DRV8308_15 Datasheet, PDF (15/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

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Feature Description (continued)
DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
MR
Pickup
1nF
0.1éF
2k
Filter components
as required ±
values may differ
in actual
application
FGIN+
4.7éF
FGIN- / TACH
820k
FGFB
100pF
5V
2.50V
2.75V
HALL_U
HALL_V
HALL_W
-
+
S
FGSEL
2
(to speed control
& stalled rotor
protection)
FG
0
1
2 ^MUX
3
+
Q
-
R
-
+
Figure 5. FG Circuit Diagram
FGOUT
The output of the FG amplifier is provided on a terminal, so the gain of the FG amplifier can be set by the user.
Filter circuits can also be implemented.
Note that the FG signal is also fed back internally to the speed control circuits.
The FG signal that the DRV8308 device uses can be generated from a PCB trace under a motor, or it can be
input from a logic-level TACH input, or it can be synthesized from the Hall sensor transitions (selectable by
register FGSEL). If generated from Hall transitions, the resulting output can be either an exclusive-or function of
the three Hall sensors, or the same as the HALL_U input, as shown in Figure 6.
Selection of FG operating mode is through the FGSEL register bits.
The FGOUT terminal is an open-drain output and requires an external pullup resistor to the logic supply.
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