Choose a small V_SET so that the power dissipation across R_SET is minimum. Choose 1 V for the bright condition, which results in an R_SET of (1 V / 20 mA) = 50 Ω. Set the DACx3701 output span to 1.8 V. The output buffer of the DAC is connected in a force-sense configuration to the MOSFET, as shown in Figure 9-1. This configuration compensates the gate-source voltage drop caused by temperature, drain current, and ageing of the MOSFET. Considering a typical gate-source voltage of 1.2 V and a power supply headroom of 200 mV, the VDD for the DAC must be a minimum of (1 V + 1.2 V + 200 mV) = 2.4 V. Use a standard 3.3-V or 5-V power supply for the DAC. A bipolar junction transistor (BJT) provides a much smaller base-emitter voltage drop, but a MOSFET has better matching between the drain and source currents. Choose a BJT over the MOSFET in case there is a less than 2.4-V supply voltage available for the DAC. Configure the MARGIN HIGH value to the code equivalent of 1 V; that is (1 V / 1.8 V) × 1024 = 569d or 0x239. The MARGIN LOW value should be the equivalent of the dim LED current that is 10 mA, which corresponds to a DAC voltage of (10 mA × 50 Ω) = 500 mV. The code for MARGN LOW is (500 mV / 1.8 V) × 1024 = 284d or 0x11C.

For control without the use of software, map the GPI to margin high-low operation as listed in Table 8-1. The rising edge of the GPI maps to the MARGIN HIGH value of the 20-mA LED current, and the falling edge maps to the MARGIN LOW value of the 10-mA LED current. When the DAC output is in the slewing condition, any change in the GPI state changes the direction of the slew after the ongoing SLEW_RATE time, as shown in the Application Curves section.

The slew time is given by (MARGIN_HIGH – MARGIN_LOW) × CODE_STEP × SLEW_RATE. For a 1.5-s slew time, CODE_STEP × SLEW_RATE = 1.5 / (569 – 284) = ~ 5 ms. Choose the CODE_STEP as 1 LSB and SLEW_RATE of 4.9152 ms. This configuration provides a slew time of 1.4 s. Adjust the MARGIN HIGH and MARGIN LOW values for more granular control.

The following pseudocode helps to get started with a light fade-in fade-out application:

//SYNTAX: WRITE <REGISTER NAME (Hex code)>, <MSB DATA>, <LSB DATA>
//Write MARGIN-HIGH code (12-bit aligned) for bright LED light
//For a 1.8-V output range, the 10-bit hex code for 1 V is 0x0239.
//With 12-bit alignment, it becomes 0x08E4
WRITE DAC_MARGIN_HIGH(0x25), 0x08, 0xE4
//Write MARGIN-LOW code (12-bit aligned) for dim LED light
//For a 1.8-V output range, the 10-bit hex code for 500 mV is 0x11C. 
//With 12-bit alignment, it becomes 0x0470
WRITE DAC_MARGIN_LOW(0x26), 0x04, 0x70
//Map GPI to margin high-low function
WRITE CONFIG2(0xD2), 0x10, 0x00
//Enable GPI
WRITE TRIGGER(0xD3), 0x04, 0x08
//Configure internal reference with 1.5x output span, and slew time and power-up the device
//CODE_STEP: 1 LSB, SLEW_RATE: 4.9152 ms
WRITE GENERAL_CONFIG(0xD1), 0x01, 0x64
//Program the EEPROM
WRITE TRIGGER(0xD3), 0x04, 0x18