The DAC sets the source current of a MOSFET using the integrated buffer, as shown in Figure 9-1. Connect the LED between the power supply and the drain of the MOSFET. This configuration allows the DAC to control or set the amount of current through the LED. The integrated buffer controls the gate-source voltage of the MOSFET inside the feedback loop, thus compensating this drop and corresponding drift due to temperature, current, and ageing of the MOSFET. Calculate the value of the LED current set by the DAC using Equation 6. In order to generate 0 mA to 20 mA from a 0-V to 2.4-V DAC output range, the value of R_SET resistor is 120-Ω. Select the internal reference with a span of 2x. Given a V_GS of 1.2 V, the V_DD of the DAC must be at least 3.6 V. Select a V_DD of 5 V to allow variation of V_GS across temperature. When the V_DD headroom is a constraint, use a bipolar junction transistor (BJT) in place of the MOSFET. BJTs have much less V_BE drop as compared to a V_GS of a MOSFET. A MOSFET provides a much better match between the current through the set register and the LED current, as compared to a BJT.

Equation 6.

The pseudocode for getting started with an LED biasing application is as follows:

//SYNTAX: WRITE <REGISTER NAME (Hex code)>, <MSB DATA>, <LSB DATA>
//Power-up the device, enable internal reference with 2x output span
WRITE GENERAL_CONFIG(0xD1), 0x11, 0xE5
//Write DAC code (12-bit aligned)
WRITE DAC_DATA(0x21), 0x07, 0xFC
//Write settings to the NVM
WRITE TRIGGER(0xD3), 0x00, 0x10