TMD27721 datasheet(28/35 Pages) TAOS

28 / 35 page

TMD2772

DIGITAL ALS

and PROXIMITY MODULE

TAOS147

E − DECEMBER 2012

The LUMENOLOGY

r Company

E 2012, TAOS Inc.

www.taosinc.com

APPLICATION INFORMATION: HARDWARE

LED Driver Pin with Proximity Detection

In a proximity sensing system, the included IR LED can be pulsed with more than 100 mA of rapidly switching

current, therefore, a few design considerations must be kept in mind to get the best performance. The key goal

is to reduce the power supply noise coupled back into the device during the LED pulses. Averaging of multiple

proximity samples is recommended to reduce the proximity noise.

The first recommendation is to use two power supplies; one for the device VDD and the other for the IR LED.

In many systems, there is a quiet analog supply and a noisy digital supply. By connecting the quiet supply to

the VDD pin and the noisy supply to the LEDA pin, the key goal can be met. Place a 1-μF low-ESR decoupling

capacitor as close as possible to the VDD pin and another at the LEDA pin, and at least 10-μF of bulk capacitance

to supply the 100-mA current surge. This may be distributed as two 4.7

μF capacitors.

TMD2772

INT

SDA

SCL

VDD

LEDA

Voltage

Regulator

Voltage

Regulator

. 10 mF

* Cap Value Per Regulator Manufacturer Recommendation

GND

VBUS

RPI

LDR

LEDK

Figure 13. Proximity Sensing Using Separate Power Supplies

If it is not possible to provide two separate power supplies, the device can be operated from a single supply.

A 22-

Ω resistor in series with the VDD supply line and a 1-μF low ESR capacitor effectively filter any power supply

noise. The previous capacitor placement considerations apply.

Voltage

Regulator

. 10 mF

TMD2772

INT

SDA

SCL

VDD

LEDA

GND

VBUS

RPI

LDR

LEDK

Figure 14. Proximity Sensing Using Single Power Supply

VBUS in the above figures refers to the I2C bus voltage which is either VDD or 1.8 V. Be sure to apply the specified

I2C bus voltage shown in the Available Options table for the specific device being used.

The I2C signals and the Interrupt are open-drain outputs and require pull−up resistors. The pull-up resistor (RP)

value is a function of the I2C bus speed, the I2C bus voltage, and the capacitive load. The TAOS EVM running

at 400 kbps, uses 1.5-k

Ω resistors. A 10-kΩ pull-up resistor (RPI) can be used for the interrupt line.

TMD27721 Datasheet(PDF) 28 Page - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS

TMD27721 Datasheet(HTML) 28 Page - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS

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