Selecting the right Hall IC for portable electronics

Selecting the right Hall IC for portable electronics

Recommendations about the proper device configuration for several popular applications

BY STEVE CHUTKA
ROHM Electronics
San Diego, CA
http://www.rohmelectronics.com

In portable consumer electronic products, Hall-effect sensors offer increased reliability as well as cost and/or performance advantages over mechanical switches. Selecting the right configuration of a Hall IC switch for a specific task requires consideration of a number of factors including electrical performance and mechanical constraints, plus manufacturing and marketing requirements. In most cases, power consumption, size and location of the magnet and sensor, product appearance, and functionality all have an impact on device selection.

Fig. 1. Block diagram of monolithic Hall-effect switch.

Hall-effect sensor topology

Hall ICs designed for portable electronics applications typically combine the Hall-effect sensing element on a single monolithic silicon chip with built-in circuitry to provide additional functions and ease of interfacing. Figure 1 depicts an idealized example of a Hall IC showing the Hall-effect element with additional functional circuitry.

There are several factors to consider, in general, in selecting the right Hall IC switch for your particular application.

Logic output configuration

• The output stage in this example shown is a CMOS push-pull circuit. Some devices provide just a single FET output. The latter may be slightly less costly but requires the addition of an external pull-up resistor that both adds to space and power consumption. In some applications, dual outputs are indicated, as described later in this article.

Sampling period

• Be sure to consider both the sampling time and sample period as this can make a big difference in the power. In many applications, all that is required is a sampling rate fast enough to detect if the device has been opened or closed.• A sampling period of 50 milliseconds or longer and sample time less than 50 µs is often more than adequate.• In such cases, the average operating current is typically less than 10 µA. • In some applications, like trackball or jog wheel position sensing, the sample rate must be higher. Operating voltage

• The lower the operating voltage, the lower the power consumption. Typical devices can operate with voltages from 1.65 to 3.3 V.

ESD immunity

• To ensure reliable operation, high electrostatic discharge (ESD) tolerance with ratings of up to 8 kV as measured in the human body model (HBM) is recommended.

Operating temperature

• A wide operating temperature is important to assure stable detection of magnetic flux density. The recommended temperature range is 40° to 85°C.

High magnetic sensitivity

• Selection and placement of the magnet is simplified if the Hall IC has high magnetic sensitivity.

Dynamic compensation circuitry

• Dynamic offset cancellation improves magnetic detection accuracy and increases the noise resistance.

Physical size

• Hall IC sizes have become much smaller. Some are available in packages as small as 1.1 x 1.1 mm with package height as low as 0.5 mm. This really aides in satisfying both mechanical and marketing requirements.

Device configuration

• Hall ICs are available in four commonly used configurations:

1. Unipolar. Operates when a magnetic field of sufficient strength and polarity is detected.

2. Omnipolar. Operates when a magnetic field of either polarity and sufficient strength is detected

3. Omnipolar with polarity discrimination. Provides two outputs, one for N-pole detection; one for S-pole.

4. Bipolar (latching). Changes state when the opposite pole is detected.

With these considerations in mind, here are some common applications and the recommended Hall IC configuration.

Open/close detection

Portable computers and flip- or jack-knife-style phones and other portable devices with a rotating hinge and clam shell design can use Hall IC switches to indicate an open or closed position. The Hall IC switch detects the presence or absence of a magnetic field and outputs a digital signal for ON/OFF. Some newer cell phones, digital cameras, and other portable instruments use a sliding mechanism where linear motion reveals the display or even a keyboard normally covered when the unit is in a standby mode.

In these applications, a unipolar Hall IC is commonly used. Omnipolar devices can also be used. The unipolar devices typically require less power to operate but require the magnet to oriented properly while the omnipolar devices will operate without regard to magnet orientation.

Screen orientation

In some portable devices, the screen pivots, allowing the front or back face of the display to be viewed, and the system has to discern which side of the display is facing the viewer. This application is also commonly found on PDAs, digital cameras, and tablet-style PCs. Some flat-panel displays for desktop computers can rotate to view a portrait versus a landscape mode. In these applications, omnipolar Hall IC switches equipped with polarity discrimination are indicated.

Function selection and control

An increasingly important function in multifunction cell phones is the jog wheel or trackball, a human interface allowing the user to scroll through a list, increase or decrease the volume or implement another function. Some lower-cost MP3 players use a jog wheel that moves clockwise or counterclockwise to select songs or scroll through a list of menu items.

Capacitive and even resistive touch sensing are also used for these applications, but the Hall-effect sensor provides a lower-cost solution. This application requires the use of two bipolar, latching Hall ICs. The jog wheel has a series of alternating N-pole and S-pole-oriented magnets. The two Hall ICs operating in combination are used to detect either clockwise or counterclockwise rotation of the wheel.

More advanced applications in phones, computers, and game handsets use a trackball that moves up and down and right and left. The design uses four Hall-effect switches to determine the direction and how fast the ball is moving. ■

For more on Hall-effect ICs, visit http://www2.electronicproducts.com/AnalogMixICs.aspx.