2014 Microchip Technology Inc. DS20005257B-page 9
MCP8063
4.0 FUNCTIONAL DESCRIPTION
The MCP8063 device generates a full-wave signal to
drive a 3-phase sensorless BLDC motor. High
efficiency and low power consumption are achieved
due to DMOS transistors and synchronous rectification
drive type. The current carrying order of the output is as
follows: OUT1 OUT2 OUT3.
4.1 Speed Control
The rotational speed of the motor can be controlled
either through the PWM digital input signal or by
varying the power supply (V
DD
). When the PWM signal
is “High” (or left open), the motor rotates at full speed.
When the PWM signal is “Low”, the motor is stopped
(and the driver outputs are set to high impedance). By
changing the PWM duty cycle, the speed can be
adjusted. Notice that the PWM frequency has no
special meaning for the motor speed and is
asynchronous with the activation of the output
transistors. Thus, the user has maximum freedom to
choose the PWM system frequency within a wide range
(from 20 Hz to 100 kHz), while the output transistor
activation always occurs at a fixed rate, which is
outside the range of audible frequencies. The typical
output frequency of MCP8063 is 23 kHz.
4.2 Frequency Generator Function
The Frequency Generator output is a “Hall-sensor
equivalent” digital output, giving information to an
external controller about the speed and phase of the
motor. The FG pin is an open-drain output, connecting
to a logical voltage level through an external pull-up
resistor. When a lock (or out-of-sync) situation is
detected by the driver, this output is set to high
impedance until the motor is restarted. Leave the pin
open when not used. The FG signal can be used to
compute the motor speed in rotations per minute
(RPM). Typically, for a four-pole BLDC fan (4P/6S), the
speed in RPMs is 30 x FG frequency (Hz).
EQUATION 1-1:
4.3 Lock-Up Protection and Automatic
Restart
If the motor is stopped (blocked) or if it loses
synchronization with the driver, a lock-up protection
circuit detects this situation and ties the outputs to GND
in order to dissipate the remaining energy from the
rotor with a minimum of self heating. After a “waiting
time” (T
WAIT
), the lock-up protection is released and
normal operation resumes for a given time (T
RUN
). In
case the motor is still blocked, a new period of waiting
time is started. T
WAIT
and T
RUN
timings are fixed
internally, so that no external capacitor is needed.
4.4 Overcurrent Limitation
The motor peak current is limited by the driver to a fixed
value (defined internally), thus limiting the maximum
power dissipation in the coils.
4.5 Thermal Shutdown
The MCP8063 device has a thermal protection function
which detects when the die temperature exceeds
T
SD
= +170°C. When this temperature is reached, the
circuit enters Thermal Shutdown mode and the outputs
OUT1, OUT2 and OUT3 are tied to GND in order to
dissipate the remaining energy from the rotor with a
minimum of self-heating. Once the junction
temperature (T
SD
) has dropped below +145°C, the
normal operation resumes (the thermal detection
circuit has +25°C hysteresis function).
FIGURE 4-1: Thermal Protection
Hysteresis.
4.6 Internal Voltage Regulator
V
BIAS
voltage is generated internally and is used to
supply internal logical blocks. The V
BIAS
pin is used to
connect an external decoupling capacitor (1 µF or
higher). Notice that this pin is for IC internal use and is
not designed to supply DC current to external blocks.
4.7 Overvoltage Shutdown
The MCP8063 device has an overvoltage protection
function which detects when the V
DD
voltage exceeds
V
OV
= +18.5V. When this temperature is reached, the
circuit enters Thermal Shutdown mode, and outputs
OUT1, OUT2 and OUT3 are disabled (high impedance).
FG 720
PS
-----------------------
Where:
P=
Total number of poles in the motor
S=Total number of slots in the motor
= Rotor speed RPM
T
SD
Thermal Shutdown
Normal operation
