TSL2771 datasheet(26/33 Pages) AMSCO

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TSL2771

LIGHT-TO-DIGITAL CONVERTER

with PROXIMITY SENSING

TAOS100B − FEBRUARY 2011

The LUMENOLOGY

r Company

E 2011, TAOS Inc.

www.taosinc.com

APPLICATION INFORMATION: HARDWARE

LED Driver Pin with Proximity Detection

In a proximity sensing system, the IR LED can be pulsed by the TSL2771 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.

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 LED, the key goal can be meet. Place a 1-μF low-ESR decoupling

capacitor as close as possible to the VDD pin and another at the LED anode, and a 22-μF capacitor at the output

of the LED voltage regulator to supply the 100-mA current surge.

TSL2771

INT

SDA

SCL

VDD

LDR

Voltage

Regulator

Voltage

Regulator

. 22 mF

* Cap Value Per Regulator Manufacturer Recommendation

IR LED

GND

VBUS

RPI

Figure 16. 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.

TSL2771

INT

SDA

SCL

VDD

LDR

Voltage

Regulator

. 22 mF

IR LED

GND

VBUS

RPI

Figure 17. 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.

TSL2771 Datasheet(PDF) 26 Page - ams AG

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