Everyone will be familiar with the problem of the voltage drop across wires, connectors or filtering components such as inductors, which force the voltage at the regulator output to be set higher than the circuit operating voltage. If the current drawn falls then the operating voltage rises above its nominal value; when the current rises, the voltage falls.
Assistance here comes in the form of the four-input MAX1804 Feedback Integrator, a novel product from Maxim (www.maximic.com). The device can be connected to your favorite voltage regulator and will intervene in its feedback loop if any of its four inputs is fed with too low a voltage. The MAX1804 manages the fine control of the regulator output voltage by increasing it until the lowest voltage of those sensed is made correct. Unused inputs to the MAX1804 are automatically disabled if the voltage present is below 90 % of the nominal value. The MAX1804 will therefore not intervene in the feedback loop if the voltage drop is too great. The device is designed for supply voltages between +2.7 V and +5.5 V. It is supplied directly from the output of the controlled regulator.
The OUT connection of the MAX1804 is connected to the feedback pin of the regulator via R3, which has a rather higher value than R1 or R2. The OUT pin draws as much current as necessary to ground to force the regulator to raise its output voltage to bring the lowest of the MAX1804 inputs to four times the voltage on the ADJ pin. The voltage on ADJ can be provided by a reference voltage generated by the regulator, or, if this is not available, by an external voltage reference or Zener diode. The available control range can be set via R3: in the limit, R3 is effectively in par010 MAX1804 IC2 COMP SHDN OUT IN1 ADJ IN2 IN3 IN4 10 6 8 1 7 5 9 ON/OFF 2 3 4 R1 R2 R3 +U Regulator IC1 REF IN OUT FB C2 470p 100n IN +U OUT2 +U OUT3 +U OUT1 RPCB R connector 014103 – 11 Regulation Setpoint +2V7…+5V5 Voltage Regulator Assistant parallel with R2.
The COMP pin is used for compensating the regulation loop. The manufacturer recommends a compensation capacitor of 470 pF.
The shutdown pin (SHDN) forces all four inputs and the output into a high-impedance state so that the MAX1804 can no longer affect the regulator.