TMD27721 datasheet(44/59 Pages) AMSCO

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ams Datasheet, Confidential

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[v1-20] 2014-Jul-21

TMD2772/ TMD2772WA − Application Information

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 1μF low-ESR

decoupling capacitor as close as possible to the VDD pin and

another at the LEDA pin, and at least 10μF of bulk capacitance

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

two 4.7μF capacitors.

Figure 50:

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 22Ω resistor in

series with the VDD supply line and a 1μF low ESR capacitor

effectively filter any power supply noise. The previous capacitor

placement considerations apply.

Application Information

1µF

VBUS

RPI

1µF

Voltage

Regulator

Voltage

Regulator

TMD2772

TMD2772WA

VDD

GND

LDR

INT

SCL

SDA

LEDA

LEDK

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