ACCELERATION OF OUTPUT ENERGY PROVISION FOR A LOAD DURING START-UP OF A SWITCHING POWER CONVERTER

[0001] This application claims the benefit under 35 U.S.C. §119(e) and 37 C.F.R. §1.78 of U.S. Provisional Application No. 61/675,399, filed on Jul. 25, 2012, which is incorporated by reference in its entirety.

[0002] 1. Field of the Invention

[0003] The present invention relates in general to the field of electronics, and more specifically to a system and method of acceleration of output energy provision for a load during start-up of a switching power converter.

[0004] 2. Description of the Related Art

[0005] Many electronic systems include circuits, such as switching power converters to provide efficient power conversion from a voltage supply into a regulated output voltage. Often, a controller controls the power conversion process of a switching power converter. The switching power converter converts input power from a supply voltage source into an amount of output power utilized by a load.

[0006] FIG. 1 depicts a flyback-type switching power converter 100 that converts the input voltage V_INT into a constant current i_S and load voltage V_LD on the side of the secondary-winding 116 of the transformer 112 and to a converter supply voltage V_DD on the side of the auxiliary-winding 124. In at least one embodiment, the input voltage V_IN is a rectified nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe and the People's Republic of China. The controller 102 generates a switch control signal CNTRL to control the flyback-type, switching power converter 104. The control signal CNTRL controls the conductivity of field effect transistor (FET) switch 106 to control the primary current i_P to meet the power demands of load 108. For an n-channel FET, the FET conducts (i.e. ON) when the switch control signal CNTRL is a logical one and is nonconductive (i.e. OFF) when the switch control signal CNTRL is a logical zero.

[0007] When the FET 106 conducts, the primary current i_P ramps up through the primary winding 110 of transformer 112. The dot convention of transformer 112 and the diode 114 prevent flow of the secondary current i_S from the secondary-winding 116 when FET 106 conducts and the primary current i_P is flowing into the primary winding 110. When the controller 102 generates the switch control signal CNTRL to turn FET 106 OFF, the primary current i_P falls to 0, and the voltage across the primary winding 110 reverses (also referred to as “flyback”). During the flyback, the secondary current i_S quickly rises and charges capacitor 118. Capacitor 118 provides an output voltage V_LD and current to the load 108. The load can be any type of load including one or more light emitting diodes. A diode and resistor-capacitor filter circuit 120 provides a path for voltage perturbations.

[0008] After the switching power converter 104 begins operation, an auxiliary power supply 122 provides the supply voltage V_DD for controller 102. The auxiliary power supply 122 includes an auxiliary-winding 124 with the same dot convention as the secondary-winding 116. The FET 126 is biased by a fixed gate voltage V_G to conduct the auxiliary current i_AUX through diode 130 and resistor 132 to the V_DD voltage node. When the controller supply voltage V_DD falls below the gate voltage V_G minus a threshold voltage V_TH of the FET 126, the FET 126 conducts and charges the V_DD node, which charges capacitor 128. When the voltage V_DD reaches V_G+V_TH, the FET 126 stops conducting. Capacitor 128 stores energy to assist in providing a relatively constant value of the controller supply voltage V_DD. Capacitor 129 provides a charge reservoir to provide charge to capacitor 128 when the FET 126 turns ON.

[0009] The controller supply voltage V_DD varies in accordance with varying power demands by controller 102. Thus, the auxiliary power supply 126 provides power to the controller 102 in accordance with the varying power demands of controller 102. When the auxiliary power supply 126 provides charge to the capacitor 128, the auxiliary power supply 126 takes charge from the primary winding 110 that would otherwise be provided to the secondary-winding 116. Since the power demands of the auxiliary power supply 122 are not monitored, the amount of power actually delivered to the secondary-winding 116 and, thus, the load 108 is not accurately known.

[0010] In one embodiment of the present invention, a method includes detecting a start-up condition of a load and causing a switching power converter to provide an amount of charge to the load at a rate in accordance with a start-up mode until reaching an energy-indicating threshold. The method further includes determining if the energy-indicating threshold has been reached and, when the energy-indicating threshold has been reached, causing the switching power converter to (i) decrease the amount of charge provided to the load relative to the charge provided during the start-up mode and (ii) operate in a distinct post-start-up-mode.

[0011] In another embodiment of the present invention, an apparatus includes a controller. The controller is configured to detect a start-up condition of a load and cause a switching power converter to provide an amount of charge to the load at a rate in accordance with a start-up mode until reaching an energy-indicating threshold. The controller is further configured to determine if the energy-indicating threshold has been reached, and, when the energy-indicating threshold has been reached, to cause the switching power converter to (i) decrease the amount of charge provided to the load relative to the charge provided during the start-up mode and (ii) operate in a distinct post-start-up-mode.

