As an example, it can be implemented with MAX17503 + MAX4211 + MAX4163 + MAX16904:
- The MAX17503 can output 12V, with a maximum current of 2.5A. This serves as the input power source. Depending on system requirements, different regulators may be chosen.
- The MAX4211 can be used to monitor the output power by checking the output voltage (tapped from load) and output current (sensed from a current-sense resistor). This part internally multiplies the two to obtain a voltage signal that is proportional to the actual output power. Some initial calibration may be required to accurately map the voltage signal to the actual power consumption.
- The MAX4163 is a SOT323 rail-to-rail operational amplifier. By comparing the difference between the target power and actual power, it adjusts the gate voltage of the MOSFET, thereby holding the output power constant. Note that the target power is represented by a constant voltage signal, which should be determined during design. A recommended method is to leverage the constant 1.21V reference voltage from MAX4211; design the system such that this reference voltage is equal to the output from the voltage multiplier in MAX4211 when the target power is consumed by the load.
- The MAX16904 supplies a 5V output that acts as the power supply for the MAX4211 and MAX4163.
In essense, this design borrows the idea from an LDO. Whereas here the extra voltage is dumped in the MOSFET in LDOs, here it is the extra power that is dumped in the MOSFET.