MCP8024 datasheet(21/46 Pages) MICROCHIP

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DS20005228A-page 21

MCP8024

4.2.2.6

Run Mode

At the end of the Ramp mode, Run mode is entered. In

Run mode, the back EMF sensor is enabled and com-

mutation is now under the control of the phase-locked

loop. Motor speed can be regulated by an outer speed

control loop.

4.2.2.7

PWM Speed Control

The inner commutation loop is a phase-locked loop,

which locks to the rotor’s position. This inner loop does

not attempt to modify the position of the rotor, but mod-

ifies the commutation times to match whatever posi-

tion the rotor has. The outer speed loop changes the

rotor velocity and the inner commutation loop locks to

the rotor’s position to commutate the phase at the cor-

rect times.

The outer speed loop pulse width modulates (PWMs)

the motor drive inverter to produce the desired wave

shape and voltage at the motor. The inductance of the

motor then integrates this PWM pattern to produce the

desired average current, thus controlling the desired

torque and speed of the motor. For a trapezoidal

BLDC motor drive with six-step commutation, the

PWM is used to generate the average voltage to pro-

duce the desired motor current and, hence, the motor

speed.

There are two basic methods to PWM the inverter

switches. The first method returns the reactive energy

in the motor inductance to the source by reversing the

voltage on the motor winding during the current decay

period. This method is referred to as fast decay or

chop-chop. The second method circulates the reactive

current in the motor with minimal voltage applied to the

inductance. This method is referred to as slow decay

or chop-coast.

The preferred control method employs a chop-chop

PWM for any situations where the motor is being

accelerated, either positively or negatively. For

improved efficiency, chop-coast PWM is employed

during steady-state conditions. The chop-chop speed

loop is implemented by hysteretic control, fixed off-

time control, or average Current mode control of the

motor current. This makes for a very robust controller

as the motor current is always in instantaneous con-

trol. The motor speed presented to the chop-chop loop

is reduced by approximately 9%. A fixed-frequency

PWM that only modulates the high-side switches

implements the chop-coast loop. The chop-coast loop

is presented with the full motor speed, so if it is able to

control the speed, the chop-chop loop will never be

satisfied and will remain saturated. The chop-chop

remains able to assume full control if the motor torque

is exceeded, either through a load change or a change

in speed that produces acceleration torque. The chop-

coast loop will remain saturated, with the chop-chop

loop in full control, during start-up and acceleration to

full speed. The bandwidth of the chop-coast loop is set

to be slower than the chop-chop loop so that any tran-

sients will be handled by the chop-chop loop and the

chop-coast loop will only be active in steady-state

operation.

4.2.3

EXTERNAL DRIVE FOR A 3-PHASE

BRIDGE WITH NMOS/NMOS

MOSFET PAIRS

Each motor phase is driven with external NMOS/

NMOS MOSFET pairs. These are controlled by a low-

side and a high-side gate driver. The gate drivers are

controlled directly by the digital input pins PWM[1:3]H/

L. A logic High turns the associated gate driver ON,

and a logic Low turns the associated gate driver OFF.

The PWM[1:3]H/L digital inputs are equipped with

internal pull-down resistors.

The low-side gate drivers are biased by the +12V LDO

output, referenced to ground. The high-side gate driv-

ers are a floating drive biased by a bootstrap capacitor

circuit. The bootstrap capacitor is charged by the +12V

LDO whenever the accompanying low-side MOSFET

is turned on.

TABLE 4-1:

COMMUTATION STATE MACHINE

STATE

OUTPUTS

BEMF

SAMPLE

CE = 0

OFF

N/A

LOCK

OFF

N/A

OFF

Phase B

OFF

Phase A

OFF

Phase C

OFF

Phase B

OFF

Phase A

OFF

Phase C

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