DRV8308_15 Datasheet, PDF (44/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
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Application Information (continued)
Since the FG trace is in close proximity to the motor coils, it is very susceptible to noise coupling from the PWM
of the motor. Noise coupling into the FG circuit causes poor speed regulation, especially at low motor speeds.
Startup is a particularly difficult situation, as the motor current is at a maximum, and the FG signal amplitude is
low (in fact, 0 at the moment of startup). If noise couples into FG during startup, the speed loop interprets the
noise as fast motor rotation, and lowers the PWM duty cycle. The result is slow startup of the motor. If this
problem is suspected, looking at the FGOUT signal with an oscilloscope during startup should reveal it.
To address this, in addition to the resistors that set the gain of the FG amplifier (R1 and R2 in Figure 28), usually
passive filter components are needed on the FG amplifier circuit.
MR
Pickup
C1
C2
R1
FGIN+
C4
FGIN±/ TACH
R2
FGFB
C3
Figure 28. FG Amplifier Circuit
Ideally, the user desires a large amount of rejection of the PWM frequency. However, the user needs to pass the
frequency that corresponds to their fastest motor speed. As an example, a motor may put out 36 FG pulses per
revolution. At 5000 RPM, this is a 3-kHz signal. If you operate the PWM at 25 kHz, you can set a single pole at 3
kHz and have significant rejection of the PWM frequency, and the higher harmonics of the PWM (which are
typically more easily coupled) are rejected even more.
Because the amplitude of the FG signal also increases with higher motor speed, it is possible to set this pole at a
much lower frequency than the maximum speed dictates. The optimal values need to be determined by testing
on the actual motor.
This pole is set by C3 in Figure 28.
In addition to rejection of high frequency, the FG winding should be AC-coupled to the amplifier to prevent any
issues with DC offsets. This capacitor (C1) must be large enough to allow the motor to start-up reliably, since the
FG frequency and amplitude are very low at startup. Typically capacitors on the order of 100 nF to 1 µF are used
here. The voltage is low, so a 6.3-V ceramic capacitor can be used.
Occasionally an additional small capacitor is used across the FG trace. This capacitor (C2 above) may not be
needed, but it can help reject very high-frequency harmonics of the PWM (glitches). Capacitors between 330 and
2200 pF are typically used.
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