[0012] In a further embodiment of the present invention, an apparatus includes a switching power converter, a load coupled to the switching power converter, and a controller, coupled to the switching power converter. The controller is configured to detect a start-up condition of a load and cause a switching power converter to provide an amount of charge to the load at a rate in accordance with a start-up mode until reaching an energy-indicating threshold. The controller is further configured to determine if the energy-indicating threshold has been reached, and, when the energy-indicating threshold has been reached, to cause the switching power converter to (i) decrease the amount of charge provided to the load relative to the charge provided during the start-up mode and (ii) operate in a distinct post-start-up-mode.

[0025] An electronic system and method include a controller to operate in a start-up mode to accelerate driving a load to an operating voltage and then operates in a post-start-up mode. In at least one embodiment, a start-up condition occurs when the controller detects that a load voltage is below a predetermined voltage threshold level. In at least one embodiment, the predetermined voltage threshold level is set so that the controller will boost the voltage to an operating value of a load voltage at a faster rate than during normal, steady-state operation. In at least one embodiment, the controller controls a switching power converter and causes the switching power converter to provide an amount of charge to the load at a rate in accordance with a start-up mode until reaching an energy-indicating threshold. When the controller determines that the energy-indicating threshold has been reached, the controller causes the switching power converter to (i) decrease the amount of charge provided to the load relative to the charge provided during the start-up mode and (ii) operate in a distinct post-start-up-mode.

[0026] In at least one embodiment, in a normal operating mode, during each cycle of an input voltage to the switching power converter, the controller provides an amount of charge to the load until target amount of charge has been provided. The target amount of charge is dependent upon a variety of circuit characteristics, such as the charge storage capacity of the load, the power utilization by the load, any phase-cut angle of the input voltage, etc. In at least one embodiment, the target amount of charge is set to smoothly transition the load voltage to an operating value of the load voltage V_LD. However, when the load voltage is below a predetermined threshold, providing charge to the load using the normal operating mode can result in an extended amount of time before the load reaches the operating value of the load voltage V_LD. For example, when the load includes a capacitor and one or more light emitting diodes (LEDs), as many as 10-13 seconds can elapse before the capacitor reaches the operating voltage value for the LEDs. In at least one embodiment, in the start-up mode, the controller disregards circuit characteristics that would otherwise limit the rate at which charge is delivered to the load, and the controller provides as much charge to the load at the highest possible rate until the load reaches energy-indicating threshold, which, in at least one embodiment, indicates that load voltage has reached the operating voltage value. In the start-up mode, the controller can, in at least one embodiment, drive the load voltage V_LD to the operating voltage value within 1-2 seconds for the LEDs.

[0027] The process of determining when the load voltage V_LD has reached the operating voltage value is a matter of design choice. In at least one embodiment, the controller detects the change of the load voltage over time, i.e. the rate of change of the load voltage. As the load voltage approaches the operating voltage value, the rate of increase of the load voltage will decrease over time. Thus, in at least one embodiment, the energy-indicating threshold represents a threshold change of the load voltage over time, and the controller begins operating in a post-start-up mode once the rate of change of the load voltage reaches the threshold rate of change. This particular embodiment provides flexibility in the controller and a lamp that includes the controller in general because the threshold change of the load voltage over time is, in at least one embodiment, not dependent on any particular load voltage level. In another embodiment, the controller compares a representation of the load voltage to a predetermined voltage threshold to determine when the load voltage is close enough to the operating voltage value to allow the controller to begin operating in a post-start-up mode. In at least one embodiment, the controller compares a time duration of a current used to charge the capacitor of the load, which represents an amount of charge provided to the load. As the load voltage nears the operating voltage value, the time duration decreases. Thus, in at least one embodiment, a minimum duration of the current used to charge the capacitor of the load represents the energy-indicating time, and the controller transitions to a post-start-up mode of operation once the time duration reaches the threshold.

[0028] FIG. 2 depicts an electronic system 200 that includes a controller 202, and the controller 202 includes start-up-mode/post-start-up-mode controller 204. The start-up-mode/post-start-up-mode controller 204 detects a start-up condition of the load 205 and increases the amount of charge provided to the load 205 relative to the amount of charge provided during normal operation after the switching power converter 212 has charged the load capacitor 207 to an operating voltage value. The load 205 includes the capacitor 207 and a load 209. The load 209 can be any type of load, such as a load that includes one or more electronic light sources such as one or more light emitting diodes (LEDs). Voltage source 206 generates a supply voltage V_SUPPLY, such as a nominally 60 Hz/110 V alternating current (AC) line voltage in the United States of America or a nominally 50 Hz/220 V AC line voltage in Europe and the People's Republic of China. An optional dimmer 208, such as a triac-based dimmer, phase cuts the supply voltage V_SUPPLY, and full-bridge rectifier 210 generates a rectified AC input voltage V_IN as an input voltage to the switching power converter 212. The switching power converter 212 can be any type of switching power converter including single or multiple stage switching power converter, a boost, buck, boost-buck, Cúk, or flyback type switching power converter.

