MCP8021 datasheet(29/66 Pages) MICROCHIP

29 / 66 page

 2020-2024 Microchip Technology Inc. and its subsidiaries

DS20006265D-page 29

MCP8021/2

4.7.1.4

Lock Mode

Before the motor can be started, the rotor should be in

a known position. In Lock mode, the microcontroller

drives Phase B low and Phases A and C high. This

aligns the rotor 30 electrical degrees before the center

of the first commutation state. Lock mode must last

long enough to allow the motor and its load to settle

into this position.

4.7.1.5

Ramp Mode

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

Ramp mode, the microcontroller steps through the

commutation state machine, increasing the step rate

linearly, until a minimum speed is reached that will

result in a usable BEMF voltage. Ramp mode is an

open-loop commutation. No knowledge of the rotor

position is used.

4.7.1.6

Run Mode

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

mode, the Back EMF sensor is enabled and commuta-

tion is now under the control of the Phase-Locked

Loop. Motor speed can be regulated by an outer speed

control loop.

4.7.1.7

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

modifies the commutation times to match whatever

position 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

correct times.

The outer speed loop pulse width modulates 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 com-

mutation, the PWM is used to generate the average

voltage to produce the desired motor current and

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,

since the motor current is always in instantaneous

control. 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 accelera-

tion 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 transients will be handled by the

chop-chop loop and the chop-coast loop will only be

active in steady-state operation.

TABLE 4-5:

COMMUTATION STATE MACHINE

State

Outputs

BEMF Phase

HSA

HSB

HSC

LSA

LSB

LSC

OE = 0

OFF

N/A

BOOTSTRAP

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

MCP8021 数据表(PDF) 29 Page - Microchip Technology

MCP8021 数据表(HTML) 29 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

数据表下载