1. Field of the Invention
The present invention relates generally to a switching regulator and more particularly to slope compensation in current-mode switching regulators.
2. Description of the Related Art
A power conversion circuit (e.g., a switching regulator) accepts a Direct Current (DC) voltage source at one level and outputs a desired DC voltage at another level. The switching regulator includes one or more semiconductor switches. The semiconductor switches alternate between connecting and disconnecting the voltage source (or input voltage) to circuits that drive the output. For example, an inductor is periodically charged and discharged in accordance with a semiconductor switch that alternately turns on and off. The output voltage level is related to the duty cycle of the switching. The switching is typically controlled by a Pulse-Width Modulation (PWM) circuit.
A switching regulator (or switched mode power supply) can be configured for voltage-mode operation or current-mode operation. In voltage-mode operation, an output voltage of the switching regulator is monitored to adjust switching duty cycle. In current-mode operation, the output voltage of the switching regulator and current through the inductor are both monitored to adjust the switching duty cycle. The transient correction performance or compensation of a current-mode switching regulator is typically superior to a voltage-mode switching regulator.
The current through the inductor follows a seesaw pattern as the inductor is alternately charged and discharged. The current-mode switching regulator typically operates with a constant peak inductor current for a steady state output load current at a given output voltage. The output voltage is determined by the switching duty cycle which is a function of the input voltage and the output voltage. The output load current is approximately equal to an average inductor current. The output voltage may undesirably oscillate when the input voltage changes because the peak inductor current and the output voltage are controlled by separate feedback loops. The current-mode switching regulator generally uses slope compensation to modify a rate at which the inductor charges to ensure that the average inductor current, and thus the output voltage, remains stable as the input voltage changes when the switching regulator operates at greater than 50% duty cycle. Without the slope compensation, the output voltage may show sub-harmonic distortion which results in a lower maximum output current and higher ripple current and voltage.
Some applications use a voltage ramping circuit to generate a slope compensation signal. The voltage ramping circuit typically uses a current source to charge a capacitor, and the current source is derived using a reference voltage and a resistor. The slope compensation signal is generally a function of inductor value, the input voltage and the output voltage. When the inductor value changes due to system requirements (e.g., output ripple voltage requirement) or when the input voltage changes during usage (e.g., in a portable device using batteries), external adjustments to the voltage ramping circuit may be needed to adjust the slope compensation signal.