[0029] During normal operation, the input voltage V_IN rises during each half cycle of the supply voltage V_SUPPLY, and the controller 202 controls the switching power converter 212 to deliver charge to the load capacitor 207 via output current i_OUT. During normal operation, and for a constant voltage, constant current load 209, the controller 202 attempts to maintain the load voltage V_LD across the load capacitor 207 at an approximately constant level. However, when the electronic system 200 is turned OFF, the load voltage V_LD falls to zero or approximately zero volts.

[0030] When the electronic system 200 is turned ON and the input voltage V_IN begins to rise during each half cycle of the supply voltage V_SUPPLY, the start-up-mode/post-start-up-mode controller 204 detects a start-up condition of the electronic system 200. The manner of detecting the start-up condition is a matter of design choice. In at least one embodiment, start-up-mode/post-start-up-mode controller 204 detects the start-up condition when the load voltage V_LD is below a predetermined voltage threshold level. In at least one embodiment, the predetermined voltage threshold level is set so that the start-up-mode/post-start-up-mode controller 204 detects a start-up condition while ignoring perturbations of the input voltage V_IN during normal operation. In at least one embodiment, the predetermined voltage threshold level for the load voltage V_LD is 70% of a nominal, operating level of the load voltage V_LD. The particular value of the predetermined threshold voltage level for the load voltage V_LD is a matter of design choice. In at least one embodiment, the predetermined threshold voltage level for the load voltage V_LD takes into account circuit operating characteristics, such as the voltage drop across the load 209, such as forward-bias voltages across LEDs when the load 209 includes LEDs. As the voltage drop across the load 209 is more tightly controlled, the percentage of the predetermined voltage threshold level for the load voltage V_LD relative to a nominal, operating level of the load voltage V_LD can increase.

[0031] To rapidly provide charge to the load capacitor 207 during start-up mode, the start-up-mode/post-start-up-mode controller 204 controls the switching power converter 212 and causes the switching power converter 212 to provide an amount of charge to the load 205 at a rate that accelerates the provision of charge to the load 205 relative to the rate of providing charge during a normal mode of operation. The start-up-mode/post-start-up-mode controller 204 continues to operate the switching power converter 212 in start-up mode until the load 205 reaches an energy-indicating threshold. When the start-up-mode/post-start-up-mode controller 204 determines that the energy-indicating threshold has been reached, the start-up-mode/post-start-up-mode controller 204 causes the switching power converter 212 to (i) decrease the amount of charge provided to the load relative to the charge provided during the start-up mode and (ii) operate in a distinct post-start-up, normal operating mode. The particular energy-indicating threshold is a matter of design choice. As subsequently described in more detail, a minimum threshold rate of change of the load voltage V_LD over time, a value of the load voltage V_LD, and a duration of the output current i_OUT during a cycle of a control switch (such as switch 312 of FIG. 3) of the switching power converter 212 represent exemplary energy-indicating thresholds.

[0032] FIG. 3 depicts an electronic system 300, which represents one embodiment of the electronic system 200. The electronic system 300 includes single stage, flyback-type switching power converter 104 that converts the input voltage V_IN and primary-side current ip into a current i_S and load voltage V_LD using the same circuit principles as previously discussed with reference to FIG. 1. The controller 302 generates a switch control signal CS to control conductivity of the switch 304, which is, for example, a field effect transistor. The start-up-mode/post-start-up-mode controller 306 represents one embodiment of the start-up-mode/post-start-up-mode controller 204. The start-up-mode/post-start-up-mode controller 306 is referred to as the “SM-PSM controller 306”. The auxiliary control circuitry 304 is optional and a matter of design choice. In at least one embodiment, the auxiliary control circuitry 304 controls the auxiliary voltage V_AUX as, for example, described in U.S. patent application Ser. No. 13/715,451, entitled “Isolation of Secondary Transformer Winding Current During Auxiliary Power Supply Generation”, filed Dec. 14, 2012, and inventors John L. Melanson, Prashanth Drakshapalli, and Siddharth Maru, which is incorporated herein by reference in its entirety.

