MCP8024 datasheet(20/46 Pages) MICROCHIP

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MCP8024

DS20005228A-page 20

 2013 Microchip Technology Inc.

approximately five time constants ~= 50 µs. The

desired current sense gain is established with an

external resistor network.

The comparator output may be employed as a current

limit. Alternatively, the current sense output can be

employed in a chop-chop PWM speed loop for any sit-

uations where the motor is being accelerated, either

positively or negatively. An analog chop-chop speed

loop can be implemented by hysteretic control or fixed

off-time of the motor current. This makes for a very

robust controller as the motor current is always in

instantaneous control.

A sense resistor in series with the bridge ground return

provides a current signal for both feedback and current

limiting. This resistor should be non-inductive to mini-

mize ringing from high di/dt. Any inductance in the

power circuit represents potential problems in the form

of additional voltage stress and ringing, as well as

increasing switching times. While impractical to elimi-

nate, careful layout and bypassing will minimize these

effects. The output stage should be as compact as

heat sinking will allow, with wide, short traces carrying

all pulsed currents. Each half-bridge should be sepa-

rately bypassed with a low ESR/ESL capacitor, decou-

pling it from the rest of the circuit. Some layouts will

allow the input filter capacitor to be split into three

smaller values, and serve double duty as the half-

bridge bypass capacitors.

The current sense resistor is chosen to establish the

peak current limit threshold, which is typically set 20%

higher than the maximum current command level to

provide overcurrent protection during abnormal condi-

tions. Under normal circumstances with a properly

compensated current loop, peak current limit will not be

exercised.

4.2.2

MOTOR CONTROL

The commutation loop of a BLDC motor control is a

Phase-Locked Loop (PLL) which locks to the rotor’s

position. Note that this inner loop does not attempt to

modify the position of the rotor, but modifies the com-

mutation times to match whatever position the rotor

has. An outer speed loop changes the rotor velocity,

and the commutation loop locks to the rotor’s position

to commutate the phases at the correct times.

4.2.2.1

Sensorless Motor Control

Many control algorithms can be implemented with the

MCP8024 in conjunction with a microcontroller. The

following discussion provides a starting point for imple-

menting the MCP8024 in a sensorless control applica-

tion of a 3-phase motor. The motor is driven by

energizing two windings at a time and sequencing the

windings in a six step per electrical revolution method.

This method leaves one winding unenergized at all

times, and the voltage on that unenergized (Back

EMF) winding can be monitored to determine the rotor

position.

4.2.2.2

Start-Up Sequence

When the motor being driven is at rest, the back EMF

is equal to zero. The motor needs to be rotating for the

back EMF sensor to lock onto the rotor position and

commutate the motor. The recommended start-up

sequence to bring the rotor from rest up to a speed

fast enough to allow back EMF sensing is comprised

of three modes: Lock or Align mode, Ramp mode, and

Run mode. Refer to the commutation state machine in

Table 4-1. The order in which the microcontroller steps

through the commutation state machine determines

the direction the motor rotates.

4.2.2.3

Disabled Mode (CE = 0)

When the driver is disabled (CE = 0), all of the drivers

are turned off.

4.2.2.4

Lock Mode

Before the motor can be started, the rotor must 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.

4.2.2.5

Ramp Mode

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

Ramp mode, the microcontroller steps through the

commutation state machine, increasing linearly, until a

minimum speed is reached. Ramp mode is an open-

loop commutation. No knowledge of the rotor position

is used.

Note:

The motor current limit comparator output

is internally ‘OR’d with the DRIVER

FAULT output of the driver logic block.

The microcontroller should monitor the

comparator output and take appropriate

actions. The motor current limit compara-

tor circuitry does not disable the motor

drivers when an overcurrent situation

occurs. Only one current limit comparator

is provided. The MCP8024 provides three

current sense amplifiers which can be

used for implementation of advanced con-

trol algorithms such as Field Oriented

Control (FOC).

Note:

With a chop-chop control, motor current

always flows through the sense resistor.

When the PWM is off, however, the fly-

back diodes, or synchronous rectifiers,

conduct, causing the current to reverse

polarity through the sense resistor.

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