Patent Publication Number: US-8970185-B1

Title: Method for maintaining high efficiency power conversion in DC-DC switching regulators over wide input supply range

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/542,691, filed on Oct. 3, 2011. The disclosure of the above application is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to a switching regulator, and more particularly to a system and method for maintaining a conduction channel of a power FET of a switching regulator. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Referring now to  FIG. 1 , a direct current to direct current (DC/DC) switching regulator  100  receives a DC input voltage (e.g., PV IN ) and outputs a DC output voltage (e.g., V OUT ). Various circuit components of the switching regulator  100  operate according to a supply voltage (e.g., V DD ), and the DC input voltage may correspond to the supply voltage, which may vary over a range. 
     The switching regulator  100  includes an output power stage including a high side driver  104  and a low side driver  108 , an inductor  112 , a bootstrap capacitor (C BST )  116 , and an output capacitor (C OUT )  120 . For example only, the drivers  104  and  108  include transistors such as NMOS power field effect transistors (FETs). A controller  124  such as a pulse width modulation (PWM) controller controls the drivers  104  and  108  via respective pre-driver buffer circuits  128  and  132 . The input voltage (e.g., V BST ) may be provided to the switching regulator  100  via a rectifier diode  136 . The inductor  112  and/or the bootstrap capacitor  116  may be located external to an integrated circuit that includes the drivers  104  and  108  and other components of the switching regulator  100 , such as the controller  124 . For example only, the bootstrap capacitor is between 10 and 50 nanoFarads. 
     SUMMARY 
     A switching regulator includes a high-side driver configured to receive an input voltage, and a low-side driver configured to receive the input voltage. The high-side driver and the low-side driver are configured to provide an output voltage based on the input voltage. A charge pump module is configured to receive a supply voltage that varies between a first voltage level and a second voltage level greater than the first voltage level, generate the input voltage based on the supply voltage, and maintain the input voltage at the second voltage level independent of variations in the supply voltage. 
     A method of operating a switching regulator includes providing an input voltage to a high-side driver and a low-side driver, providing an output voltage based on the input voltage using the high-side driver and the low-side driver, receiving a supply voltage that varies between a first voltage level and a second voltage level greater than the first voltage level, generating the input voltage based on the supply voltage, and maintaining the input voltage at the second voltage level independent of variations in the supply voltage. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  illustrates a switching regulator according to the prior art; 
         FIG. 2  illustrates a switching regulator including a charge pump module according to the principles of the present disclosure; and 
         FIG. 3  illustrates an example charge pump module according to the principles of the present disclosure. 
     
    
    
     DESCRIPTION 
     Direct current to direct current (DC/DC) switching regulators generally include a high side driver and a low side driver. The high side driver and the low side driver include transistors such as NMOS power field effect transistors (FETs). An output conductance (e.g., GDS) of the power FETs is proportional to a difference between a threshold voltage (e.g., V TH ) and a gate-source voltage (V GS ) of the power FETs. A threshold voltage is generally in the range of 0.65V-0.8V. An inverse of the output conductance corresponds to an on resistance (e.g., R DSon ) of the power FETs. 
     Switching regulators typically operate according to a variable power input voltage (e.g., PV IN ), which corresponds to a variable supply voltage (e.g., V DD ). For example only, the supply voltage range may be approximately 2.3V to 5.5V. The power input voltage is input to the power FETs (e.g., to respective drains of the power FETs). When a power FET is ON, the gate-source voltage is approximately equal to PV IN  (or, V DD ). Accordingly, as the supply voltage varies, the output conductance and the on resistance of the power FETs also vary. For example, the on resistance of each of the power FETs corresponds to: R DSon =1/GDS ˜ 1/(V GS −V TH ). 
     For example only, if the supply voltage is 2.3V and the threshold voltage is 0.8V, then the on resistance of the power FET is proportional to 1/(2.3−0.8), or 1/1.5. Conversely, if the supply voltage is 5.5V, then the on resistance is proportional to 1/(5.5-0.8), or 1/4.7. Accordingly, as the supply voltage varies from 2.3V to 5.5V, the on resistance varies by more than a factor of 3. More specifically, as the supply voltage decreases, the on resistance increases. As the on resistance increases, power loss of the switching regulator increases (i.e., efficiency of the switching regulator decreases). 