[0033] FIG. 4 depicts a start-up mode/post-start-up mode process 400 (referred to herein as “SM-PSM process 400”). In at least one embodiment, the SM-PSM controller 306 operates in accordance with the SM-PSM process 400. FIG. 5 depicts exemplary waveforms 500 associated with the electronic system 300 and the SM-PSM process 400. Referring to FIGS. 3, 4, and 5, operation 402 determines if the electronic system 300 is in a start-up condition by monitoring a value of the mode select signal MODE_SEL. The value of the mode select signal MODE_SEL signal indicates whether the electronic system 300 is in a start-up condition or the electronic system 300 is in a normal-operating mode condition. The source of the mode select signal MODE_SEL is a matter of design choices. Exemplary generation of the Mode select signal MODE_SEL is subsequently discussed with reference to FIGS. 6-10.

[0034] At time t₀, the input voltage V_IN has been at 0V for the equivalent of multiple cycles of an active input voltage V_IN, and, thus, the electronic system 300 is OFF. In at least one embodiment, the SM-PSM process 400 does not operate when the electronic system 300 does not operate. At time t₁, the dimmer 208 conducts current, which generates an exemplary leading edge of the input voltage V_IN, and the electronic system 300 turns ON. When the electronic system 300 turns ON, the SM-PSM controller 306 performs the SM-PSM process 400.

[0035] At time t₁, a value of the mode select signal MODE_SEL indicates that the electronic system 300 is in start-up mode, and the start-up operation 402 proceeds to operation 404, which initializes the start-up mode period 502 of the SM-PSM controller 306. The amount of time that elapses between when the load capacitor 207 is at 0V and when the load capacitor 207 reaches an operating voltage directly depends on the rate of the amount of charge delivered to the load capacitor 207. During the start-up mode period 502, the SM-PSM controller 306 controls the switch 304 so that the switching power converter 104 provides the secondary current i_S with a start-up value to charge the load capacitor 207 and decrease the duration T₀ of the start-up mode period 502 relative to conventional processes. In at least one embodiment, during operation 404, the SM-PSM controller 306 ignores any synthetic limitations, such as a target charge level (which is subsequently discussed), on the amount of charge transferred to the load capacitor 207 and transfers more charge than the amount of charge that correlates to the phase angle of the input voltage V_IN. In at least one embodiment, the SM-PSM controller 306 causes the switching power converter 112 to provide as much charge as possible to the load capacitor 207 during the start-up mode period 502. Operation 406 monitors whether or not an energy-indicating threshold has been reached. In at least one embodiment, a change of the mode select signal MODE_SEL state indicates reaching the energy-indicating threshold. The particular energy-indicating threshold is a matter of design choice, such as a derivative of a voltage representing the load voltage V_LD across the load capacitor 207, a threshold value of the load voltage V_LD, and/or a duration T_OFF—CS of an ‘off’ portion of the control signal CS to switch 304. These exemplary energy-indicating thresholds are subsequently discussed in more detail.

[0036] When operation 406 determines that the energy-indicating threshold has been reached and, thus, in at least one embodiment, the load voltage V_LD has reached or approximately reached an operating level, the SM-PSM process 400 transitions to the post-start-up mode period 504 of post-start-up operation 408. During the post-start-up mode of operation 402, the SM-PSM controller 306 provides an amount of charge to the load capacitor 207 in direct correlation to the phase angle of the input voltage V_IN so that the power delivered to the LED(s) 308 also tracks the phase angle. For example, as seen in the Q_TRANSFERRED waveform of FIG. 5, during the post-start-up mode period 502, the SM-PSM controller 306 limits the amount of charge transferred to the load capacitor 207 to the determined target charge level Q_TARGET. As subsequently described in more detail, the SM-PSM controller 306 establishes the target charge level Q_TARGET to correlate with the phase angle of the input voltage V_IN. When the phase angle of the input voltage V_IN is less than 180 degrees, the target charge level Q_TARGET is set below the maximum amount of charge that can be delivered to the load capacitor 207 by the switching power converter 104.

