Active dropout optimization for current mode LDOs
A DC/DC converter has a linear voltage regulator for reducing or eliminating the output ripple of the converter with a minimum loss of efficiency. The converter comprises a converter stage with a supply voltage input, a converted voltage output and a control input, a regulator stage having an input connected to the converted voltage output of the converter stage and an output connected to a load, and a tracking circuit with inputs for a voltage at the converted voltage output of the converter stage, a voltage at the output of the regulator stage and a load sense current, and an output connected to the control input of the converter stage. The tracking circuit controls the converter stage so as to increase the converted voltage with an increasing load sense current and vice versa. The output voltage of the converter is always just sufficient to eliminate the ripple without having to operate the regulator's pass transistor in its linear range.
1. A DC/DC converter, comprising:
a converter stage having (i) a converter input for connection to a supply voltage, (ii) a converter output for an output converted voltage, and (iii) a converter control input for adjusting the output converted voltage;
a regulator stage for ripple control having (i) a regulator input connected to said converter output, (ii) a regulator output for connection to a load, (iii) a pass transistor connected between said regulator input and said regulator output, said pass transistor controlled by an error of a voltage at said regulator output from a reference voltage; and
a tracking circuit having (i) a tracking input for a sense current which is proportional to a current through said pass transistor and (ii) a tracking output connected to said converter control input, wherein the tracking circuit controls the converter stage to increase the output converted voltage for an increased pass transistor current, and to decrease the output converted voltage for a decreased pass transistor current,
wherein said tracking circuit includes a comparator with (i) an inverting input connected to said regulator output, (ii) a non-inverting input connected by a resistor to said converter output, and (iii) an output connected to said tracking circuit output, and wherein said sense circuit current is mirrored to said non-inverting input.