MCP8026 datasheet(30/64 Pages) MICROCHIP

30 / 64 page

MCP8025/6

DS20005339B-page 30

 2016 Microchip Technology Inc.

4.3

Motor Control Unit

The motor control unit is comprised of the following:

• External drive for a 3-phase bridge with

NMOS/NMOS MOSFET pairs

• Back EMF sampler with phase multiplexer and

neutral simulator (MCP8025)

• Motor current sense amplifier and comparator

• Two additional current sense amplifiers

(MCP8026)

4.3.1

MOTOR CURRENT SENSE

CIRCUITRY

The internal motor current sense circuitry consists of an

operational amplifier and a comparator. The amplifier

output is presented to the inverting comparator input

and as an output to the microcontroller. The

non-inverting comparator input is connected to an

internally programmable 8-bit DAC. A selectable motor

current limit threshold may be set with a SET_ILIMIT

message from the host to the MCP8025/6 via the DE2

communication link. The DACREF<7:0> bits in the

CFG1 register contain the DAC current reference

value. The dual-purpose ILIMIT_OUT pin handles the

current limit output as well as system fault outputs. The

8-bit DAC is powered by the 5V supply. The DAC

output voltage ranges from 0.991V to 4.503V. The DAC

has a bit value of (4.503V – 0.991V)/(28 – 1) = 13.77 mV/bit.

A DAC input of 00H yields a DAC output voltage of

0.991V. The default power-up DAC value is 40H

(1.872V). The DAC uses a 100 kHz filter. Input code

to output voltage delay is 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

situations 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

minimize 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

eliminate, 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

separately bypassed with a low ESR/ESL capacitor,

decoupling 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

conditions. Under normal circumstances with a

properly compensated current loop, peak current limit

will not be exercised.

The current sense operational amplifier is disabled

when CE = 0.

4.3.2

BACK EMF SAMPLER WITH PHASE

MULTIPLEXER AND NEUTRAL

SIMULATOR (MCP8025)

The commutation loop of a BLDC motor control is a

phase lock loop (PLL) that locks to the rotor’s position.

Note that 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. 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.

The Back EMF sensor consists of the motor, a

Back EMF sampler, a phase multiplexer and a neutral

simulator.

The Back EMF sampler takes the motor phase

voltages and calculates the neutral point of the motor

by using Equation 4-2.

EQUATION 4-2:

NEUTRAL POINT

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

comparator circuitry does not disable the

motor drivers when an overcurrent

situation occurs. Only one current limit

comparator is provided. The MCP8026

provides three current sense amplifiers

which can be used for implementation of

advanced control 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

flyback diodes or synchronous rectifiers

conduct, causing the current to reverse

polarity through the sense resistor.

NEUTRAL

A B C

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