau region regardless of the load current.
Figure 3. Classic gate driver circuit vs. L648x gate driver
1.2
Turn-on sequence
When one of the external MOSFET must be turned-on the gate driving circuitry disables the
respective Miller clamp switch and starts forcing the programmed current into the gate. After
the controlled current time expires, the high-side and the low-side drivers act differently: the
low-side drivers do not limit the gate current; the high-side ones instead limit the gate
current to about 1 mA (see
Figure 4).
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