Datasheet

 2011-2013 Microchip Technology Inc. DS20002281B-page 11
MTD6505
4.0 FUNCTIONAL DESCRIPTION
The MTD6505 generates a full-wave signal to drive a
3-phase BLDC motor. High efficiency and low-power
consumption are achieved due to CMOS transistors
and synchronous rectification drive type.
4.1 Speed Control
The rotational speed of the motor can be controlled
either through the PWM digital input signal or by acting
directly on 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 IC outputs are
set to high-impedance and the motor is stopped.
By changing the PWM duty cycle, the speed can be
adjusted. Thus, the user has freedom to choose the
PWM system frequency within a wide range (from
1 kHz to 100 kHz).
Since the PWM pin has an internal pull-up resistor
connected to V
DD
, it is recommended to drive it
between 0V and High-Z. The PWM driver must be able
to support the pull-up resistor current, to drive the pin.
See “PWM Internal Pull-Up Resistor” in Section 1.0,
Electrical Characteristics.
The output transistor activation always occurs at a fixed
rate of 30 kHz, which is outside the range of audible
frequencies.
4.2 Motor Rotation Direction
The current-carrying order of the outputs depends on
the DIR pin state (“Rotation Direction”) and is described
in Table 4-1. The DIR pin is not designed for dynamic
direction change during operation.
4.3 Frequency Generator Function
The Frequency Generator output (FG) is a “Hall effect
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 an 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 it is not used.
4.4 Lockup Protection and Automatic
Restart
If the motor is blocked and cannot rotate freely, a lock-
up protection circuit detects it and disables the driver by
setting its outputs to high-impedance to prevent the
motor coil from burnout. After a “waiting time” (T
WAIT
),
the lock-up protection is released and normal operation
resumes for a given time (T
RUN
). If 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 required.
4.5 Overcurrent Protection
The motor peak current is limited by the driver to
750 mA (standard value), thus limiting the maximum
power dissipation in the coils.
4.6 Thermal Shutdown
The MTD6505 device has a thermal protection function
which detects when the die temperature exceeds
T
J
= +170°C. When this temperature is reached, the
circuit enters the Thermal Shutdown mode, and the
outputs OUT1, OUT2 and OUT3 are disabled (high-
impedance), avoiding the IC destruction and allowing
the circuit to cool down. When the junction temperature
(T
J
) drops below +145°C, normal operation resumes.
The thermal detection circuit has +25°C hysteresis.
FIGURE 4-1: Thermal Protection
Hysteresis.
Note 1: The PWM frequency has no direct effect
on the motor speed and is asynchronous
with the activation of the output
transistors.
Note 2: The standard output frequency is 30 kHz.
A 20 kHz output frequency option is
available upon request.
TABLE 4-1: MOTOR ROTATION
DIRECTION OPTIONS
(DIR PIN)
DIR Pin State
Rotation
Direction
Outputs Activation
Sequence
Connected
to GND
or Floating
Forward OUT1 → OUT2 → OUT3
Connected
to V
BIAS
Reverse OUT3 → OUT2 → OUT1
T
J
+145°
Thermal shutdown
+170°
Normal
operation