MICRO POWER VOLTAGE REGULATOR

MO7VHOLA Technical Developments

MICRO POWER VOLTAGE REGULATOR

by H. D. Bush

  At times when minimum battery drain is required and a regulated voltage is needed to supply a circuit, such as a real time clock, commercially available regulators may draw many times the load current as their quiescient operating current. This circuit was developed to reduce the total current while providing a regulated voltage to a relatively con- stant load current.

  The basic approach is to sample the load volt. age periodically and adjust the charge on a holding capacitor which sets the gate voltage on a FET. The output voltage is the sum of the capacitor voltage and the FET gate voltage required for the load cur- rent. When the load current changes, the required FET gate voltage changes and the circuit adjusts the voltage on the holding capacitor to compensate for the voltage difference, maintaining the desired output voltage.

  Refer to the schematic for part designation. Ql is a junction FET with a pinchoff below the desired regulated voltage but above the minimum needed to operate the LTC1040 comparator. When the bat- tery is connected, the LTC1040 starts to oscillate at a frequency determined by R6 and C3 (about once per second). The LTC1040 is a comparator which samples the input lines and sets the on/off output to high or low, depending on the measurement. Each sample requires about 80 microseconds and occurs at a rate determined by the oscillator RC time con- stant (R6C3). During the non-sample time, the cur- rent requirement is very low and is primarily the charging current into C3, set by R6.

  When the LTC1040 samples, Vpp is switched to the supply and powers the sensing bridge and also connects the holding capacitor Cl to R5 via an...