SLVUA41E
10
2.5 If the GUI has slow performance
Some computers exhibit slow performance on the third GUI tab. 2 common causes are:
1. Too many Ports in Device Manager. Go there by accessing Start
Control Panel
System
Device Manager. Under Ports, the one for the EVM is named
“USB Serial Port”. If there are
others listed, try disabling them with a right-click Disable. Then restart the GUI.
2. A slow computer with many programs running in the background. Try closing all extra
programs and disk scanners.
3. Tuning the speed control system
The DRV8308 Clock Frequency Mode for speed control performs best when the device registers are
tuned for a particular
system’s motor, inertia, speed, and load torque.
3.1 Key registers
This first set of registers must be set correctly:
MOD120
– Always set this to 3970.
SPEED
– This is the open-loop gain during spin-up, before LOCKn goes Low. It simply must
be high enough in order to reach the commanded RPM.
AUTOGAIN
– When enabled, the DRV8308 scales LOOPGAIN with RPM, because more gain is
needed for more speed. This is a valuable feature and should generally be left
enabled. Then
ScaledGain = LOOPGAIN * f
CLKIN
/AG_SETPT
.
AG_SETPT
– This setpoint effectively sets the range for AUTOGAIN. Simply use the setting that
is closest to f
CLKIN
(see the DRV8308 datasheet register descriptions).
AUTOADV
– When enabled, the DRV8308 more aggressively scales ADVANCE with RPM:
ScaledAdvance = ADVANCE * f
HALLU
/AA_SETPT
. Note that without this,
ADVANCE by itself already scales with speed since each value shifts timing by
0.1% of the Hall U period. Disable AUTOADV initially; once a system is well-tuned,
you can experiment with AUTOADV to potentially improve performance.
AA_SETPT
– This only applies when AUTOADV is enabled. Use the setting closest to the
frequency of Hall U.
IDRIVE
– This sets the predriver output current and directly controls the external FET V
GS
turn-on time. Unless high current FETs are used, lower values are generally better,
to minimize switching noise.
FGSEL
– If an FG Sense trace is not used, it is generally best to set FGSEL to 0 (HALL_U).
