SLASEW8A October 2020 – September 2023 DAC43701-Q1 , DAC53701-Q1
PRODUCTION DATA
Choose a 5-V Zener diode of less than 50 μA of reverse current. The DACx3701-Q1consumes a quiescent current of around 200 μA. Assume the load current of the DAC to be maximum 500 μA. Thus, the total current needed for the DAC and the Zener diode is 750 μA. Considering a 2 × derating factor, design the circuit for 1.5 mA. Hence, the value of the current limiting resistor RZ is (12 V – 5 V) / 1.5 mA = 4.7 kΩ. The worst-case power dissipation across the Zener diode is (5 V × 1.5 mA) = 7.5 mW. Any standard Zener diode can be used with this power rating.
Choose a negative temperature coefficient (NTC) thermistor with nominal resistance of 10 kΩ at 25°C. At the foldback point of 30°C, the resistance of the NTC, RNTC is approximately 7 kΩ. At 105°C, the resistance of the NTC is approximately 800 Ω. Choose the value of R after simulating the linearity plot along with the correct NTC part number. For the sake of calculation, take R as 5 kΩ. In this case, VFB is 2.08 V at 25°C (RNTC = 7 kΩ). For 10‑bit resolution and a full-scale output of 5 V, 2.08 V corresponds to the DAC code (2.08 × 1024 / 5) = 426. Similarly, the LED must be cut off at 105°C, which refers to an RNTC of 800 Ω and a corresponding VFB of 4.31 V, or the DAC code 883. Thus, the margin-high value is 883, or 0x373, and the margin-low value is 426, or 0x1AA.
For voltage output, use an RC filter as shown in Figure 8-4. The resistor divider comprised of R1, R2, and R3 define the voltage below the foldback temperature. This voltage divider is useful in case the maximum LED current is set below the 100% value.