     A switching regulator according to the principles of the present disclosure includes a charge pump module. The charge pump module maintains a desired power input voltage regardless of variations in the supply voltage. For example, the charge pump module maintains the power input voltage at 5.5V regardless of whether the supply voltage decreases to 2.3V. Further, the charge pump module may include a capacitive charge pump circuit that uses the existing bootstrap capacitor of the switching regulator. The charge pump module can be used to boost the voltage of the bootstrap capacitor. 
     The boost voltage level (at V BST ) of the switching regulator can be set in accordance with the size of a high side driver and the size of a bootstrap capacitor. The effective input voltage to a pre-driver is reduced (from boost voltage level at V BST ) due to the charge sharing effect between the bootstrap capacitor and the gate capacitance of the driver. The voltage reduction at V BST  is dependent upon the ratio of the gate capacitance of the driver to the capacitance of the bootstrap capacitor. If the capacitance of the bootstrap capacitor is significantly greater than the gate capacitance of the driver, then the charge sharing effect is negligible. For example, in typical applications, the bootstrap capacitor is an external capacitor in the range of 10 nF to 50 nF, while the gate capacitance of the driver is in a range of hundreds of pico Farads. In this circumstance, the reduction at V BST  is relatively small (e.g., 10-50 mV). Conversely, the charge pump module accordingly to the present disclosure sets V BST  independently of the supply voltage V DD . The charge pump module can be set to boost V BST  to a voltage level of, for example only, 7.5V instead of 5V, allowing the use of a smaller bootstrap capacitor, and reducing the charge sharing effect. The smaller size of the bootstrap capacitor allows the bootstrap capacitor to be integrated with other components of the switching regulator (e.g., the power FETs) on an integrated circuit (IC), eliminating the need for an extra pin on the IC for communication with an external bootstrap capacitor. 
     Referring now to  FIG. 2 , a direct current to direct current (DC/DC) switching regulator  200  receives supply voltages (e.g., V dd  and V ss ) and outputs a DC output voltage (e.g., V OUT ). The supply voltages may vary over a range. Various circuit components of the switching regulator  200  operate according to a DC input voltage which may correspond to the supply voltages. 
     The switching regulator  200  includes an output power stage including a high side driver  204  and a low side driver  208 , an inductor  212 , a bootstrap capacitor (C BST )  216 , and an output capacitor (C OUT )  220 . For example only, the drivers  204  and  208  include transistors such as NMOS power field effect transistors (FETs). A controller  224  such as a pulse width modulation (PWM) controller controls the drivers  204  and  208  via respective pre-drivers  228  and  232 . The input voltage is provided to the low side driver  208  via a switch (e.g., a PMOS transistor)  236 . The controller  224  includes non-overlapping algorithms that control the pre-drivers  228  and  232 . Furthermore, the controller  224  may implement any voltage level shifter required to control the gate drive of the switch  236 . 
     A charge pump module  240  receives the supply voltages V DD  and V SS  and outputs the input voltage. In particular, the charge pump module  240  outputs the input voltage at a predetermined voltage (e.g., 5.5V) regardless of variation in the supply voltage V DD . For example, the predetermined voltage 5.5V may correspond to a desired on resistance of the drivers  204  and  208  (i.e., to maintain a minimal on resistance of the drivers  204  and  208 ). Accordingly, if the supply voltage V DD  decreases below 5.5V, the charge pump module  240  still outputs the input voltage at 5.5V. 
     The charge pump module  240  receives a feedback voltage  244  and a reference voltage V REF . The feedback voltage  244  indicates the input voltage output from the charge pump module  240 . Conversely, the reference voltage indicates a desired input voltage. The charge pump module  240  compares the feedback voltage  244  and the reference voltage V REF  to maintain the input voltage based on the desired input voltage. Accordingly, the input voltage output from the charge pump module  240  can be increased or decreased according to the reference voltage. 
     The charge pump module  240  charges the bootstrap capacitor  216  using the input voltage. For example only, the input voltage provides a boost voltage V BST  to the bootstrap capacitor  216 . The bootstrap capacitor  216  retains a voltage charge provided by the boost voltage V BST  and functions as a holding capacitor for the charge pump module  240 . Accordingly, the charge pump module  240  does not require a separate holding capacitor. Further, the charge pump module  240  may charge the bootstrap capacitor  216  to a voltage greater than 5.5V. For example, the reference voltage can be increased to 7V, 8V, or another value greater than the 5.5V needed to minimize the on resistances of the drivers  204  and  208 . Consequently, the size of the bootstrap capacitor  216  can be reduced. For example, the size of the bootstrap capacitor  216  can be decreased enough to allow the bootstrap capacitor  216  to be located on the same integrated circuit as the drivers  204  and  208  and other components of the switching regulator  200 . Accordingly, a dedicated pin that would be needed otherwise for communicating with a bootstrap capacitor located external to the integrated circuit is eliminated. 
