DRV8601 Datasheet, PDF (8/20 Pages) Texas Instruments – Haptic Driver for DC Motors (ERMs) and Linear Vibrators (LRAs) with Ultra-Fast Turn-On

DRV8601
SLOS629B – JULY 2010 – REVISED JANUARY 2012
www.ti.com
PSEUDO-DIFFERENTIAL FEEDBACK WITH INTERNAL REFERENCE
In the pseudo-differential feedback configuration (Figure 15), feedback is taken from only one of the output pins,
thereby reducing the number of external components required for the solution. The DRV8601 has an internal
reference voltage generator which keeps the REFOUT voltage at VDD/2. The internal reference voltage can be
used if and only if the PWM voltage is the same as the supply voltage of the DRV8601 (i.e., if V_PWM = VDD, as
assumed in this section).
Having V_PWM= VDD ensures that there is no voltage signal applied to the motor at a PWM duty cycle of 50%.
This is a convenient way of temporarily stopping the motor without powering off the DRV8601. Also, this
configuration ensures that the direction of rotation of the motor changes when crossing a PWM duty cycle of 50%
in both directions. For example, if an ERM motor rotates in the clockwise direction at 20% duty cycle, it will rotate
in the counter-clockwise direction at 80% duty cycle at nearly the same speed.
Mathematically, the output voltage is given by Equation 1 (where s is the Laplace Transform variable and V_IN is
the single-ended input voltage):
V_O,DIFF
=2
´
æ
çè ^VIN
-
Vdd
2
ö
÷ø
´
R_F
R_I
´
1
1 + sR_FC_F
(1)
R_F is normally set equal to R_I (R_F = R_I) so that an overdrive voltage of VDD is achieved when the PWM duty
cycle is set to 100%. The optional feedback capacitor C_F forms a low-pass filter together with the feedback
resistor R_F, and therefore, the output differential voltage is a function of the average value of the input PWM
signal. When driving a motor, design the cutoff frequency of the low-pass filter to be sufficiently lower than the
PWM frequency in order to eliminate the PWM frequency and its harmonics from entering the motor. This is
desirable when driving motors which do not sufficiently reject the PWM frequency by themselves. When driving a
linear vibrator in this configuration, if the feedback capacitor C_F is used, care must be taken to make sure that the
low-pass cutoff frequency is higher than the resonant frequency of the linear vibrator.
When driving motors which can sufficiently reject the PWM frequency by themselves, the feedback capacitor
may be eliminated. For this example, the output voltage is given by:
V_O,DIFF
=2
´
æ
çè ^VIN
-
Vdd ö
2 ^÷ø
´
R_F
R_I
(2)
where the only difference from Equation 1 is that the filtering action of the capacitor is not present.
Same Voltage as
PWM I/O Supply
C_R
REFOUT VDD
Shutdown
Control
SE PWM
IN2
OUT-
EN ^DRV8601
R_I
IN1
OUT+
GND
+ _{LRA or}
– ^{DC Motor}
R_F
C_F
Figure 15. Pseudo-Differential Feedback with Internal Reference
8
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