1. Field of the Invention
The present invention relates to a switching control circuit.
2. Description of the Related Art
A step-down DC-DC converter for generating a target level output voltage lower than an input voltage is incorporated in various electronic devices. FIG. 8 depicts the general configuration of a step-down DC-DC converter. The DC-DC converter 100 includes N-channel MOSFETs 110 and 111, an inductor 120, and a capacitor 121. An input voltage Vin is applied to the drain of the N-channel MOSFET 110. When the N-channel MOSFET 110 is turned on and the N-channel MOSFET 111 is turned off, the input voltage Vin is applied to the inductor 120, which charges the capacitor 121, thus raises an output voltage Vout. Subsequently, when the N-channel MOSFET 110 is turned off and the N-channel MOSFET 111 is turned on, accumulated energy on the inductor 120 causes current to flow through a loop formed of the N-channel MOSFET 111, the inductor 120, and the capacitor 121. This causes the capacitor 121 to discharge, thus lowers the output voltage Vout. In this manner, in the DC-DC converter 100, the N-channel MOSFETs 110 and 111 are turned on and off in proper timing to control the output voltage Vout to turn it into the target level voltage.
The DC-DC converter 100 also includes resistors 125 and 126, an error amplifying circuit 130, a capacitor 131, a resistor 132, a power source 135, a current source 136, a capacitor 137, a triangular wave generator 140, a comparator 150, a buffer 151, and an inverter 152. These components serve as a circuit that controls switching by the N-channel MOSFETs 110 and 111.
To a negative input terminal of the error amplifying circuit 130, a feedback voltage Vf is applied, which is obtained by dividing the output voltage Vout with the resistors 125 and 126. To one positive input terminal of the error amplifying circuit 130, a reference voltage Vref from the power source 135 is applied, which reference voltage Vref is a reference for the target voltage level. To the other positive input terminal of the error amplifying circuit 130, a voltage Vss is applied, which is generated as a result of charging the capacitor 137 with currents from the current source 136. The error amplifying circuit 130 outputs a voltage Ve that is given by amplifying an error between a lower voltage selected out of two voltages applied to two positive input terminals and the feedback voltage Vf applied to the negative input terminal. The capacitor 131 and the resistor 132 are provided to cause the error amplifying circuit 130 to make an integral action.
The comparator 150 compares the level of a voltage Vt, which is output from the triangular wave generator 140 and changes in a shape of a triangular wave, with the level of the error voltage Ve output from the error amplifying circuit 130. The comparator 150 keeps outputting an H level signal while the error voltage Ve is higher than the voltage Vt, and keeps outputting an L level signal while the error voltage Ve is lower than the voltage Vt. When the comparator 150 outputs an H level signal, the H level signal is input to the gate of the N-channel MOSFET 110 via the buffer 151 to turn on the N-channel MOSFET 110 as an L level signal is input to the N-channel MOSFET 111 via the inverter 152 to turn off the N-channel MOSFET 111. When the comparator 150 outputs an L level signal, on the other hand, the L level signal is input to the gate of the N-channel MOSFET 110 via the buffer 151 to turn off the N-channel MOSFET 110 as an H level signal is input to the N-channel MOSFET 111 via the inverter 152 to turn on the N-channel MOSFET 111.
Specifically, when the feedback voltage Vf is lower than the reference voltage Vref or the voltage Vss, the voltage Ve rises to increase a ratio of output of an H level signal from the comparator 150, which leads to a rise in the output voltage Vout. When the feedback voltage Vf is higher than the reference voltage Vref or the voltage Vss, the voltage Ve falls to increase a ratio of output of an L level signal from the comparator 150, which leads to a fall in the output voltage Vout. In this manner, in the DC-DC converter 100, a signal output from the comparator 150 is put under PWM (Pulse Width Modulation) control so as to turn the feedback voltage Vf into a lower voltage selected out of the voltage Vref and the Voltage Vss.
