DRV8308_15 Datasheet, PDF (14/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
www.ti.com
8.3 Feature Description
8.3.1 Hall Comparators
Three comparators are provided to process the raw signals from Hall effect transducers to commutate the motor.
The Hall amplifiers sense zero crossings of the differential inputs and pass the information to digital logic.
The Hall amplifiers have hysteresis, and their detect threshold is centered at 0. Note, hysteresis is defined as
shown in Figure 4:
Hall
Differential
Voltage
0V
V_HYS
Hall Amplifier
Output
(Internal)
Figure 4. Hall Amplifier Hysteresis
In addition to the hysteresis, the Hall inputs are deglitched with a circuit that ignores any extra Hall transitions for
a period of 20 μs after sensing a valid transition. This prevents PWM noise from being coupled into the Hall
inputs, which can result in erroneous commutation.
If excessive noise is still coupled into the Hall comparator inputs, it may be necessary to add capacitors between
the + and – inputs of the Hall comparators, and (or) between the input or inputs and ground.
The ESD protection circuitry on the Hall inputs implements a diode to VREG. Because of this diode, the voltage
on the Hall inputs should not exceed the VREG voltage.
Since VREG is disabled in standby mode (ENABLE inactive), the Hall inputs should not be driven by external
voltages in standby mode. If the Hall sensors are powered from VREG or from VSW, this is specified by the
DRV8308 device; however, if the Hall sensors are powered externally, they should be disabled if the DRV8308 is
put into standby mode. In addition, they should be powered-up before enabling the motor, or an invalid Hall state
may cause a delay in motor operation.
8.3.2 FG Amplifier, Comparator, and FG Output
An FG amplifier and comparator provide rotational feedback from an external magnetic reluctance sensor. A
diagram of the FG circuit is shown in Figure 5:
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