PWM modulation circuit and class D amplifier using such PWM modulation circuit
View Patent ↗The PWM modulation circuit according to the invention is connected to an input terminal for an analog baseband signal and a staircase generating circuit generating a staircase wave, and carries out PWM modulation to a staircase wave input from the staircase generating circuit based on an analog baseband signal input from the input terminal. The staircase generating circuit includes a plurality of constant current source circuits, a plurality of switches connected to the plurality of constant current source circuits on a one-to-one basis, a serial-parallel conversion circuit that carries out serial-parallel conversion to a clock signal input from a clock signal input terminal and outputs the result to the plurality of switches, and an output voltage control circuit that converts a constant current input from the plurality of constant current source circuits into a constant voltage at an arbitrary value and outputs the result. The plurality of switches switch between their on and off states in response to a clock signal input from the serial-parallel conversion circuit. The output voltage control circuit converts a constant current input from the plurality of constant current source circuits connected to the plurality of switches in an on state into a constant voltage in a prescribed ratio, so that a staircase wave is generated.
1. A PWM modulation circuit, connected to an input terminal for an analog baseband signal, and a staircase generating circuit generating a staircase wave, the modulation circuit carrying out PWM modulation to a staircase wave input from the staircase generating circuit based on an analog baseband signal input from the input terminal;
wherein the staircase generating circuit comprises:
a plurality of constant voltage source circuits;
a plurality of switches connected to the plurality of constant voltage source circuits on a one-to-one basis; and
a serial-parallel conversion circuit, carrying out serial-parallel conversion to a clock signal input from a clock signal input terminal and outputs the result to the plurality of switches; and
the plurality of switches switch between their on and off states in response to a clock signal input from the serial-parallel conversion circuit;
the plurality of constant voltage source circuits which are connected to the plurality of switches in an on state, output a constant voltage in a prescribed ratio, so that a staircase wave is generated,
wherein the plurality of constant voltage source circuits comprise:
a DC-DC converter circuit that can output voltages at a plurality of different voltage values in response to the on/off states of the plurality of switches; and
a voltage control circuit that arbitrarily sets the plurality of voltage values, and
in the staircase generating circuit, the DC-DC converter circuit outputs the plurality of voltages based on the voltage value in the prescribed ratio set by the voltage control circuit, so that a staircase wave is generated.
2. The PWM modulation circuit according to claim 1 , wherein the serial-parallel conversion circuit comprises:
a frequency-dividing circuit that frequency-divides a clock signal input from the clock signal input terminal and outputs the result; and
a plurality of delay circuits having different delay amounts that delay a signal input from the frequency dividing circuit and output the result, and
the plurality of switches are connected to the plurality of delay circuits on a one-to-one basis.
3. The PWM modulation circuit according to claim 1 , wherein as the prescribed ratio, the amplitude ratio of the staircase wave generated by the staircase generating circuit is 1: √2 or a value close to the ratio.
4. A class D amplifier, comprising:
the PWM modulation circuit according to claim 1 ;
an output stage amplifier that amplifies a PWM modulation signal input from the PWM modulation circuit and outputs the result; and
a speaker that outputs the signal input from the output stage amplifier as an audio sound signal.