DRV2624 Datasheet, PDF (11/79 Pages) Texas Instruments – DRV2624 Ultra Low Power Closed-Loop LRA/ERM Haptic Driver with Internal Memory

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DRV2624
SLOS893A – DECEMBER 2015 – REVISED DECEMBER 2015
Feature Description (continued)
8.3.2.1 Auto-Resonance Engine for LRA
The DRV2624 auto-resonance engine tracks the resonant frequency of an LRA in real time, effectively locking
onto the resonance frequency after half of a cycle. If the resonant frequency shifts in the middle of a waveform
for any reason, the engine tracks the frequency from cycle to cycle. The auto-resonance engine accomplishes
tracking by constantly monitoring the back-EMF of the actuator. Note that the auto-resonance engine is not
affected by the auto calibration process, which is only used for level calibration. No calibration is required for the
auto resonance engine.
8.3.2.2 Real-Time Resonance-Frequency Reporting for LRA
The smart-loop architecture makes the resonant frequency of the LRA available through I²C. Because frequency
reporting occurs in real time, it must be polled while the DRV2624 device synchronizes with the LRA. The polled
data should not be polled when the actuator is idle or braking.
8.3.2.3 Automatic Switch to Open-Loop for LRA
In the event that an LRA produces a non-valid back-EMF signal, the DRV2624 device automatically switches to
open-loop operation and continues to deliver energy to the actuator in overdrive mode at a default and
configurable frequency. If the LRA begins to produce a valid back-EMF signal, the auto-resonance engine
automatically takes control and continues to track the resonant frequency in real time. When synchronized, this
mode uses all of the benefits of the smart-loop architecture.
¦_{(LRA_NO-BEMF)} |
1
u W_{(DRIVE_TIME[4:0])} ± W_{(ZC _ DET _ TIME[1:0])}
(1)
The DRV2624 device offers an automatic transition to open-loop mode without the re-synchronization option.
This feature is enabled by setting the LRA_AUTO_OPEN_LOOP bit. The transition to open-loop mode only
occurs when the driver fails to synchronize with the LRA. The AUTO_OL_CNT[1:0] parameter can be adjusted to
set the amount of non-synchronized cycles allowed before the transition to the open-loop mode. Note that the
open-loop mode does not receive benefits from the smart-loop architecture, such as automatic overdrive and
braking.
¦_{(LRA_OL)}
1
OL_LRA_PERIOD[6:0] × 97.56 × 10^±
(2)
8.3.2.4 Automatic Overdrive and Braking
A key feature of the DRV2624 is the smart-loop architecture which employs actuator feedback control for both
ERMs and LRAs. The feedback control desensitizes the input waveform from the motor-response behavior by
providing automatic overdrive and automatic braking.
An open-loop haptic system typically drives an overdrive voltage at startup that is higher than the steady-state
rated voltage of the actuator to decrease the startup latency of the actuator. Likewise, a braking algorithm must
be employed for effective braking. When using an open-loop driver, these behaviors must be contained in the
input waveform data. Consider the example of an ERM actuator of Motor A and another of Motor B. The ideal
input waveform in open loop is different (see Figure 16). In contrast, by using the smart-loop technology with
automatic overdrive and braking, the same input waveform will work optimally for both actuators (see Figure 17).
The smart-loop architecture works equally well for LRAs with a combination of feedback control and an auto-
resonance engine.
Ideal Open-Loop Waveform for Motor A
Ideal Open-Loop Waveform for Motor B
Input and output
Accleration
Output with feedback
Figure 16. Typical Open Loop Waveform
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