Method for Operational Amplifier Output Clamping for Switching Regulators
A switching regulator includes a feedback network that generates a voltage V ea that is proportional to the difference between the output of the switching regulator and a reference voltage. A comparator is used to compare V ea to a sawtooth voltage to generate a voltage V pwm . The voltage V pwm drives a high-side switching transistor that controls the output of the regulator. A first clamping transistor that provides a variable gain connection between the voltage V ea and ground. As V ea rises above a predefined level, the clamping circuit increases the connection between V ea and ground, preventing V ea from substantially exceeding V t . As a result, transient performance of the regulator is improved.
1 . A method for controlling a switching regulator that includes a high-side switching transistor, the method comprising:
generating a voltage V ea that is proportional to the difference between the output of the switching regulator and a reference voltage;
generating a sawtooth voltage V t that is proportional to the current passing through the high-side switching transistor;
modulating a variable gain connection between the voltage V ea and ground as a function of V ea to limit the degree to which V ea is allowed to exceed V t ; and
comparing V ea and V t to generate a PWM signal to drive the high-side switching transistor.
2 . A method as recited in claim 1 that further comprises: modulating a variable gain connection between the voltage V ea and the input voltage to the switching regulator as a function of V ea to limit the degree to which V ea is allowed to fall below V t .
3 . A method as recited in claim 1 in which the step or modulating a variable gain connection between the voltage V ea and ground further comprises:
generating a voltage proportional to the difference between V ea and a voltage generated by a current source; and
using the proportional voltage to control a transistor that connects V ea to ground.
4 . A method as recited in claim 2 in which the step or modulating a variable gain connection between the voltage V ea and the input voltage to the switching regulator further comprises:
generating a voltage proportional to the difference between V ea and a voltage generated by a current source; and
using the proportional voltage to control a transistor that connects V ea to the input voltage to the switching regulator.
5 . A circuit for controlling a switching regulator that includes a high-side switching transistor, the circuit comprising:
a first op amp that generates a voltage V ea that is proportional to the difference between the output of the switching regulator and a reference voltage;
a current sensing circuit that that generates a sawtooth voltage V t that is proportional to the current passing through the high-side switching transistor;
a first clamping transistor that provides a variable gain connection between the voltage V ea and ground;
a second op amp that drives the first clamping transistor as a function of V ea to limit the degree to which V ea is allowed to exceed V t ; and
a third op amp that compares V ea and V t to generate a voltage V pwm to drive the high-side switching transistor.
6 . A circuit as recited in claim 5 that further comprises:
a second clamping transistor that provides a variable gain connection between the voltage V ea and the input voltage to the switching regulator; and
a fourth op amp that drives the second clamping transistor as a function of V ea to limit the degree to which V ea is allowed to fall below V t .
7 . A switching regulator that comprises:
a high-side switching transistor;
a feedback network that generates a voltage V ea that is proportional to the difference between the output of the switching regulator and a reference voltage;
a comparator that compares V ea to a sawtooth voltage to generate a voltage V pwm to drive the high-side switching transistor;
a first clamping transistor that provides a variable gain connection between the voltage V ea and ground; and
a first op amp that drives the first clamping transistor as a function of V ea to limit the degree to which V ea is allowed to exceed V t .
8 . A switching regulator as recited in claim 7 that further comprises:
a second clamping transistor that provides a variable gain connection between the voltage V ea and the input voltage to the switching regulator; and
a second op amp that drives the second clamping transistor as a function of V ea to limit the degree to which V ea is allowed to fall below V t .