MIC4605 Datasheet, PDF (20/25 Pages) Micrel Semiconductor – 85V Half-Bridge MOSFET Drivers

Micrel, Inc.
Placement of the decoupling capacitors is critical. The
bypass capacitor for VDD should be placed as close as
possible between the VDD and VSS pins. The bypass
capacitor (C_B) for the HB supply pin must be located as
close as possible between the HB and HS pins. The etch
connections must be short, wide, and direct. The use of a
ground plane to minimize connection impedance is
recommended (refer to the Grounding, Component
Placement, and Circuit Layout section for more
information).
The voltage on the bootstrap capacitor drops each time it
delivers charge to turn on the MOSFET. The voltage drop
depends on the gate charge required by the MOSFET.
Most MOSFET specifications specify gate charge versus
V_GS voltage. Based on this information and a
recommended ΔV_HB of less than 0.1V, the minimum
value of bootstrap capacitance is calculated as:
C_B
≥
Q_GATE
∆V_HB
Eq. 12
Where:
Q_GATE = Total gate charge at V_HB
∆V_HB = Voltage drop at the HB pin
The decoupling capacitor for the VDD input may be
calculated in with the same formula; however, the two
capacitors are usually equal in value.
DC Motor Applications
MIC4605 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 three-
phase brushless motors. As shown in Figure 13, Figure
14, and Figure 15, 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. Three-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 MIC4605 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.
November 11, 2013
20
MIC4605
Figure 13. Half-Bridge DC Motor
Figure 14. Full-Bridge DC Motor
Revision 1.0