The DACx3701 features a Hi-Z power-down mode that is set by default at power-up, unless the device is programmed otherwise using the NVM. When the DAC output is at Hi-Z, the current through R₃ is zero and the SMPS is set at the nominal output voltage of 3.3 V. To have the same nominal condition when the DAC powers up, bring up the device at the same output as V_FB (that is 0.6 V). This configuration makes sure there is no current through R₃ even at power-up. Calculate R₁ as (V_OUT – V_FB) / 100 µA = 27 kΩ.

To achieve ±10% margin-high and margin-low conditions, the DAC must sink or source additional current through R₁. Calculate the current from the DAC (I_MARGIN) using Equation 6 as 12 µA.

Equation 6.

where

• I_MARGIN is the margin current sourced or sinked from the DAC.
• MARGIN is the percentage margin value divided by 100.
• I_NOMINAL is the nominal current through R₁ and R₂.

To calculate the value of R₃, first decide the DAC output range, and make sure to avoid the codes near zero-scale and full-scale for safe operation in the linear region. A DAC output of 20 mV is a safe consideration as the minimum output, and (1.8 V – 0.6 V – 20 mV = 1.18 V) as the maximum output. When the DAC output is at 20 mV, the power supply goes to margin high, and when the DAC output is at 1.18 V, the power supply goes to margin low. Calculate the value of R₃ using Equation 7 as 48.3 kΩ. Choose a standard resistor value and adjust the DAC outputs. Choosing R₃ = 47 kΩ makes the DAC margin high code as 1.164 V and the DAC margin low code as 36 mV.

Equation 7.

The DACx3701 have a slew rate feature that is used to toggle between margin high, margin low, and nominal outputs with a defined slew rate. See the Section 8.6.2 for the slew rate setting details.

The pseudocode for getting started with a power-supply control application is as follows:

//SYNTAX: WRITE <REGISTER NAME (Hex code)>, <MSB DATA>, <LSB DATA>
//Write DAC code (12-bit aligned) for nominal output
//For a 1.8-V output range, the 10-bit hex code for 0.6 V is 0x0155. With 12-bit alignment, it becomes 0x0554
WRITE DAC_DATA(0x21), 0x05, 0x54
//Write DAC code (12-bit aligned) for margin-low output at the power supply
//For a 1.8-V output range, the 10-bit hex code for 1.164 V is 0x0296. With 12-bit alignment, it becomes 0x0A58
WRITE DAC_MARGIN_HIGH(0x25), 0x0A, 0x58
//Write DAC code (12-bit aligned) for margin-high output at the power supply
//For a 1.8-V output range, the 10-bit hex code for 36 mV is 0x14. With 12-bit alignment, it becomes 0x50
WRITE DAC_MARGIN_LOW(0x26), 0x00, 0x50
//Power-up the device with enable internal reference with 1.5x output span. This will output the nominal voltage (0.6 V)
//CODE_STEP: 2 LSB, SLEW_RATE: 25.6 µs
WRITE GENERAL_CONFIG(0xD1), 0x12, 0x14
//Trigger margin-low output at the power supply
WRITE TRIGGER(0xD3), 0x00, 0x80
//Trigger margin-high output at the power supply
WRITE TRIGGER(0xD3), 0x00, 0x40
//Write back DAC code (12-bit aligned) for nominal output
WRITE DAC_DATA(0x21), 0x05, 0x54