Texas Instruments PMP15034 Manuale utente

Test Report

July 2017

PMP15034 Test Results

System Description

A piezoelectric sounder is a loudspeaker that uses the piezoelectric effect to generate sound. It is widely used in

the application where sound volume and high pitch are more important than sound quality, such as toys, air

conditioner, microwave oven and alarm equipment.

The piezoelectric sounder normally required a 1-KHz to 5-KHz rectangular voltage to operate. The sound press

mainly depends on the amplitude of the driving voltage. The higher driving voltage, the higher sound press will be.

For example, the PKM13EPYH4002 from Murata requires 0-V to 30-V or +15-V to −15-V rectangular voltage. The

power supply in these applications could be two cell alkaline battery, lithium-ion battery or 3.3-V, 5-V power rail,

which is much lower than the target voltage. Therefore, a high performance, simple boost converter TLV61046A

is required for this application.

TLV61046A is 1.8-V - 5.5-V input, up to 28-V output, full integrated boost converter. It integrates low side

MOSFET, high side rectifier diode and the isolation MOSFET. The isolation MOSFET disconnects the pass-

through path between input and output when the device is disabled. This function helps to reduce the battery

power loss when the piezoelectric sounder doesn’t work.

Based on TLV61046A, the reference design introduces two driving circuit of piezoelectric sounder. With PWM

signal from MCU, one circuit provides rectangular wave driving voltage between 0-V and 28-V; the other circuit

provides ±15-V rectangular wave driving voltage.

The block diagram of the 28-V driver circuit is shown in Figure 1. The output voltage of the TLV61046A is set to

28V. A PWM signal from MCU drives Q1 to control the voltage across the piezoelectric sounder LS. When PWM

signal is logic high, the Q1 turns on, so 28V is applied to LS. When PWM signal is logic low and Q1 turns off, the

LS is discharged by the resistor in parallel. The frequency of the PWM signal should be equal to the

recommended frequency of the sounder. The duty of the PWM should be as low as possible to reduce the power

loss when the sound press is acceptable.

1.8V~5.5V

TLV61046A

Boost Converter

VIN

GND

VOUT

PWM

28V

GND

Figure 1: Block Diagram of the 28-V Driver Circuit

The block diagram of ±15-V driving circuit is shown in Figure 2. The output of the TLV61046A is set to 15V. An

additional charge pump circuit doubles the output voltage to approximately 30V. When Q1 turn off and Q2 turn on,

the voltage according the sounder LS is 15V; when Q1 turns on and Q1 turns off, the voltage according LS is -

15V. The frequency of driving voltage follows the PWM input from MCU. By modifying the feedback divider of the

TLV61046A and selecting proper Q1, Q2, this circuit supports ±28 V for the sound. The resistor in parallel with LS

can discharge the voltage to zero after the boost converter is disabled.

Lit. Number

2PMP15034 Test Results

1.8V~5.5V

TLV61046A

Boost Converter

VIN

GND

VOUT

PWM

15V

GND

Charge Pump

30V Q1

Figure 2: Block Diagram of ±15-V Driver Circuit

Design Theory

The schematic of the 28-V driver circuit is shown in Figure 3. The output voltage of the boost converter is

configured by R1 and R3. This voltage should be set to as high as possible to get highest sound.

Q1 is a NPN device driven by the PWM signal from the MCU. The sounder TS1 is charged to 28V when Q1 turns

on, and discharged by the R2 to 0V when Q1 turns off. The frequency of the PWM should be equal to the

recommended operating frequency of the sounder, which is 4KHz in this reference design.

The duty cycle of the PWM and the R2 resistance impact the sound press of TS1. They also impact the power

loss of the driver circuit because the current flows through R2 to ground when Q1 turns on.

The power loss of R2 is calculated by equation (1), where D is duty cycle of the input PWM signal.

PR2 = VOUT2 × D ÷ R2 (1)

Follows are two steps to find suitable R2 and duty cycle to compromise the sound press and power loss:

Setting the duty cycle to 50%, select a 1-KΩ to 10-KΩ resistor that minimize the power loss without

suffering the sound press

Using R2 selected by step 1, slowly decrease the duty cycle to minimize the power loss if the sound press

requirement is met.

Figure 3: 28-V Rectangular Waveform Driver Circuit

Lit. Number

PMP15034 Test Results 3

The schematic of the ±15-V driver circuit is shown in Figure 4. the output voltage of the TLV61046A is set to 15V.

The additional charge pump circuit doubles the output voltage to approximately 30V. The NPN Q2 and PNP Q3

form a totem pole driver circuit. The Q4 and R5 form a simple level shift circuit. When PWM is high voltage, the

Q4 turns on. So the totem pole circuit outputs 0.7V. The voltage according LS2 is -14.3V. When the PWM is low

voltage, the output voltage of totem pole circuit is 29.3V. So the LS2 voltage is 14.3V.

Figure 4: ±15-V Rectangular Waveform Driver Circuit

The current flowing through R5 to ground increases the power loss of the circuit. It is suggested to minimize the

duty cycle of the PWM signal if the sound press meets the requirement.

The R6 in parallel with LS2 discharge the DC voltage at LS2 to zero after the TLV61046A is disabled.

Test Result

Figure 5 shows the startup waveform the 28-V circuit by EN signal. The device finishes startup process in 7ms. It

is suggested that the PWM input signal is 7ms later than EN high logic.

VOUT 5V/div

EN 2V/div

Figure 5: Startup at VIN=3.3V

Figure 6 shows operating waveform when the PWM input signal is 4KH with 30% duty and VIN = 3.3V. The

sounder voltage is charged to 28V when the Q1 turns on, and discharged to almost 0 V by the resistor when the

Q1 turns off

Lit. Number

4PMP15034 Test Results

Driving Voltage 10V/div

PWM 5V/div

Figure 6: Operating Waveform with 4-KHz PWM

The startup by EN waveform of the ±15-V circuit is shown in Figure 7, the two output voltage ramps up to target

value within 7ms.

VOUT 5V/div

EN 2V/div

Double VOUT 10V/div

Figure 7: Startup at VIN=3.3V

Figure 8 shows the operating waveform with 4-KHz, 30% duty PWM input when VIN=3.3V. The rising and falling

time is very short because the totem pole has strong driving capability.

Driving Voltage 10V/div

PWM 5V/div

Figure 8: Operating Waveform with 4-KHz PWM

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