Royce Higashi and Tony Doy, Maxim Integrated Products
EDN
Low-profile piezoelectric speakers can provide quality sound for portable electronic devices, but they require voltage swings greater than 8 V p-p across the speaker element. Yet, most portable devices include only a low-voltage power source, and conventional amplifiers operating from batteries cannot provide enough voltage swing to drive a piezoelectric speaker.
One approach to this problem is to use IC1 in Figure 1, which you can configure to drive a piezoelectric speaker with as much as 12 V p-p and operate from a single 3 V supply.
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| Figure 1. | This bridge-tied-load configuration multiplies the amplifiers’ voltage-swing capability. | |
IC1, a MAX4410, combines a stereo-headphone driver with an inverting charge pump that derives a negative –3 V supply from the positive 3 V supply. Thus, providing the drive amplifiers with an internal ±3 V supply allows each output of IC1 to swing 6 V p-p.
Configuring IC1 as a BTL (bridge-tied load) driver again doubles the maximum swing at the load to 12 V p-p. In the BTL configuration, IC1’s right channel serves as the master amplifier. It sets the gain of the device, drives one side of the speaker, and provides a signal to the left channel. If you configure IC1 as a unity-gain follower, the left channel inverts the output of the right channel and drives the other leg of the speaker. To ensure low distortion and good matching, you should set the left-channel gain using precision resistors.
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| Figure 2. | Testing the circuit yields this THD+N versus output voltage for the Figure 1 circuit. |
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We tested the circuit with a Panasonic WM-R57A piezoelectric speaker, yielding the THD+N (total-harmonic-distortion-plus noise) curves (figures 2 and 3). Note that THD+N increases as frequency increases in both graphs. Because the speaker appears to the amplifier as a capacitor, the speaker’s impedance decreases as frequency increases, resulting in a larger current draw from the amplifier.
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| Figure 3. | Testing the circuit yields this THD+N versus frequency for the Figure 1 circuit. |
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IC1 remains stable with the speaker, but a speaker with different characteristics might cause instability (Figure 4). In that case, you can isolate the speaker’s capacitance from the amplifier by adding a simple inductor/resistor network in series with the speaker (within the dotted lines on Figure 1). The network maintains stability by maintaining a minimum high-frequency load of approximately 10 W at the IC’s output.
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| Figure 4. | Step response at the OUTR output of IC1 in Figure 1, which drives a WM-R57A piezoelectric speaker, shows that IC1 remains stable with the speaker. |
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