SLYT878 April   2026 DAC8771 , DAC8775 , LMR51606 , LMR54406 , TLV9301 , XTR200

 

  1.   1
  2.   2
  3. 1 Choosing the right DC/DC
  4. 2 Controlling the DC/DC output
  5. 3 Example circuit using sourcing current
  6. 4 Example circuit using voltage feedback
  7. 5 Measurements and performance
  8. 6 Precision and noise
  9. 7 Settling time and dynamic performance
  10. 8 Conclusion
  11. 9 Additional resources
  12. 10About the author

3 Example circuit using sourcing current

Figure 4 shows the construction of a high-side current source using an operational amplifier, PMOS transistor M1, and a resistor. Equation 8 calculates the current generated as:

Equation 7. I t a = ( V S - V O U T ) / R c

You will need to consider the input/output and supply range of the operational amplifier and the maximum gate-to-source voltage (VGS) of M1. Further simplifying the circuit by removing the operational amplifier, Equation 8 calculates the current generated as:

Equation 8. I t a = ( V S - V O U T + V t h ) / R c

This saves power, cost and area, with some inaccuracy of current from the variation in threshold voltage (Vth).

Current source feedback circuit. Figure 4 Current source feedback circuit.

The TI XTR200 is a 4-20 mA current transmitter with VS from 8V to 60V and VH of 3V. If the load is up to 800Ω, VOUT goes up to 16V with 20mA of current. This VS has to track the output. At VOUT = 0V, VS = 8V, and at VOUT = 16V, VS = 19V. Use Equation 8 and Equation 5 to calculate resistors Rt, Rb and Rc. You will find that it is not possible to maintain VH >3V without increasing the headroom for the low VOUT.

Values Rt = 80kΩ, Rb = 3kΩ and Rc = 60kΩ produce the output-supply curve shown in Figure 5. Headroom is dependent on the output because this simple design uses only Rc as design variable. More complex circuits can overcome this limitation. But even with this simple circuit, the maximum power dissipation drops to half or less compared to the nonadaptive case. Any low-power rail-to-rail operational amplifier such as the OPA2990 will work in place of U2, as shown in Figure 6.

VS-VOUT, VH-VOUT relation. Figure 5 VS-VOUT, VH-VOUT relation.
Output stage using the XTR200 with an adaptive supply. Figure 6 Output stage using the XTR200 with an adaptive supply.
Note: Simulation: The switching regulator simulation is quite long. Replacing the DC/DC with a low-dropout regulator (LDO) that has a similar VREF and similar input and output ranges can speed up DC simulation and enable easy creation of the transfer function graph. If the LDO has a different VREF, insert a voltage-controlled voltage source (VCVS) between the feedback node and the LDO's actual feedback node. For example, if VREF = 1.2V, and you want to design for the TI LMR54406 buck converter, which has a VREF = 0.8V, you can add a VCVS with gain of 1.5 to convert the 0.8V into 1.2V.