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DS20005853E-page 21

MIC4605

7.9

Decoupling and Bootstrap

Capacitor Selection

Decoupling capacitors are required for both the

low-side (VDD) and high-side (HB) supply pins. These

capacitors supply the charge necessary to drive the

external MOSFETs and also minimize the voltage

ripple on these pins. The capacitor from HB to HS has

two functions: it provides decoupling for the high-side

circuitry and also provides current to the high-side

circuit while the high-side external MOSFET is on.

Ceramic capacitors are recommended because of their

low impedance and small size. Z5U-type ceramic

capacitor dielectrics are not recommended because of

the large change in capacitance over temperature and

voltage. A minimum value of 0.1 µF is required for each

of the capacitors, regardless of the MOSFETs being

driven. Larger MOSFETs may require larger capaci-

tance values for proper operation. The voltage rating of

the capacitors depends on the supply voltage, ambient

temperature and the voltage derating used for reliabil-

ity. 25V rated X5R or X7R ceramic capacitors are

recommended for most applications. The minimum

capacitance value should be increased if low-voltage

capacitors are used, because even good quality dielec-

tric capacitors, such as X5R, will lose 40% to 70% of

their capacitance value at the rated voltage.

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 (CB) 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 Section 7.12

“Grounding, Component Placement and Circuit

Layout”

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

VGS voltage. Based on this information and a recom-

mended ΔVHB of less than 0.1V, the minimum value of

bootstrap capacitance is calculated as:

EQUATION 7-13:

The decoupling capacitor for the VDD input may be

calculated in with the same formula; however, the two

capacitors are usually equal in value.

7.10

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 3-phase brushless motors. As shown in

Figure 7-8, Figure 7-9 and Figure 7-10, 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 as com-

pared 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 engi-

neer a 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.

GATE



-----------------



Where:

QGATE = Total Gate Charge at VHB

ΔV

= Voltage Drop at the HB Pin

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