4 Waveform Test Results and Analysis (Audio-to-Haptic Mode)

First, the DRV2605L’s output in Audio-to-Haptic (A-V) mode was examined using controlled audio inputs. The goal was to see how effectively the driver converts different frequency audio signals into LRA vibrations, and how the vibration amplitude scales with audio amplitude. Three test tones were used – 100Hz, 200Hz, and 300Hz – which represent audio content below, at, and above the typical LRA resonant frequency (about 180Hz). These can be thought of as a low bass rumble (100Hz), a near-resonance buzz (200Hz), and a higher-pitched hum (300Hz). Each tone was tested at four amplitude levels: 25%, 50%, 75%, and 100% (where 100% corresponds to the system volume being set to maximum). For example, 25% corresponds to setting the system volume to 25, 50% to volume level 50, and so on. This configuration allows us to observe the linearity of the DRV2605L's output at different input volume levels and determine whether any saturation occurs in the vibration response.

For each test, the audio was played in A-V mode and the steady-state waveforms of the LRA drive (differential output) and the audio input were captured. The oscilloscope screen shots (Figure 4-1 through Figure 4-12) show the results. In all these figures, the blue trace (labeled C2) is the audio input voltage, and the magenta or yellow traces (C1 and C3, or the mathematical difference M3) represent the differential voltage across the LRA as driven by the DRV2605L. Essentially, the colored PWM wave on each graph is the driver output, and the envelope or amplitude corresponds to the vibration strength. The measured amplitude of the audio input for each case (in millivolts RMS) is indicated in the figure captions for reference.

100Hz Input (Below Resonance): At 100Hz, the LRA is being driven below the resonant frequency. The vibration is expected to be weaker for a given voltage compared to at resonance, but the DRV2605L can still drive proportionally to the audio amplitude.

DRV2605L output for a 100Hz sine wave input at 25% volume (2.1mV). At this low audio level, the differential voltage applied to the LRA is small, resulting in a very slight vibration. The waveform shows the PWM pulses (pink or yellow) at the driver output; the amplitude of these pulses is low, just enough to produce a barely perceptible buzz.

Figure 4-1 100Hz Input at 25% Volume – Minimal Vibration

DRV2605L output for 100Hz input at 50% volume (7.1mV). With a moderate audio level, the driver delivers a higher voltage to the LRA (roughly double the amplitude of the 25% case). The LRA’s vibration is now noticeable. The scope trace shows a clear 100Hz envelope in the LRA’s drive voltage (blue trace oscillating at 100Hz, and PWM output following that envelope). The vibration amplitude has roughly doubled from the 25% case, indicating an approximately linear response in this range.

Figure 4-2 100Hz Input at 50% Volume – Noticeable Vibration

DRV2605L output for 100Hz input at 75% volume (14.1mV). At this higher audio level, the driver’s output PWM swings are larger, driving the LRA with greater force. The vibration is strong and still follows the 100Hz audio waveform envelope.

Figure 4-3 100Hz Input at 75% Volume – Strong Vibration

DRV2605L output for 100Hz input at 100% volume (21.8mV). With maximum audio input, the driver outputs the full-scale differential voltage (yellow or magenta PWM traces). The LRA receives a strong 100Hz-driven vibration. The driver successfully drives the LRA without clipping.

Figure 4-4 100Hz Input at 100% Volume – Full-Scale Vibration

200Hz Input (Near Resonance): 200Hz is in the vicinity of our test LRA’s resonant frequency. The LRA is expected to respond most strongly around this frequency, achieving larger vibration for less input voltage.

DRV2605L output for a 200Hz sine wave input at 25% volume (2.1mV). There is no PWM output on M3 trace.

Figure 4-5 200Hz Input at 25% Volume – No PWM Output

DRV2605L output for 200Hz input at 50% volume (7.1mV). With a half-scale audio input at resonance, the LRA drive (yellow or magenta) is more pronounced. The blue trace indicates the 200Hz audio waveform.

Figure 4-6 200Hz Input at 50% Volume – Moderate Vibration

DRV2605L output for 200Hz input at 75% volume (14.1mV). At 75% audio level, the output waveform shows large PWM swings.

Figure 4-7 200Hz Input at 75% Volume – High PWM Activity

DRV2605L output for 200Hz input at 100% volume (21.8mV). Full-scale input at resonance drives the LRA at maximum intensity. The output waveform (magenta or yellow) exhibits the highest amplitude pulses observed, and the blue trace confirms the 200Hz audio is continuous. The LRA’s movement is at the peak – this corresponds to a very strong vibration (a deep buzz).

Figure 4-8 200Hz Input at 100% Volume – Maximum Vibration

At resonance, the DRV2605L’s closed-loop control was especially valuable. As the LRA’s motion intensified, the back-EMF from the LRA also increases (LRAs generate a voltage as the LRA move).

300Hz Input (Above Resonance): 300Hz is higher frequency. In this regime, the driver is pushing the LRA less stronger, so the vibration output is expected to drop off.

DRV2605L output for a 300Hz input at 25% volume (2.1mV). At a quarter volume, the audio is a 300Hz tone, but the LRA’s differential voltage (M3, red trace) is zero.

Figure 4-9 300Hz Input at 25% Volume – No Vibration

DRV2605L output for 300Hz input at 50% volume (7.0mV). With a moderate 300Hz input, the driver again outputs has no vibration.

Figure 4-10 300Hz Input at 50% Volume – No Vibration

Indeed, at 300Hz, the vibrations were quite weak. The DRV2605L appears to handle the input by converting this into a lower-frequency vibration but at an amplitude that reflects the difficulty of a 300Hz excitation. As the amplitude was increased:

DRV2605L output for 300Hz input at 75% volume (14.1mV). At 75% volume, the scope shows substantial PWM activity. The LRA does vibrate, but the vibration feels softer than the equivalent 75% at 200Hz. The output waveform is well-behaved (no erratic behavior), indicating the driver is in control – the driver simply cannot create as strong a vibration at that frequency.

Figure 4-11 300Hz Input at 75% Volume – Soft Vibration

DRV2605L output for 300Hz input at 100% volume (21.0mV). With maximum audio amplitude at 300Hz, the driver outputs as much as the driver can. The scope shows a high-frequency content in the audio (blue) but the LRA drive (magenta or yellow) manifests as an amplitude-modulated waveform primarily at the LRA’s resonance. The resulting vibration is present and roughly correlates to the audio’s intensity, but this is noticeably weaker than the 100Hz or 200Hz full-volume cases.

Figure 4-12 300Hz Input at 100% Volume – Weak Vibration Response

Higher frequency, some tactile feedback is still generated – the control makes sure the waveforms remained clean in all cases (no unintended distortion or prolonged ringing). Note that very short audio bursts (less than approximately 10–20ms) do not produce much tactile sensation if below the human tactile threshold. The DRV2605L’s algorithm likely incorporates filtering that can ignore extremely brief blips of audio to avoid spurious ticks.

Overall, these steady-state tests demonstrated that the DRV2605L effectively converts audio input into LRA vibrations, especially around the LRA’s resonant frequency. The device provided a roughly proportional vibration response to the audio amplitude up the limits, with maintaining stability.