MP6531A – 5V - 60V, THREE-PHASE BLDC MOTOR PRE-DRIVER

MP6531A Rev. 1.02

www.MonolithicPower.com

13

6/20/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Input Voltage (VIN)

VIN supplies all power to the device. VIN must

be properly bypassed with a capacitor to

ground. The normal operating range of VIN is

between 5V and 60V.

VIN should never be allowed to exceed the

absolute maximum ratings, even in a short-term

transient condition, or damage to the device

may result. In some cases, especially where

mechanical energy can turn a motor into a

generator, it may be necessary to use some

form of over-voltage protection, such as a TVS

diode, between VIN and ground.

Component Selection

MOSFET Selection

Correctly selecting the power MOSFETs used

to drive a motor is crucial to designing a

successful motor drive. The MOSFET must

have a VDS breakdown voltage higher than the

supply voltage. A considerable margin (10-15V)

should be added to prevent MOSFET damage

from transient voltages caused by parasitic

inductances in the PCB layout and wiring. For

example, for 24V power supply applications,

MOSFETs with a breakdown voltage of 40-60V

minimum are recommended. More margin is

desirable in high-current applications, since the

transients caused by parasitic inductances may

be larger. There are also conditions such as

regenerative braking that can inject current

back into the power supply. Care must be taken

so that this does not increase the power supply

voltage enough to damage components.

The MOSFETs must be able to safely pass the

current needed to run the motor. The highest

current condition (which is normally when the

motor is first started or stalled) must be

supported. This is called the stall current of the

motor.

Similar to the current capability of the MOSFET

MOSFET when it is fully turned on. The

MOSFET dissipates power proportional to the

Equation (1):

P = I

(1)

motor current, the heat generated in this power

can be dissipated safely. In some cases, this

may require special PCB design considerations

and/or external heatsinks to be used for the

MOSFETs.

Consideration should be made for the safe

operating area (SOA) of the MOSFETs during

fault conditions such as a short circuit. The IC

acts quickly in the event of a short, but there is

still a very short amount of time (about 3µs)

where large currents can flow in the MOSFETs

while the protection circuits recognize the fault

and disable the outputs.

External Capacitor Selection

The MP6531A can provide a gate drive voltage

(VREG) of 10-12V, even if the input supply

voltage drops as low as 5V. This gate drive

voltage is generated by a charge pump inside

the part, which uses external capacitors.

have a capacitance of 470nF. The capacitor

must be rated to withstand the maximum VIN

power supply voltage. An X7R or X5R ceramic

capacitor is recommended. With a 470nF

capacitor, VREG can output approximately

10mA when VIN is 5V. If operation below 10V is

not needed, a 220nF capacitor can be used. To

provide the large peak currents needed to turn

on the HS-FET, use bootstrap capacitors.

These capacitors are charged when the output

is driven low, and then the charge in the

bootstrap capacitor is used to turn on the HS-

FET when the output is driven high. Note that

an internal charge pump keeps the bootstrap

capacitor charged when the output is held high

for an extended period.

Bootstrap capacitors are selected depending on

the MOSFET total gate charge. When the HS-

FET is turned on, the charge stored in the

bootstrap capacitor is transferred to the HS-

FET gate. As a simplified approximation, the

minimum

bootstrap

capacitance

can

be

estimated with Equation (2):

(2)