     Referring now to  FIG. 3 , the switching regulator  200  is shown with an example charge pump module  300 . The charge pump module  300  includes a charge pump control module  304 , switches  308 - 1 ,  308 - 2 ,  308 - 3 , and  308 - 4 , referred to collectively as switches  308 , capacitor  312 , comparator  316 , and resistors  320  and  324 . The charge pump control module  304  controls the switches  308  to selectively charge the bootstrap capacitor  216  based on an output of the comparator  316 . Furthermore, the charge pump module  300  may also implement any voltage level shifter required to control the gates of the switches  308 - 1  and  308 - 3 . For example, the comparator  316  compares the feedback voltage  244  (or V BST ) and the reference voltage V REF . The output of the comparator  316  indicates whether the charge pump control module  304  should control the switches  308  to charge the bootstrap capacitor  216 . For example, if the feedback voltage  244  (or V BST ) is less than the reference voltage V REF e, the charge pump control module  304  charges the bootstrap capacitor  216 . The switch  308 - 4  may be connected to either V DD  or V OUT  at a node  328 . 
     For example, in a first phase (e.g., a charge phase), the switches  308 - 1  and  308 - 2  are closed to charge the capacitor  312 . When the switches  308 - 1  and  308 - 2  are closed, current flows between the supply voltages V DD  and V SS  to charge the capacitor  312 . In a second phase (e.g., a pump phase), the switches  308 - 1  and  308 - 2  are opened and the switches  308 - 3  and  308 - 4  are closed to pump the charge to the bootstrap capacitor  216 . In other words, the charge pump control module  304  selectively opens and closes the switches  308  to sequentially charge the capacitor  312  and the bootstrap capacitor  216  using the supply voltages. 
     When the input voltage V BST  is sufficiently high (i.e., greater than or equal to the reference voltage V REF ), the switches  308 - 1  and  308 - 3  are closed to provide the supply voltage V DD  as the input voltage. In other words, when the input voltage V BST  is high, operation of the charge pump module  300  to charge the bootstrap capacitor  216  is not necessary. Conversely, when the input voltage V BST  is less than the reference voltage V REF , the charge pump control module  304  controls the switches  308  to charge the bootstrap capacitor  216 . In this manner, the charge pump module  300  provides at least a desired input voltage (e.g., 5.5V) to the high side driver  204  and the low side driver  208 . 
     The controller  224  controls the high side driver  204  and the low side driver  208  in a complementary and non-overlapping fashion. In other words, when the high side driver  204  is ON (i.e., closed), the low side driver  208  is OFF (i.e., open). Conversely, when the low side driver  208  is ON, the high side driver  204  is OFF. 
     When the low side driver  208  is ON, the charge pump module  300  is operated to provide the desired input voltage to the low side driver  208 , and the controller  224  turns the switch  2360 N (e.g., using a voltage level shifter) to provide the input voltage to the pre-driver  232 . Accordingly, the low side driver  208  is driven by the input voltage, which is regulated at, for example, 5.5V, instead of the variable supply voltage V DD . In other words, the low side driver  208  is driven according to the input voltage (e.g., V BST ), which is independent of the supply voltage V DD . 
     Conversely, when the high side driver  204  is on, the charge stored in the bootstrap capacitor  216  maintains the input voltage at a high side of the high side driver  204 . For example, the bootstrap capacitor  216  maintains the input voltage at 5.5V. The voltage at the high side of the high side driver  204  and a voltage at a low side of the high side driver  204  drives the high side driver  204 . Accordingly, the high side driver  204  is driven according to the input voltage (e.g., V BST ), which is independent of the supply voltage V DD . 
     The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. 
     As used herein, the term module may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor (shared, dedicated, or group) that executes code; analog circuits; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. The term module may include memory (shared, dedicated, or group) that stores code executed by the processor. The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, and/or objects. The term shared, as used above, means that some or all code from multiple modules may be executed using a single (shared) processor. In addition, some or all code from multiple modules may be stored by a single (shared) memory. The term group, as used above, means that some or all code from a single module may be executed using a group of processors. In addition, some or all code from a single module may be stored using a group of memories.