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TMD2671
DIGITAL PROXIMITY DETECTOR
TAOS144B ? SEPTEMBER 2012
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 V DD 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 V DD pin and the noisy supply to the LEDA pin, the key goal can be met. Place a 1- μ F low-ESR decoupling
capacitor as close as possible to the V DD pin and another at the LEDA pin, and a 22- μ F capacitor at the output
of the LED voltage regulator to supply the 100-mA current surge.
V BUS
Voltage
Regulator
V DD
LEDK
C*
1 m F
LDR
R P
R P
R PI
GND
TMD2671
INT
Voltage
Regulator
. 22 m F
1 m F
LEDA
SCL
SDA
* Cap Value Per Regulator Manufacturer Recommendation
Figure 12. 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 V DD supply line and a 1- μ F low ESR capacitor effectively filter any power supply
noise. The previous capacitor placement considerations apply.
V BUS
Voltage
Regulator
22 W
V DD
LEDK
. 22 m F
1 m F
LDR
R P
R P
R PI
GND
LEDA
TMD2671
INT
SCL
SDA
1 m F
Figure 13. Proximity Sensing Using Single Power Supply
V BUS in the above figures refers to the I 2 C bus voltage which is either V DD or 1.8 V. Be sure to apply the specified
I 2 C bus voltage shown in the Available Options table for the specific device being used.
The I 2 C signals and the Interrupt are open-drain outputs and require pull?up resistors. The pull-up resistor (R P )
value is a function of the I 2 C bus speed, the I 2 C bus voltage, and the capacitive load. The TAOS EVM running
at 400 kbps, uses 1.5-k Ω resistors. A 10-k Ω pull-up resistor (R PI ) can be used for the interrupt line.
The LUMENOLOGY r Company
r
www.taosinc.com
r
Copyright E 2012, TAOS Inc.
21
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