MIC4605 Datasheet, PDF (15/25 Pages) Micrel Semiconductor – 85V Half-Bridge MOSFET Drivers

Micrel, Inc.
Application Information
Adaptive Dead Time
The MIC4605 Door Lock/Unlock Module diagram
illustrates how the MIC4605 drives the power stage of a
DC motor. It is important that only one of the two
MOSFETs is on at any given time. If both MOSFETs on
the same side of the half bridge are simultaneously on,
VIN will short to ground. The high current from the
shorted VIN supply will then “shoot through” the
MOSFETs into ground. Excessive shoot-through causes
higher power dissipation in the MOSFETs, voltage spikes
and ringing in the circuit. The high current and voltage
ringing generate conducted and radiated EMI. Table 1
illustrates truth tables for both the MIC4605-1 (dual TTL
inputs) and MIC4605-2 (single PWM input) that details
the “first on” priority as well as the failsafe delay.
Table 1. MIC4605-1 and MIC4605-2 Truth Tables
LI
HI LO HO Comments
0
0
0 0 Both outputs off.
0
1
0
1
HO will not go high until LO
falls below 1.9V.
LO will be delayed an extra
1
0
1 0 240ns if HS never falls below
2.2V.
1
1
X
X
First ON stays on until input of
same goes low.
PWM LO HO Comments
0
1 0 LO will be delayed an extra
240ns if HS never falls below
2.2V.
1
0 1 HO will not go high until LO
falls below 1.9V.
MIC4605
the driver cannot monitor the gate voltage inside the
MOSFET. Figure 7 shows an equivalent circuit of the
gate driver section, including parasitics.
Figure 7. MIC4605 Driving an External MOSFET
The internal gate resistance (R_{G_FET}) and any external
damping resistor (R_G) isolate the MOSFET’s gate from
the driver output. There is a delay between when the
driver output goes low and the MOSFET turns off. This
turn-off delay is usually specified in the MOSFET data
sheet. This delay increases when an external damping
resistor is used.
The MIC4605 uses a combination of active sensing and
passive delay to ensure that both MOSFETs are not on at
the same time, minimizing shoot-through current. Figure
8 illustrates how the adaptive dead time circuitry works.
Minimizing shoot-through can be done passively, actively
or through a combination of both. Passive shoot-through
protection can be achieved by implementing delays
between the high and low gate drivers to prevent both
MOSFETs from being on at the same time. These delays
can be adjusted for different applications. Although
simple, the disadvantage of this approach is requires long
delays to account for process and temperature variations
in the MOSFET and MOSFET driver.
Adaptive dead time monitors voltages on the gate drive
outputs and switch node to determine when to switch the
MOSFETs on and off. This active approach adjusts the
delays to account for some of the variations, but it too
has its disadvantages. High currents and fast switching
voltages in the gate drive and return paths can cause
parasitic ringing to turn the MOSFETs back on even while
the gate driver output is low. Another disadvantage is that
Figure 8. Adaptive Dead Time Logic Diagram (PWM)
Figure 9 shows the dead time (<20ns) between the gate
drive output transitions as the low-side driver transitions
from on-to-off while the high-side driver transitions from
off-to-on.
November 11, 2013
15
Revision 1.0