If control is started to turn the reference voltage Vf into the voltage Vref at the start of operation of the DC-DC converter 100, a process of a sharp increase in the output voltage Vout causes an excess current, which breaks the N-channel MOSFETs 110 and 111. To prevent this, the Vss voltage is used in the DC-DC converter 100 to achieve soft start through which the output voltage Vout is gradually raised.
A state where the output voltage Vout is not at zero level, i.e., a pre-bias state may occur at the start of the DC-DC converter 100. The pre-bias state results, for example, when the capacitor 121 discharges incompletely following the end of the previous operation of the DC-DC converter 100 or when current leaks from a device connected to the output side of the DC-DC converter 100.
If the DC-DC converter 100 is started in the pre-bias state, the output voltage Vout falls because the feedback voltage Vf is higher than the voltage Vss in the pre-bias state, so that the N-channel MOSFET 111 is turned on and the N-channel MOSFET 110 is turned off. As a result, current flows through the loop formed of the capacitor 121, the inductor 120, and the N-channel MOSFET 111 to cause the capacitor 121 to discharge, which lowers the output voltage Vout. Then, when the N-channel MOSFET 110 is turned on and the N-channel MOSFET 111 is turned off, accumulated energy on the inductor 120 causes current to flow backward from the inductor 120 toward the drain of the N-channel MOSFET 110 at the input side of the DC-DC converter 100. This action of energy backflow from the output side to the input side is called a regenerative action.
When the regenerative action is made, the direction of the voltage of the inductor 120 is the same as that of a pre-bias voltage, so that a voltage higher than the pre-bias voltage is generated at the input side. At the start of the DC-DC converter 100, the voltage Vss compared with the feedback voltage Vf is low, because of which a ratio of turning on the N-channel MOSFET 111 is high while a ratio of turning on the N-channel MOSFET 110 is low. This results in turning on of the N-channel MOSFET 111 for a long time, which accumulates greater energy on the inductor 120, causing an extremely greater voltage increase at the input side when the regenerative action occurs. Extremely high voltage at the input side leads to such troubles as the breakage of the DC-DC converter 100 and malfunction of an excess voltage protective circuit that monitors the input voltage Vin to the DC-DC converter 100.
For prevention of the regenerative action, a method of stopping switching operation by transistors at the start of a DC-DC converter has been suggested (e.g., “low-input voltage mode synchronous rectification back controller” released by Japan Texas Instruments Incorporated in November 2001, <URL: http//www.tij.co.jp/jsc/ds/SLUS585A.pdf>). The DC-DC converter 100 is provided with a comparator 160 that serves as a circuit that prevents such regenerative action. The comparator 160 compares the feedback voltage Vf with the voltage Vss, and outputs an L level signal when the feedback voltage Vf is higher than the voltage Vss and outputs an H level signal when the feedback voltage Vf is lower than the voltage Vss. In other words, when the feedback voltage Vf is higher than the voltage Vss because of the pre-bias state, the comparator 160 outputs an L level signal. In this case, both N-channel MOSFETs 110 and 111 are controlled to become off in the DC-DC converter 100. As time goes by, the voltage Vss rises, and the feedback voltage Vf becomes lower than the voltage Vss. At this point, the comparator 160 outputs an H level signal, which leads to the start of complementary switching operation by the N-channel MOSFETs 110 and 111.
In recent years, a ripple converter has received much attention as a highly responsive self-exciting DC-DC converter (e.g., Japanese Patent Application Laid-Open Publication No. 2006-14559).
The DC-DC converter 100 needs the comparator 160 to prevent the regenerative action. This brings a demand for a switching control circuit that is smaller in circuit scale and less in cost in comparison with a method using such a comparator.
The present invention was conceived in view of the above problems, and it is therefore the object of the present invention is to provide a switching control circuit that can prevent a regenerative action and that has a small circuit scale.