A4960 Datasheet, PDF (14/34 Pages) Allegro MicroSystems – The A4960 is a three-phase, sensorless, brushless DC (BLDC) motor controller for use with external N-channel power MOSFETs and is specifically designed for automotive applications.

A4960
Automotive, Sensorless BLDC Controller
or high-to-low) zero crossing and latches it until the next state
change. This latching action, combined with precise comparator
hysteresis, provides a robust sensing system.
There are three variables that effect the BEMF sensing, these are:
• Commutation Blank Time, t_CB
• BEMF Hysteresis, V_BHYS
• BEMF Window, t_BW
Commutation Blank Time
The BEMF detectors are inhibited for t_CB following a commuta-
tion event. This prevents any commutation transients and winding
demagnetization periods from disturbing the BEMF sensing
system. The commutation blank time is shown in figure 5 and is
selected by CB[1:0] (Config0 bits 11:10 ).
BEMF Window
The BEMF window is the length of time after any PWM change,
low-to-high or high-to-low, during which the phase voltage and
the output of the BEMF comparator is ignored, as shown in
figure 6. It is selected using BW[2:0] (Run bits 9:7). The BEMF
window is effectively the BEMF comparator blank time.
If the PWM-on time is less than the BEMF window, then the
phase voltage is ignored for the whole PWM-on time. If the
PWM-off time is less than the BEMF window, then the phase
voltage is ignored for the whole PWM-off time.
BEMF Hysteresis
The BEMF hysteresis is the amount by which the BEMF voltage,
measured on the undriven phase, must exceed the normal zero-
crossing value before zero crossing is detected and TACHO goes
high. This is illustrated in figures 7 and 8.
If the BEMF is falling, then the zero crossing will be detected
when the BEMF voltage is lower than the normal zero-crossing
value minus the BEMF hysteresis (figure 7).
If the BEMF is rising, then the zero crossing will be detected
when the BEMF voltage is higher than the normal zero-crossing
value plus BEMF hysteresis (figure 8).
The BEMF hysteresis is selected using BH[1:0] (Run bits
11:10). It can be set to zero, low (typically 60 mV), high (typi-
cally 240 mV) or Auto. Auto sets the hysteresis to the high level
during start-up and reduces it to the low level during running.
This provides added security during start-up in achieving a stable
BEMF detection but permits the motor to run slower or at a lower
voltage when BEMF detection is achieved.
Start-up
In order to correctly detect the zero crossing, the changing motor
BEMF on any phase must be detectable when that phase is not
being driven. When the motor is running at a relatively constant
speed, this is ensured by the adaptive commutation scheme used.
However, during start-up, particularly when the motor load has a
high friction component, the motor must be accelerated from rest
in such a way that the BEMF zero crossing can be detected. Ini-
tially, as the motor is started, there is no rotor position informa-
tion from the BEMF sensor circuits and the motor must be driven
in an open loop 3-phase stepper mode. Unlike a true stepper
motor, which is designed for open loop operation, most 3-phase
BLDC motors are unstable when driven in this way and will
overshoot the intended step point by a large margin. To overcome
this limitation the motor must be accelerated such that the motor
movement and the phase step sequence can synchronize to allow
correct BEMF zero crossing detection.
The initial start speed, the acceleration rate, and the accelerat-
ing torque must be adjusted for each combination of motor and
mechanical load. These parameters can be programmed in the
A4960 through the serial interface. Configuration registers Con-
fig4 and Config5 provide the following programmable param-
eters:
• Config4 bits SC[3:0] – set the start of ramp speed
• Config4 bits EC[3:0] – set the end of ramp speed
• Config5 bits RR[3:0] – set the ramp rate
• Config5 bits RQ[3:0] – set the acceleration torque
To ensure that the motor start-up and sensorless BEMF capture
is consistent, the start sequencer always forces the motor to a
known start position. The time during which the motor is forced
into the start position can be programmed through the serial inter-
face using the four hold time bits, HT[3:0] (Config3 bits 3:0).
The torque applied during this hold time is programmed using
the four hold torque bits, HQ[3:0], (Config3 bits 7:4). These two
variables allow a stable start condition to be achieved for differ-
ent motor and attached mechanical loads.
As soon as a valid BEMF zero crossing is detected during the
start sequence, the A4960 will transition to full BEMF commuta-
tion and the start sequencer will be reset. The TACHO output
Allegro MicroSystems, Inc.
14
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com