MIC4604 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – 85V Half Bridge MOSFET Drivers with up to 16V Programmable Gate Drive

Micrel, Inc.
Current in the high-side driver is sourced from capacitor C_B
and flows into the HB pin and out the HO pin, into the gate
of the high side MOSFET. The return path for the current
is from the source of the MOSFET and back to capacitor
C_B. The high-side circuit return path usually does not have
a low-impedance ground plane so the etch connections in
this critical path should be short and wide to minimize
parasitic inductance. As with the low-side circuit,
impedance between the MOSFET source and the
decoupling capacitor causes negative voltage feedback
that fights the turn-on of the MOSFET.
It is important to note that capacitor C_B must be placed
close to the HB and HS pins. This capacitor not only
provides all the energy for turn-on but it must also keep HB
pin noise and ripple low for proper operation of the high-
side drive circuitry.
MIC4604
Figure 9. Turn-Off Current Paths
Figure 8. Turn-On Current Paths
Figure 9 shows the critical current paths when the driver
outputs go low and turn off the external MOSFETs. Short,
low-impedance connections are important during turn-off
for the same reasons given in the turn-on explanation.
Current flowing through the internal diode replenishes
charge in the bootstrap capacitor, C_B.
DC Motor Applications
MIC4604 MOSFET drivers are widely used in DC motor
applications. They address brushed motors in both half-
bridge and full-bridge motor topologies as well as 3-phase
brushless motors. As shown in Figure 10, Figure 11, and
Figure 12, the drivers switch the MOSFETs at variable
duty cycles that modulate the voltage to control motor
speed. In the half-bridge topology, the motor turns in one
direction only. The full-bridge topology allows for
bidirectional control. 3-Phase motors are more efficient
compared to the brushed motors but require three half-
bridge switches and additional circuitry to sense the
position of the rotor.
The MIC4604 85V operating voltage offers the engineer
margin to protect against Back Electromotive Force (EMF)
which is a voltage spike caused by the rotation of the rotor.
The Back EMF voltage amplitude depends on the speed of
the rotation. It is good practice to have at least twice the
HV voltage of the motor supply. 85V is plenty of margin for
12V, 24V, and 40V motors.
June 25, 2013
13
Revision 1.0