130 in switching voltage regulator 100.

At 222, the output voltage V_OUTdrops in response to the switching voltage regulator going into a dropout condition due to the input voltage V_IN102 falling below the specified nominal output voltage V_OUT(nom). When the input voltage V_IN102 falls below the specified nominal output voltage V_OUT(nom), it causes the output feedback voltage V_FB133 to fall below the reference voltage V_REF138, which causes the compensation voltage V_COMP135 to rise to its upper clamp voltage at 212. During the period from 222 to 224, the voltage control loop is out of control, so the output voltage V_OUTis not being properly regulated and will follow the input voltage V_IN. The highest voltage that the output voltage V_OUTcan reach in a buck voltage regulator is equal to the input voltage V_IN.

At 224, the input voltage V_INrecovers and the switching voltage regulator comes out of the dropout condition. The output voltage V_OUTrises, but does not stop rising at the specified nominal output voltage V_OUT(nom). Instead, the output voltage V_OUTovershoots at 226 to a peak voltage significantly higher than the specified nominal output voltage V_OUT(nom). As the switching voltage regulator comes out of the dropout condition, the compensation voltage V_COMP135 begins to drop from the clamp voltage at 214. However, a recovery time is required for the compensation voltage V_COMP135 to discharge from the clamp voltage to its steady-state value at 216. The time between 214 and 216 is the recovery time.

When the compensation voltage V_COMP135 reaches its steady-state value, the voltage control loop regains control, and the output voltage V_OUTwill reach the specified nominal output voltage V_OUT(nom)at 228. In this case, the compensation voltage V_COMP135 has to discharge from 2.1 V to 0.65V, which requires a recovery time of over 150 microseconds. During this recovery time, the output voltage V_OUTovershot its specified nominal output voltage of 5V by more than 3V. This overshoot problem occurred during the recovery time and was due to the significant voltage difference between the clamp voltage and the steady-state voltage of the compensation voltage V_COMP135.

One possible solution to the overshoot problem is to make the clamp voltage dynamic, adjusting to changing conditions rather than setting the clamp voltage at a fixed voltage. Adequate information is provided by the switching voltage regulator circuit to determine and generate a dynamic compensation clamp voltage that limits the amount of voltage discharge required on the compensation voltage V_COMP135, which helps to limit the overshoot on the output voltage V_O130.

The clamp voltage V_Clampcan be set to dynamically remain just above the voltage that the compensation voltage V_COMPshould be at during normal operation. This helps ensure that the voltage clamp has no effect on circuit performance during normal operation, but clamps the compensation voltage V_COMPwhen the switching voltage regulator gets into a dropout condition. This allows the compensation voltage V_COMPto recover from a dropout condition more quickly. The compensation voltage V_COMPcan recover more quickly because it only has to move a smaller amount (e.g. 100 mV instead of 1-2V) than it does with a fixed voltage clamp circuit.

The clamp voltage V_Clampthat limits the compensation voltage V_COMPcan be set so that recovery from a dropout condition can occur more quickly with less disruption to the output voltage V_OUTby setting the clamp voltage V_Clampto dynamically track and remain just above the compensation voltage V_COMP. Then, if the switching voltage regulator goes into a dropout condition, the compensation voltage V_COMPrecovers more quickly coming out of the dropout condition because the compensation voltage V_COMPis not required to discharge as much to regain control of the voltage control loop.

This can be done by making the clamp voltage V_Clampdependent on the sensed current flowing through inductor 110, which is sensed by the current sense circuit 118. A minimum compensation voltage V_COMP135 is necessary to trigger switching in the voltage regulator. In one example case, this minimum compensation voltage, V_{COMP_Offset}, to trigger switching was determined to be 600 mV. However, in other systems, V_{COMP_Offset}may be at a different voltage. The maximum voltage of the Slope Comp signal from Slope Comp generator 116 when running at or near 100% duty cycle, V_{MAX_Slope_Comp}, can be determined for any given system. In one example system, V_{MAX_Slope_Comp}was determined to be 340 mV. However, the magnitude for V_{MAX_Slope_Comp}can be different in other systems.

The base voltage for the compensation voltage clamp, V_{Base_Clamp}, can be determined by adding the minimum compensation voltage, V_{COMP_Offset}to the maximum slope compensation voltage V_{MAX_Slope_Comp}. In some examples, a safety margin voltage, V_margin, may be added to ensure that the clamp on the compensation voltage never interferes with normal operation of the voltage regulator. In cases where a safety margin voltage is added, the base voltage for the compensation voltage clamp V_{Base_Clamp}is equal to the sum of the minimum compensation voltage V_{COMP_Offset}, the maximum slope compensation voltage when running at 100% duty cycle V_{MAX_Slope_Comp}, and the safety margin voltage V_margin.

To determine the clamp voltage, the inductor current ripple is sensed and added to the base voltage for the compensation voltage clamp V_{Base_Clamp}. In at least one example system, the sensed inductor current signal is rectified before being added to the base voltage for the compensation voltage clamp V_{Base_Clamp}. Rectifying the inductor current signal may improve performance because it is the peak inductor current that is being regulated. So, the compensation voltage V_COMPshould remain higher than the peak. Ideally, the clamp voltage V_Clamptracks the peak inductor current because the information from the remainder of the slope compensation waveform is not needed. So, a more accurate clamp voltage can be determined by rectifying the sensed inductor current signal prior to adding it to the base voltage for the compensation voltage clamp V_{Base_Clamp}, but, this is not required.

FIG. 3 shows a graph 300 of voltage versus time for signals of an example dynamic voltage clamp circuit. Voltage 310 is V_{COMP_Offset}which is the minimum compensation voltage V_COMP135 required to trigger switching in the voltage regulator. Voltage 320 is V_{MAX_Slope_Comp}, which is the maximum voltage of the Slope Comp signal from Slope Comp generator 116 when the voltage regulator is running at 100% duty cycle. Voltage 330 is V_margin, which is a safety margin voltage that may be added in some cases to ensure that the clamp on the compensation voltage never interferes with normal operation of the voltage regulator. The addition of a voltage V_margin