2 DRV2605L Audio-to-Haptic Mode Overview and Advantages

The DRV2605L is a specialized haptic driver for both Linear Resonant Actuators (LRAs) and Eccentric Rotating Mass motors (ERMs). This includes a closed-loop control architecture and an internal library of haptic effects. This document focuses on the Audio-to-Haptic capability, a feature that automatically converts an audio input signal into meaningful tactile effects. Key advantages of using the DRV2605L Audio-to-Haptic mode include:

• Automatic Audio Conversion to Haptics: In Audio-to-Haptic mode, the DRV2605L continuously monitors an analog audio input and generates a corresponding drive output on the LRA. Low-frequency components in the audio (for example, a 50–200Hz bass rhythm or an impact sound) are detected and translated into vibration. This happens in real time, so even if a game does not explicitly issue a rumble command, the audio produces a tactile response. The conversion algorithm is tuned so that the vibrations feel relevant to the content – sharp transient sounds create crisp taps, while sustained low tones create a buzzing or rumble – rather than a one-size-fits-all buzz.
• Resonance Tracking and Smart-Loop™ Closed-Loop Control: The DRV2605L employs a closed-loop control system with automatic resonance tracking for LRAs. An LRA has a natural resonant frequency (typically about 170–200Hz); driving this at resonance maximizes efficiency and vibrational strength, but the resonant frequency can shift slightly with temperature, aging, or if the device is not exactly mounted. The DRV2605L’s Smart-Loop™ algorithm constantly adjusts to stay on the LRA’s resonance and applies active braking when needed. This yields more consistent acceleration and faster rise/fall times for vibrations. In practice, closed-loop control prevents the LRA from ringing or overshooting when the audio signal stops, and makes sure each pulse is crisp. (By contrast, an open-loop drive can let the LRA ring and take longer to settle.)
• Built-In Vibration Effect Library: Aside from Audio-to-Haptic mode, the DRV2605L contains an internal ROM library of over 100 predefined haptic waveforms and effects (licensed from Immersion). While not the focus of this note, this library can be very useful when games do provide specific haptic events – the host processor can trigger library waveforms through I²C (for example, playing a double click vibration or an explosion rumble). Having this library on-chip means developers don’t need to craft custom vibration waveforms for common effects. This design primarily used Audio-to-Haptic for automatic background feedback, but the library effects are available for on-demand events (like a game menu selection or a special in-game action). Later in the document demonstrates switching between Audio-to-Haptic and a library effect.

Note that the DRV2605L’s Audio-to-Haptic engine drives the LRA designed for tactile feel, which does not always exactly mimic the input audio waveform. The goal is to maximize the vibration relevance rather than literally reproducing the audio frequencies (especially since an LRA cannot respond well to very high frequencies). The device effectively extracts the amplitude envelope of the audio and drives the LRA at the resonant frequency, modulating the intensity. This makes sure of strong feedback even if the audio frequency is above or below the LRA’s designed for range.

To illustrate the benefit of the closed-loop Smart-Loop control, Figure 2-1 shows a scope capture of an open-loop drive scenario. Here, a built-in library waveform (a double-click effect) was played to an LRA with the driver’s closed-loop features disabled. The upper trace shows the driver’s raw PWM output to the LRA, and the lower trace shows the actual voltage across the LRA. Without adaptive control, the LRA’s response is not tightly controlled – you can observe some variation in amplitude and a slower damping of the vibration after each pulse. Enabling the DRV2605L’s Smart-Loop closed-loop mode makes these pulses more consistent and sharply delineated (not shown in this figure, but observed in our testing).

Open-loop LRA drive waveform for a double-click haptic effect (using the DRV2605L’s internal library).The upper waveform (yellow/magenta) is the differential output from the driver, and the lower waveform (red) is the resulting voltage across the LRA. Without closed-loop damping and resonance control, the LRA motion shows slight overshoot and inconsistent amplitude between pulses.

Figure 2-1 Open-Loop LRA Waveform for Double-Click Effect

In summary, the DRV2605L’s Audio-to-Haptic mode simplifies the addition of ambient haptics by leveraging existing audio, while the closed-loop control and effect library makes sure that the vibrations are high-quality and that specific tactile events can still be played as needed.