Piezo haptic driver extends battery life in portables

By Tim Dhuyvetter, principal staff engineer, and Nazzareno (Reno) Rossetti, analog and power management expert, Maxim Integrated

During your last hike, you may have taken a picture of a beautiful and wild creature: a snake! Were you tempted to move your fingers over its scales to feel its smooth shape? Before long, you may be able to do just that — without the risk of getting bit.

With the help of haptics, the tactile feedback technology that exploits the sense of touch by applying forces, vibrations, or motions to the user is growing by leaps and bounds and will soon render the feeling of the complex textures described above. More mundanely, studies show that, on average, virtual keyboard users will complete typing tasks significantly faster when provided with tactile feedback.

Fig. 1: Virtual and safe touch with haptics (Image: Shutterstock)

To accomplish this, haptic technology is moving beyond the basic options possible with a vibrating motor and by exploiting the flexibility of piezoelectric actuator technology.

The piezo actuator
The piezoelectric actuator converts an electrical signal into a precisely controlled physical displacement. Signals of hundreds of volts at frequencies of a few hundred hertz are necessary to produce a meaningful displacement (Fig. 2 ). Just as the human ear perceives certain frequency sounds to be louder than others, vibrations are perceived differently based on their intensity.

As in the hearing process, the intensity of the tactile experience is proportional to the frequency and amplitude of the vibrating signal. Changing the vibration intensity and pattern creates a virtually limitless number of combinations, rhythms, or messages. When operated below their electrical resonant frequencies, piezo actuators are capacitive in nature.

Piezo-based haptics offer several advantages over competing technologies, including fast response time, thin profile, low power, and a wide range of available piezo characteristics and mounting technique.

Fig. 2: Typical piezo haptic driver waveform

The haptic system
Fig. 3 shows a high-level block diagram of a multi-channel haptic system. As an example, when the user strikes a key, a capacitive touch sensor IC detects the action and reports the information to the microprocessor, which instructs the piezo haptic driver to apply the appropriate vibration pattern to the piezo element selected with the high-voltage multiplexer.

Accordingly, the user receives a physical sensation of having struck the key. Naturally, for this to work, the loop response must be faster than the operator detection time lag. A haptic event with a delay of 5 ms or less will be perceived by the user as simultaneous.

Fig. 3. Multi-channel haptic system example

Typical solution
Fig. 4 shows a typical piezo haptic driver IC implementation for portables. A boost converter steps up the lithium-ion battery voltage to 60 V, providing the necessary high-voltage power supply to the Class AB amplifier.

Fig. 4: Typical haptic driver solution

This solution requires significant input power, which is dissipated in the IC. High power dissipation is problematic in portable applications. First, the Class AB linear amplifier is inherently dissipative (say, 60% efficiency), and the boost converter adds to the losses with its own finite efficiency (say, 85%).

As an example, a 1-µF capacitance with 100 V_P-P at 200 Hz adds up to a load of 1 W:

With a system efficiency of 50% (085 × 0.6), the input power from the battery adds up to 2 W, power dissipated on the IC.

Ideal solution
An ideal solution is one that recovers the capacitor energy as opposed to dissipating it. One solution is to use a Class D amplifier instead of a Class AB one. This way, the amplifier losses are minimized, but the boost converter losses remain. Additionally, a Class D amplifier solution requires a complicated output filter that is bulky and introduces additional losses.

The next step is to combine the amplification and the boost function into one single circuit, as shown in Fig. 5 . Notice that the high-voltage, 200-Hz waveform is very slow compared to the boost converter clock frequency (approximately 500 kHz), resulting in a switching regulator with a slowly varying output.

Fig. 5 also shows the haptic voltage and current waveforms with their amplitude, sign, and phase relationship. During the positive half-wave period, the inductor current charges the capacitor/piezo. During the negative half-wave, the current is returned to the input capacitor through the rectifying MOSFET transistor T₁ , conducting reverse current when on. The result is an energy-recycling scheme that yields virtually no losses, with the exception of the switching losses and finite R_DS(ON) of the MOSFET transistors.

Notice how the controller IC is a cost-effective low-voltage device and only the external MOSFETs and passives are high-voltage. The capacitor C_HV implements the necessary level translation between the low-voltage driver and the high-voltage MOSFET gate.

Fig. 5. Boost converter regenerative solution

High-efficiency piezo haptic actuator driver
As an example, the MAX77501 is a high-efficiency driver for piezo haptic actuators and is optimized for driving up to 2-μF piezo elements. It can generate a single-ended haptic waveform of up to 110-V_P-P amplitude from a 2.8-V to 5.5-V input power supply or a single-cell Li+ battery. Memory playback (RAM) and real-time streaming (FIFO) of haptic waveforms are supported.

A 25-MHz SPI interface provides full system access and control, including fault reporting and monitoring. This al­lows for a rapid 600-μs playback startup time from shutdown. The on-board memory can be dynamically allocated as multiple waveform storage or as a FIFO buffer. The IC also implements an ultra-low-power boost architecture that provides the lowest power consumption solu­tion for a haptic actuator driver. Built-in undervoltage lockout (UVLO), cycle-by-cycle overcurrent limit, overvoltage, and thermal shutdown protections ensure safe operation under abnormal operating conditions. The IC is available in a 30-bump, 0.4-mm-pitch, wafer-level package (WLP).

Conclusion
Piezoelectric actuators are a key element of modern haptic sensing systems, helping provide tactile feedback to the user, who is interacting with an electronic display in battery-operated, handheld, and wearable devices.

Typical piezo haptic drivers are complex and power-hungry. This design solution reviews a typical implementation, in which significant power is dissipated in the Class AB amplifier driving the piezo and in the boost converter generating the high-voltage power supply. Subsequently, it introduces a novel, regenerative implementation in which the boost converter is both the high-voltage source and the amplifier that drives the capacitive piezo element drawing AC current. The boost converter inductor current, when positive, charges the piezo capacitance. When it is negative, the energy is returned to the input capacitor, effectively implementing a regenerative mechanism that minimizes power losses, thereby maximizing portable battery life.

About the authors
Tim Dhuyvetter is principal staff engineer in the Mobile Product group at Maxim Integrated. Tim has more than 43 years of experience in IC design and systems architecture for power management and audio circuit systems.

Nazzareno (Reno) Rossetti is an analog and power management expert at Maxim Integrated. He is a published author and holds several patents in this field. Reno holds a doctorate in electrical engineering from Politecnico di Torino, Italy.