Design Notes

The DACx3204 12-Bit, 10-Bit, and 8-Bit, Quad Voltage and Current Output Smart DACs With Auto-Detected I2C, PMBus™, or SPI Interface Data Sheet recommends using a 100nF decoupling capacitor for the VDD pin and a 1.5µF or greater bypass capacitor for the CAP pin. The CAP pin is connected to the internal LDO. Place these capacitors close to the device pins.
The nominal voltage of the SMPS is set by resistors R₁ and R₂. The SMPS uses an internal 600mV reference voltage at the FB pin to determine the voltage at the output. Calculate R₁ and R₂ using the following equations:
$R_{1} = \frac{V_{N O M I N A L} - V_{F B}}{I_{N O M I N A L}}$
$R_{2} = \frac{R_{1} \times V_{F B}}{V_{N O M I N A L} - V_{F B}}$
A nominal current of 100µA through R₁ and R₂ and 3.3V nominal output voltage is used. With these values the equations become:

$R_{1} = \frac{3.3 V - 0.6 V}{100 µ A} = 27 k Ω$
$R_{2} = \frac{27 k Ω \times 0.6 V}{3.3 V - 0.6 V} = 6 k Ω$
To achieve the desired margin, the DAC43204 must sink or source additional current through R₁. This current (I_MARGIN) is calculated by:
$I_{M A R G I N} = \frac{V_{N O M I N A L} \times (1 + M A R G I N) - V_{F B}}{R_{1}} - I_{N O M I N A L}$
For a ±10% margin, the equation becomes:
$I_{M A R G I N} = \frac{3.3 V \times (1 + 0.10) - 0.6 V}{27 k Ω} - 100 µ A = 12 µ A$
The DAC codes for ±I_MARGIN are stored in the DAC-MARGIN-HIGH and DAC-MARGIN-LOW registers. The codes programmed to these registers, in decimal, is calculated using:
$D A C_M A R G I N_H I G H = \frac{I_{D A C, M A X} - I_{M I N}}{I_{M A X} - I_{M I N}} \times 256$
$D A C_M A R G I N_L O W = \frac{I_{D A C, M I N} - I_{M I N}}{I_{M A X} - I_{M I N}} \times 256$
Using an I_OUT range of ±25µA, the equation becomes:
$D A C_M A R G I N_H I G H = \frac{12 µ A - (- 25 µ A)}{25 µ A - (- 25 µ A)} \times 256 = 189.44 d$
$D A C_M A R G I N_L O W = \frac{- 12 µ A - (- 25 µ A)}{25 µ A - (- 25 µ A)} \times 256 = 66.56 d$
This is rounded to 189d and 67d to give a I_DAC,MAX of 11.9µA and a I_DAC,MIN of –11.9µA.
In this design, GPI is used for Margin High, Low function. A high on GPI sets the DAC output to I_DAC,MAX and the SMPS V_OUT to margin low, or 2.97V. A low on GPI sets the DAC output to I_DAC,MIN and the SMPS V_OUT to margin high, or 3.63V.
The DAC43204 can be programmed with the initial register settings described in the Register Settings section using I²C or SPI. Save the initial register settings in the NVM by writing a 1 to the NVM-PROG field of the COMMON-TRIGGER register. After programming the NVM, the device loads all registers with the values stored in the NVM after a reset or a power cycle.