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ADJUSTING ON-TIME FOR A
DISCONTINUOUS SWITCHING VOLTAGE
REGULATOR
CROSS REFERENCE TO RELATED PATENTS 5
AND PATENT APPLICATIONS
This application is related to U.S. patent application Ser.
No. 11/393,425 filed on Mar. 30, 2006, entitled "SWITCH-
ING VOLTAGE REGULATOR COMPRISING A CYCLE 10
COMPARATOR FOR DYNAMIC VOLTAGE SCALING"
the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
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1. Field of the Invention
The present invention relates to voltage regulators. More particularly, the present invention relates to adjusting the on-time for a discontinuous switching voltage regulator.
2. Description of the Prior Art 20 Discontinuous switching voltage regulators comprise a
charging element that is charged/discharged and then tristated over a cycle that begins when an error signal exceeds a threshold. For example, the charging time may begin when an output voltage falls below a reference volt- 25 age, or when a propagation oscillation signal representing a propagation delay of a digital circuit falls below a reference oscillation signal. In a discontinuous buck mode switching voltage regulator, the charging element typically comprises an inductor that is connected to a source voltage through an 30 upper switch (e.g., a field effect transistor or FET) while charging during an "on-time," and connected to ground through a lower switch (e.g., using a diode or FET) while discharging during a "discharge time." Once the inductor discharges, both the upper and lower switches are opened to 35 tristate the inductor during a "tristate time" in order to prevent an output capacitor from discharging through the inductor to ground. The inductor remains in tristate until the error signal exceeds the threshold which starts a new cycle.
The on-time of the discontinuous mode cycle may be 40 established relative to the current flowing through the inductor such that the upper switch is turned off when the current exceeds a peak current threshold. The prior art has suggested to adjust the peak current threshold according to a linear transfer function of the estimated load current to improve 45 power efficiency and reduce ripple voltage. However, employing a linear transfer function to adjust the peak current threshold may decrease power efficiency due to an increase in the transient response time, and it may also lead to instability. In addition, employing an analog current 50 sensor and a programmable analog current comparator can increase the cost and complexity of the switching voltage regulator.
There is, therefore, a need to adjust the on-time of a discontinuous switching voltage regulator to improve power 55 efficiency while maintaining an acceptable transient response. There is also a need to avoid the cost and complexity of a current controlled loop using an analog current sensor and a programmable analog current comparator.
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SUMMARY OF THE INVENTION
An embodiment of the present invention comprises a discontinuous switching voltage regulator including a charging element operable to generate an output voltage, switch- 65 ing circuitry coupled to the charging element, and switch control circuitry operable to control the switching circuitry
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to configure the charging element during a cycle, including to charge the charging element for an on-time, discharge the charging element for a discharge time, and tristate the charging element for a tristate time. In operation, the on-time is initialized to a first on-time, and a first switch time is measured comprising the first on-time and a first discharge time of a first cycle. A first tristate time of the first cycle is measured, and a first ratio of the first tristate time to the first switch time is determined. The first ratio is compared to a first ratio threshold, and the on-time is adjusted to a second on-time if the first ratio exceeds the first ratio threshold.
In one embodiment, the first cycle is started in response to an error signal. In one embodiment, the error signal comprises a difference between the output voltage and a reference voltage. In another embodiment, the error signal comprises a difference between a propagation oscillation signal and a reference oscillation signal, wherein the propagation oscillation signal represents a propagation delay of a digital circuit. In yet another embodiment, when the error signal crosses an error threshold, the switching circuitry is configured to charge the charging element for the on-time.
In one embodiment, the charging element comprises an inductor.
In another embodiment, after the on time, the switching circuitry is configured to discharge the charging element for the discharge time, and in one embodiment, after the discharge time, the switching circuitry is configured to tristate the charging element for the tristate time.
In yet another embodiment, if the first ratio is greater than the first ratio threshold, the second on-time is less than the first on-time, and in one embodiment, if the first ratio is less than the first ratio threshold, the second on-time is greater than the first on-time.
In still another embodiment, the switch control circuitry is further operable to measure a second switch time comprising the second on-time and a second discharge time of a second cycle, measure a second tristate time of the second cycle, determine a second ratio of the second tristate time to the second switch time, compare the second ratio to a second ratio threshold, and decrease the on-time to a third on-time if the second ratio is greater than the second ratio threshold. In one embodiment, the second on-time is approximately 1/N times the first on-time, and the third on-time is approximately 1/N times the second on-time.
In another embodiment, the switch control circuitry is further operable to measure a third switch time comprising the third on-time and a third discharge time of a third cycle, measure a third tristate time of the third cycle, determine a third ratio of the third tristate time to the third switch time, compare the third ratio to a third ratio threshold, and reset the on-time to the first on-time if the third ratio is less than the third ratio threshold.
In one embodiment, the switch control circuitry comprises a digital timer for timing the on-time; in another embodiment, the switch control circuitry comprises a digital timer for timing the discharge time, and in yet another embodiment, the switch control circuitry comprises a digital timer for timing the tristate time.
Another embodiment of the present invention comprises a method of operating a discontinuous switching voltage regulator. The discontinuous switching voltage regulator comprises a charging element operable to generate an output voltage, and switching circuitry coupled to the charging element, wherein the switching circuitry is configured during a cycle, including to charge the charging element for an on-time, discharge the charging element for a discharge time, and tristate the charging element for a tristate time. The
