AN842 Datasheet, PDF (2/4 Pages) Silicon Laboratories – Use a Self-Powered Op Amp to Create a Low-Leakage Rectifier

AN842
where VDS is the drain-to-source voltage and VGS is the gate-to-source voltage. Combine these equations to
relate the MOSFET’s gate drive to a function of the drain-to-source voltage:
Figure 2.
If you make R2 12 times larger in value than R1, a 40 mV voltage drop across the MOSFET’s drain-to-source
voltage is sufficient to turn on the MOSFET at low drain currents (Figure 2). You could choose a higher ratio to
further reduce the voltage drop within the limits of the op amp’s worst-case input-offset voltage of 6 mV. The op
amp is powered from output-reservoir capacitor C1. The amplifier has rail-to-rail inputs and outputs and no phase
inversion when operating near the rails. The amplifier operates at power-supply voltages as low as 0.8 V. You
directly connect the op amp’s non-inverting input to the VDD rail and the amp’s output to the gate of the MOSFET.
The circuit consumes slightly more than 1 µA when actively rectifying a 100 Hz sine wave, less current leakage
than that of most Schottky diodes. The BSH205 supports milliamp-level currents at a gate-to-source voltage of
0.8 V.
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Rev. 1.0