LMG3410 Datasheet, PDF (16/33 Pages) Texas Instruments – 600-V 12-A Single Channel GaN Power Stage

LMG3410
SNOSD10A – APRIL 2016 – REVISED JUNE 2016
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However, with the LMG3410, during the boost converter startup, significant shoot-through current can occur for
high drain slew rates while starting up. This shoot-through current is approximately 1.25 µC per switching event
at 50 V/ns, and is comparable to a reverse-recovery event. If this shoot-through current is undesirable, the drain
slew rate of the low-side device must be reduced during startup. In Figure 11, the FAULT output from the high-
side device is used to gate MOSFET Q1. When FAULT from the high-side is high, once the device is powered
up, Q1 turns on and reduces the effective resistance connected to RDRV on the low-side LMG3410. With this
circuit, the dv/dt of the low-side device can be held low to reduce power dissipation and reduce ringing during
high-side startup, but then increase to reduce switching loss during normal operation.
9.2.2.2 Signal Level-Shifting
As the LMG3410 is a single-channel power stage, two devices are used to construct a half-bridge converter,
such as the one shown in Figure 11. A high-voltage level shifter or digital isolator must be used to provide
signals to the high-side device. Using an isolator for the low-side device is optional but will equalize propagation
delays between the high-side and low-side signal path, as well as providing the ability to use different grounds for
the power stage and the controller. If an isolator is not used on the low-side device, the control ground and the
power ground must be connected at the LMG3410, as described in Layout Guidelines, and nowhere else on the
board. With the high current slew rate of the fast-switching GaN device, any ground-plane inductance common
with the power path may cause oscillation or instability in the power stage without the use of an isolator.
Choosing a digital isolator for level-shifting is an important consideration for fault-free operation. Because GaN
switches very quickly, exceeding 50 V/ns in hard-switching applications, isolators with high common-mode
transient immunity (CMTI) are required. If an isolator suffers from a CMTI issue, it can output a false pulse or
signal which can cause shoot-through. In addition, choosing an isolator that is not edge-triggered can improve
circuit robustness. In an edge-triggered isolator, a high dv/dt event can cause the isolator to flip states and cause
circuit malfunctioning.
On/off keyed isolators are preferred, such as the TI ISO78xxF series, as a high CMTI event would only cause a
short (few nanosecond) false pulse, which can be filtered out. To allow for filtering of these false pulses, an R-C
filter at the driver input is recommended to ensure these false pulses can be filtered. If issues are observed,
values of 1 kΩ and 22 pF can be used to filter out any false pulses.
9.2.2.3 Buck-Boost Converter Design
The Buck-boost converter generates the negative voltage necessary to turn off the direct-drive GaN FET. While it
is controlled internally, it requires an external power inductor and output capacitor. The converter is designed to
use a 22 µH inductor and a 2.2 µF output capacitor. As the peak current of the buck-boost is limited to less than
350 mA, the inductor chosen must have a saturation current above 350 mA. A Taiyo-Yuden BRC2518T220K 22
µH SMT inductor in a 0806 package is recommended. This inductor is connected between the BBSW pin and
ground. A 2.2 µF, 25V 0805 bypass capacitor is required between V_NEG and ground. Due to the voltage
coefficient of X7R capacitors, a 2.2 µF capacitor will provide the required minimum 1.0 µF capacitance when
operating.
9.2.3 Application Curves
V_DS (50 V/div)
I_D (2.5 A/div)
V_OUT = 400V
I_L = 5 A
R_DRV = 40 k:
V_DS (50 V/div)
I_D (1.25 A/div)
V_BUS = 400 V
I_L = 5 A
R_DRV = 40 k:
Time (5 ns/div)
D006
Figure 12. Turn-on Waveform in Application Example
Time (5 ns/div)
D007
Figure 13. Turn-off Waveform in Application Example
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