Micrel, Inc.

MIC4604

June 25, 2013

13

Revision 1.0

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

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.

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.

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

Figure 9. Turn-Off Current Paths

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.