AFE4490 datasheet(16/85 Pages) TI2

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AFE

TX_SUP

TX3

TX2

TX1

TX_SUP

LEDs

AFE4404

SBAS689D – JUNE 2015 – REVISED DECEMBER 2016

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Feature Description (continued)

8.3.1.1.1

LED Current Setting

The default LED current range is from 0 mA to 50 mA. The individual currents of each of the three LEDs can be

controlled independently, each with a separate 6-bit control.

Taken as a decimal number, the 6-bit setting provides 63 equal steps between 0 mA and 50 mA. Each increment

of the ILED 6-bit code causes the LED current setting to increment by approximately 0.8 mA. For details, see

The LED current range can be doubled by setting the ILED_2X bit to 1. The accuracy of higher current settings

close to 100 mA can be low because of current saturation of the driver. Each increment of the ILED 6-bit code

causes the LED current to increment by approximately 1.6 mA when ILED_2X is set to 1.

8.3.1.2 TIA Gain Settings

The TIA gain is set by programming the value of Rf (the feedback resistor of the TIA). The Rf setting is controlled

using the TIA_GAIN register bit. For details see register 21h.

By default, the same TIA_GAIN setting is applied for all four phases of the receiver. Separate gains can be set

for two of the four phases by setting the EN_SEP_GAIN bit. When the EN_SEP_GAIN bit is enabled, the

TIA_GAIN register controls the Rf1 setting and the TIA_GAIN_SEP register controls the Rf2 settings.

Mapping of the Rf1 / Rf2 values to the two sets of 3-bit controls is described in Table 50.

8.3.1.3 TIA Bandwidth Settings

TIA bandwidth settings are similar to TIA gain settings. The TIA bandwidth is set by programming the value of Cf

(the feedback capacitance of the TIA). The product of Rf and Cf gives the time constant of the TIA and must be

set approximately 1/5th (or less) of the LED or sampling pulse durations. This choice of time constant allows the

TIA to pass the incoming pulses from the photodiode.

Cf is controlled using the TIA_CF register bit. For details, see register 21.

By default, the same TIA_CF setting is applied for all four phases of the receiver. Similar to the TIA gain settings,

a separate Cf can be set for two of the four phases by setting the EN_SEP_GAIN bit. When the EN_SEP_GAIN

bit is enabled, the TIA_CF register controls the Cf1 settings and TIA_CF_SEP controls the Cf2 settings. Mapping

the Cf1 / Cf2 values to the two sets of 3-bit controls is the same as illustrated in Table 51.

8.3.2 Power Management

The AFE has three independent supplies for the transmitter, receiver, and I/O.

8.3.2.1 Transmitter Supply (TX_SUP)

The transmitter supply has a range of 3.0 V to 5.25 V. In the most common arrangement, this supply can be the

same supply that the anodes of the LEDs are tied to, as shown in Figure 25.

Figure 25. LED to Pin Connections

When the LEDs must be tied to a different supply, care must be taken to ensure that the LED supply is within

0.3 V of TX_SUP. This consideration of the LED supply voltage prevents the electrostatic discharge (ESD)

diodes inside the AFE from turning on during the off state of the LEDs.

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