DC-DC converter and semiconductor integrated circuit for controlling power source
A control technology which eliminates the need for changing the switching frequency even under light load where the on-time of a drive switching element becomes shorter than a minimum on-time dependent on the characteristics of the circuit in a synchronous rectification switching regulator. The synchronous rectification switching regulator includes a drive switching element for storing energy in a coil by applying a DC input voltage from a DC power supply to an inductor and permitting a current to flow, and a rectification switching element for rectifying the current of the inductor during an energy discharge period where the drive switching element is turned off. The timing for turning off the rectification switching element under light load is delayed so as to store energy in the inductor from the output, and the on-time is controlled to become longer as the load becomes lighter by the output from an error amplifier.
1. A DC-DC converter, comprising:
an inductor for converting voltage;
a driving switching element to control an electric current flowing through the inductor;
a rectifying switching element to be turned on and off complementarily to the driving switching element; and
a switching control circuit to generate control signals of the driving switching element and the rectifying switching element,
wherein the switching control circuit includes:
an error amplifier to output a voltage according to a potential difference between a feedback voltage from an output side and a reference voltage;
a PWM control pulse generating circuit to receive an output of the error amplifier and generate PWM control pulses for controlling the driving switching element and the rectifying switching element by PWM control;
a minimum on-time pulse generating circuit to generate a minimum on-time pulse having a pulse width corresponding to a minimum on-time of the driving switching element; and
an off-timing delaying circuit to delay a switching timing of the rectifying switching element from its on-state to its off-state by the control signal of the rectifying switching element for a longer time as a load becomes lighter according to the output of the error amplifier at a time of a light load at which the PWM control pulses become narrower than a pulse width of the minimum on-time pulse.
2. The DC-DC converter according to claim 1 , wherein the switching control circuit includes a reverse current detecting circuit to detect a flow of a current in the inductor in a direction reverse to a direction at an on-time time of the driving switching element, and
wherein the switching control circuit is configured to make the off-timing delaying circuit or an output thereof effective based on detection of a reverse direction current by the reverse current detecting circuit.
3. The DC-DC converter according to claim 1 , further comprising a waveform generating circuit to generate a triangular wave of a predetermined frequency,
wherein the PWM control pulse generating circuit includes a PWM comparator to compare a first triangular wave generated by the waveform generating circuit and the output of the error amplifier to generate the PWM control pulse, and
wherein the off-timing delaying circuit includes a second comparator to compare a second triangular wave vertically symmetrical to the first triangular wave, generated by the waveform generating circuit, and the output of the error amplifier to generate a signal having an off-timing.
4. The DC-DC converter according to claim 1 , further comprising a waveform generating circuit to generate a triangular wave of a predetermined frequency,
wherein the PWM control pulse generating circuit includes a PWM comparator to compare the triangular wave generated by the waveform generating circuit and the output of the error amplifier to generate the PWM control pulse, and
wherein the off-timing delaying circuit includes a timer circuit and a second comparator, the timer circuit including a current source to make an electric current according to the output of the error amplifier flow and a capacitor element to be charged by the current source, the timer circuit using the minimum on-time pulse generated by the minimum on-time pulse generating circuit as a reset signal, and the second comparator comparing an output potential of the timer circuit and a predetermined reference voltage to generate a signal having an off-timing.
5. The DC-DC converter according to claim 1 , wherein:
the driving switching element is connected between a direct-current voltage input terminal, to which the input direct-current voltage is applied, and one terminal of the inductor;
the rectifying switching element is connected between the one terminal of the inductor and a ground point; and
a smoothing capacitor is connected between another terminal of the inductor and the ground point, the smoothing capacitor outputting a voltage which is a stepped-down input direct-current voltage.