Datasheet

AFE4490

SBAS602F –DECEMBER 2012–REVISED OCTOBER 2013

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TRANSMIT SECTION

The transmit section integrates the LED driver and the LED current control section with 8-bit resolution. This

integration is designed to meet a dynamic range of better than 105 dB (based on a 1-sigma LED current noise).

The RED and IR LED reference currents can be independently set. The current source (I

LED

) locally regulates

and ensures that the actual LED current tracks the specified reference. The transmitter section uses a reference

voltage for operation. This reference voltage is available on the REF_TX pin and must be decoupled to ground

with a 2.2-μF capacitor. The TX_REF voltage is derived from the TX_CTRL_SUP. The maximum LED current

setting depends on the transmitter reference voltage. By default, after reset, this voltage is 0.75 V and supports

up to a 150-mA LED current. For higher LED currents up to 200 mA, the reference can be programmed to 1.0 V

(using the LED_RANGE[1:0] register bits).

The minimum LED_DRV_SUP voltage required for operation depends on the:

• Voltage drop across the LED (V

LED

• Voltage drop across the external cable, connector, and any other component in series with the LED (V

CABLE

and

• Transmitter reference voltage.

Using the default reference voltage of 0.75 V, the minimum LED_DRV_SUP voltage can be as low as 3.25 V,

provided that Equation 4 is met. Refer to the Recommended Operating Conditions table.

3.25 V – (V

LED

+ V

CABLE

) > 1.4 V (4)

To lower the minimum LED_DRV_SUP voltage even further, the transmitter reference voltage can be

programmed to 0.5 V. By doing so, the minimum LED_DRV_SUP voltage can be reduced to 3.0 V, provided that

Equation 5 is met. Refer to the Recommended Operating Conditions table.

3.0 V – (V

LED

+ V

CABLE

) > 1.4 V (5)

Note that with the 0.5-V transmitter reference voltage, the maximum LED current supported is 100 mA.

Two LED driver schemes are supported:

• An H-bridge drive for a two-terminal, back-to-back LED package; see Figure 71.

• A push-pull drive for a three-terminal, common-anode LED package; see Figure 72.

Product Folder Links: AFE4490