1. Technical Field
The present invention generally relates to power conversion, and particularly relates to charge pump DC-to-DC converters that provide parallel modes of operation.
2. Background
Charge pumps include switching circuitry that use capacitors to store and transfer energy to an output. Charge pumps frequently include regulation control loops to maintain their output voltages at a desired voltage (or range of voltages.) Different charge pump configurations allow various modes of buck (step-down) and boost operation.
Modal DC-to-DC converters may be constructed using different charge pump output circuits, or different configurations of such circuits. For example, so-called fractional mode DC-to-DC converters typically offer a “1×” mode of operation wherein the converter regulates a linear pass output circuit to supply output power from the converter at a desired output voltage, and at least one “non-1×” mode wherein the converter regulates a charge pump output circuit to supply output power from the converter at a desired output voltage. The converter generally is programmed or otherwise configured to switch between the 1× and non-1× modes of operation responsive to changing operating conditions. For boost-mode charge pump circuits, typical non-1× modes may be any one or more of 1.5× and 2× modes, while typical non-1× modes for buck-mode charge pump circuits may be any one or more of ½×, ⅔×, and ¾× modes.
Generally, one of the available modes will be more efficient than the others, for given operating conditions. As a working example, a given DC-to-DC converter may be configured to maintain the same desired output voltage over a varying range of input voltages, e.g., a changing battery voltage. For one range of battery voltages, it may be more efficient (or required, given the input/output voltages involved) for the converter to operate in the 1× mode, while for another range of battery voltages, it may be more efficient (or required) for the converter to operate in a non-1× mode.
A significant difficulty in realizing a working modal DC-to-DC converter in actual circuitry is that the output voltage from the DC-to-DC converter generally cannot reach the ideal input/output ratio for a given charge pump configuration due to the series resistance and parasitic capacitance of the transistor switches and the capacitors used to implement the charge pump output circuit. Consequently, the maximum output voltage to input voltage ratio that can be delivered by the converter will decrease as the output load current increases. A first order parameter for calculating how much the output voltage will drop is the effective output resistance of the charge pump output circuit. For example, if the output voltage is 1V below its ideal input/output ratio when delivering 1 A of output current, then the active output circuit of the DC-to-DC converter has an output resistance of 1V/1 A=1 Ohm under those conditions.
This effective output resistance presents several design challenges, including efficiency loss and difficulty in making the decision as to when to switch between modes of the DC-to-DC converter. The efficiency losses arise because, at higher output current loads, the output voltage drop incurred because of the effective output resistance means that the DC-to-DC converter needs to switch to a less efficient configuration at higher input voltages in order to reliably deliver the rated output voltage. Moreover, once the DC-to-DC converter has switched to the less efficient mode, it generally has no direct way to tell if it will be able to switch back to the more efficient mode and reliably deliver all required output power. Therefore, the switchover return decision is typically made by estimating the series resistance of the transistor switches and capacitors, then adding some margin onto the calculated result before deciding to switch back to the more efficient mode.
Such margin-based decision logic causes the DC-to-DC converter to remain in the less efficient mode longer than may be actually necessary. Of course, switching to a more efficient mode at an input voltage that is not high enough to deliver the needed output current at the rated voltage is bad, too, because it may cause the DC-to-DC converter to ping-pong between modes, where it rapidly switches back and forth between operating modes, because each time that it switches to the more efficient mode the output voltage drops, causing transition back to the less efficient mode.
The above challenges encourage conservative switchover control. For example, designers may design a given DC-to-DC converter to remain in one mode longer than optimal efficiency would dictate, to allow for switchover detection threshold errors, ping-pong avoidance, etc. In other words, given the potential problems associated with prematurely switching from one mode to another, and with switching under dynamic load current conditions, the tendency is to use more conservative values (e.g., voltage and/or current thresholds) to trigger mode switchover. Efficiency consequently suffers because the DC-to-DC converter may defer switching into a more efficient operating mode until worst-case detection values are satisfied.