Datasheet

DA7280
LRA/ERM Haptic Driver with Multiple Input Triggers,
Integrated Waveform Memory and Wideband Support
Datasheet
Revision 3.0
30-Jul-2019
CFR0011-120-00
23 of 76
© 2019 Dialog Semiconductor
5.3 Resonant Frequency Tracking
LRAs are high-Q systems that have to be driven exactly at resonance to achieve maximum possible
output acceleration. DA7280 supports continuous resonant frequency tracking via BEMF sensing
during playback to achieve optimum LRA acceleration output across manufacturing spread,
operating temperature range, external damping, and actuator aging.
When the FREQ_TRACK_EN is high, a digital resonant frequency tracking loop locks onto the LRA
resonant frequency in real time by adjusting the drive period. This ensures that the actuator is always
driven at the optimum frequency for the highest efficiency electrical to mechanical energy conversion.
The loop range of 50 Hz to 300 Hz covers existing narrowband LRAs; typical resonant frequency lock
accuracy is 0.5 Hz.
To increase absolute accuracy of the lock during playback, D7280 supports automatic scaling of the
frequency tracking controller gain. The feature is enabled via FREQ_TRACKING_AUTO_ADJ and
becomes active after the device has achieved initial lock, see Section 5.7.1.
The resonant frequency tracking algorithm is designed to converge to the correct value from up to
25 % offset between the initial nominal datasheet value and the actual resonant frequency. This
range is conservative in order to prevent unwanted behavior. A fault will trigger if the actuator
resonant frequency is outside the 50 Hz to 300 Hz range. To block these two features, set
FREQ_TRACKING_FORCE_ON = 1, see Section 5.7.1.
5.4 Active Acceleration and Rapid Stop
Mechanical systems such as LRAs and ERMs accelerate and decelerate exponentially and the time
between transitions (for example from stopping of the drive signal to the actuator coming to a
complete rest) can be perceptibly slow for the user. DA7280 features Active Acceleration and Rapid
Stop to overcome this latency, which enables stronger clicks and a higher fidelity playback in both
LRAs and ERMs. This capability offers a distinct advantage over legacy systems, which do not sense
BEMF, because it allows the use of cheaper, slower response time actuators while keeping haptic
effects crisp.
Active Acceleration employs relative drive architecture based on BEMF sensing, which enables
temporary overdrive on all level changes reducing the time required to achieve a target drive level.
The DA7280 Active Acceleration algorithm does not require dedicated calibration procedures and
enables accurate overdrive and underdrive throughout the lifetime of an actuator. The feature
removes the need for a separate calibration sequence to determine the correct overdrive duration,
see Figure 10 and Figure 11.
Enabling Active Acceleration typically reduces the time to achieve the target drive level by a factor of
two on sequence level changes. The Rapid Stop feature typically reduces the time to achieve a zero
drive level by a factor of three when enabled. Figure 10 shows a drive sequence without the features
enabled and Figure 11 illustrates the reduced time to target when Active Acceleration and Rapid
Stop are enabled.
Note: The Active Acceleration and Rapid Stop features require frequency tracking to be enabled.
Figure 12 demonstrates the system with an actual LRA for an equivalent duration sequence without
and with the Active Acceleration and Rapid Stop features. The nominal actuator acceleration is
achieved faster and the stopping time is reduced by a factor of approximately eight.
Active Acceleration and Rapid Stop are enabled using ACCELERATION_EN and RAPID_STOP_EN.