Technical Bulletin
Transformer Solutions for Ultrasonic Sensing
Overview
Ultrasonic sensing technology has been around a long timeand is a cost-effective solution for short distance objectdetection. As shown in Figure 1, the technique measures
the time interval between the sending of an ultrasonic signal
and the return of the signal’s reflection to detect the locationof the target objects.
More accurate and reliable high-performance ultrasonicsensors are becoming available, and the demand for thistechnology (and its essential components) is growing, especially for advanced driver-assistance systems (ADAS)
and automated driving systems in the automotive industry.
More and more sensors are needed as the level of systemcomplexity increases. Automotive parking assist and collision warning are just two of the lead systems in this sensor
growth. Convenience items like trunk-opening sensing arealso driving demand as OEMs look at ways to deploy thiscost-effective technology. Ultrasonic sensing also plays animportant role in many industrial automation applications likeliquid level detecting and anti-collision on work transportation platforms. Other fields of applications include medical,energy, manufacturing, and smart grid.
Automotive Applications• Automated parking assist• Blind-spot warning• Presence and proximity detection• Automotive kick-to-open lift gates (smart trunk opener)

Industrial Applications• Liquid level sensing and inventory management• Robotics landing assist and obstacle detection• Anti-collision on aerial work platforms• Forklift monitoring and control• Industrial door-opening sensing

Automotive Ultrasonic Transducer Driver
Semiconductor manufacturers are now offering highly
integrated system-on-chip (SoC) products which
integrate the transducer driver, signal conditioner,
diagnostics and advanced DSP core in a single compact
package. These devices have a complementary low-side
driver pair that can drive a transducer either in a transformer-based topology using a step-up transformer or in a
direct-drive topology using external high-side FETS. Thesefull-featured products provide the designer with a great dealof flexibility and their high level of integration results in thesmallest solution size possible.
A transformer-driven solution may use a center-tappedboost transformer to drive a large excitation voltage acrossthe ultrasonic transducer and, thus, create the desired
sound pressure level (SPL). This method is recommended
for most ultrasonic applications today to achieve maximum
ranging distance.
Transducer

Transmitted Wave

ObjectTransmitted Signal

Transformer Driven
Ultrasonic Sensing

TOF

Received Signal
Reflected Wave

Figure 1. Ultrasonic sensing schematicDocument 1510

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Technical Bulletin
Transformer Solutions for
Ultrasonic Sensing
Application Examples
VIN

VPWR

Ultrasonic Park Assist

OUTA
CT

To ECU/BCM

IO

OUTB
100 µFTXDTo MCU(optional)

RXD

INP

C(INP)INNDECPL

AVDD

C(INN)IOREG

100 nF
100 nF

GNDP

GND

Figure 2. Typical Application Diagram (Transformer Drive)

Coilcraft Transformers for UltrasonicSensing

Inductance (mH)

Traditionally, tunable-inductance transformers have beenused with ultrasonic transducer drivers. But the disadvantage of tunable transformers is that the inductancetends to drift with temperatures, possibly making thesystem unstable. Coilcraft’s ultrasonic transformer has
a fixed inductance value and is stable up to ambient
temperature TA = 125°C (Figure 3). This feature is criticalfor accurate and reliable system performance, especially in
automotive environments.
3.3
3.2
3.1
3.0
2.9
2.8
2.7
2.6
2.5
2.4
-40˚C-20˚C0˚CFixed20˚C

40˚C

60˚C80˚C

The Parking Assist System (PAS) identifies and measuresdistance between a vehicle and objects in relatively close
proximity. The distance information can then be communicated to the driver with audio and/or visual indications. Thisapplication uses ultrasonic sensors with transformer-driven
transducers almost exclusively.
The PGA460-Q1 on-chip ultrasonic transducer driver from
Texas Instruments can accurately detect objects from lessthan 10 cm away to more than 7 meters, a much wider range
than today’s typical requirements.
To achieve long-distance ranging, an ultrasonic boost
transformer is recommended to excite the transducer tothe maximum desired SPL level. The transformer-driven
solution offers both superior long- distance ranging and thecapability to discern objects at close proximity accurately
by reducing and fine tuning the transducer decay ring time.
If the ring time is too long, the ringing interferes with thereceiver’s ability to pick up the reflected signal.
The TIDA-01597 reference design from Texas Instruments
uses three of their PGA460-Q1 ultrasonic transducer drivers and three ultrasonic transformers to provide object
detection at a distance from 25 cm to 2.5 meters. Typically,
these systems have required between four and 16 sensors
strategically placed around the vehicle to provide the desired
detection.

100˚C 120˚C

TunableFigure 3. Coilcraft’s ultrasonic transformer has a fixedinductance value and is stable up to 125°C ambient
Coilcraft’s WA8351-AL SMT Transformer is optimized for
the PGA460-Q1 Automotive Ultrasonic Signal Processor
and Transducer Driver from Texas Instruments.
• Compact, low-profile package – just 8.2 × 6.6 × 5.4 mm• Optimized for a variety of ultrasonic transducers• High-performance for ultrasonic time-of-flight (TOF)
sensing• Ideal for automotive ADAS and industrial applications(e.g., liquid level sensing, flow sensing, or fluid ID)• AEC-Q200 Grade 1 (–40°C to +125°C ambient)Document 1510

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Technical Bulletin
Transformer Solutions for
Ultrasonic Sensing
The Kick-to-Open System
The Kick-to-Open, or Smart Trunk Opener (STO), is being
adopted by many automotive OEMs as a popular optionalfeature. It allows hands-free access to open the trunk witha simple kicking motion near the rear bumper.

Ultrasonic sensor technology has advantages over the
traditional capacitive sensing method to provide better
sensitivity. Since the system must detect the kicking motionof the operator, the near-field detection must be optimized.
Because of this requirement, the transformer drive topology (Figure 2) is also recommend here since the ultrasonicringing decay profile is more predictable and provides moreaccurate short-range performance.

Summary
Coilcraft’s high quality ultrasonic transformers are key
components that can help to achieve wide-ranging, accurate ultrasonic distance sensing. These transformers aretemperature-stable and AEC-Q200 qualified, making theman important part of many ultrasonic ranging applications.

STO allows hands-free openingof the trunk with a simple kicking motion

Document 1510

Revised 10/17/18