[0037] In at least one embodiment, the controller 302 controls switch 304 so that the switching power converter 104 transfers charge to the secondary-winding 116 until a pre-determined charge target (Q_target) is met. Controller 302 determines the amount of charge transferred in each cycle of the switch 304 in accordance with Equation 1:

$\begin{array}{cc}{Q}_{\mathrm{transferred}}=\left(\frac{N1}{2}\right)\times I_{\mathrm{peak}}\times T_2& \mathrm{Equation}1\end{array}$

N1 is the turns ratio between the secondary-winding 116 and the primary-winding 110, I_peak is the peak value of the primary-side current i_P, and T₂ is the off time of switch 304 until the primary-side current i_P decays to zero or until a new cycle of the control signal CS begins, whichever occurs first. Controller 206 determines the accumulated, transferred charge for 1 through M cycles of the input voltage V_IN in accordance with Equation 2, where M is a positive integer:

$\begin{array}{cc}{Q}_{\mathrm{total\_transferred}}=\sum _1^M\left(\frac{N1}{2}\right)\times I_{\mathrm{peak}}\times T_2& \mathrm{Equation}2\end{array}$

[0038] Controller 302 continues to transfer charge to the secondary-winding 116 until the accumulated, transferred charge equals Q_target. By isolating the transfer of charge to the secondary-winding 116 and the auxiliary-winding 124, the controller 306 can determine the accumulated amount of charge transferred to the load capacitor 207. The particular manner of operating the electronic system 300 during the post-start-up mode period 504 is a matter of design choice and is further illustratively described in U.S. patent application Ser. No. 13/715,914, entitled “Multi-mode Flyback Control for a Switching Power Converter”, filed Dec. 14, 2012, and inventors Siddharth Maru, Zhaohui He, and Mohit Sood, which is hereby incorporated by reference in its entirety.

[0039] The manner of generating the control signal CS is a matter of design choice. In at least one embodiment, the control signal CS is generated as described in U.S. patent application Ser. No. 12/919,086, entitled “Primary-Side Control of a Switching Power Converter With Feed Forward Delay Compensation”, inventors Zhaohui He, et al., and filing date Jun. 1, 2012, which is hereby incorporated by reference in its entirety.

[0040] Operation 402 continues to monitor the mode select signal MODE_SEL, and when the mode select signal MODE_SEL indicates that the electronic system has returned to start-up conditions, the SM-PSM process 400 repeats as previously described.

[0041] The particular implementation of controller 302 is a matter of design choice. For example, controller 302 can be (i) implemented as an integrated circuit including, for example, a processor to execute software or firmware instructions stored in a memory, (ii) implemented using discrete components, or (iii) implemented using any combination of the foregoing. Additionally, in at least one embodiment, all of the components in electronic system 300, except the voltage supply 206 and the dimmer 208, are included in a lamp.

[0042] FIG. 6 depicts an exemplary mode select signal MODE_SEL generator 600. Referring to FIGS. 3 and 6, resistors 310 and 312 form a voltage divider so that the auxiliary feedback voltage FBAUX represents the auxiliary voltage V_AUX as shown in Equation 3:

FBAUX=R1/(R1+R2)·V_AUX   Equation 3

where R1 equals the resistance value of resistor 312, and R2 equals the resistance value of the resistor 310. In at least one embodiment, the values of R1 and R2 are set so that auxiliary feedback voltage FBAUX is 1V when the load voltage V_LD is at a nominal operating voltage level. The auxiliary voltage V_AUX is proportional to the load voltage V_LD as shown in Equation 4:

V_AUX=N2/N1·(V_LD+V_FD)   Equation 4

where N2 is the turns ratio between auxiliary-winding 124 and the primary-side winding 110, N1 is the turns ratio between the secondary-side winding 116 and the primary-side winding 110, and V_FD is the forward bias voltage of the diode 114. By substitution with Equation 3 and Equation 4, the auxiliary feedback voltage FBAUX is also proportional to the load voltage V_LD as shown in Equation 5:

FBAUX=R1/(R1+R2)·N2/N1·(V_LD+V_FD)   Equation 5

[0043] FIG. 7 depicts exemplary waveforms 700 associated with the electronic system 300 and mode select signal MODE_SEL generator 600. The mode select signal MODE_SEL generator 600 determines the derivative dFBAUX/dt of the feedback voltage FBAUX over time and changes the state of the mode select signal MODE_SEL from indicating the start-up mode to indicating the post-start-up mode when the dFBAUX/dt reaches a threshold value dFBAUX/dt_TH. The start-up mode period 502 (FIG. 5) begins at time t₁, and the feedback auxiliary voltage FBAUX rises approximately linearly until time t₂ as the switching power converter 104 provides charge to the load capacitor 207 to raise the load voltage V_LD. Thus, the derivative dFBAUX/dt of the auxiliary feedback voltage FBAUX over time is relatively constant. At time t2, the load voltage V_LD approaches an operating voltage, and the rate of increase of the load voltage V_LD and, thus, the feedback voltage FBAUX and the derivative dFBAUX/dt decreases. When the derivative dFBAUX/dt reaches the predetermined threshold value derivative dFBAUX/dt _TH at time t₃, the load voltage V_LD has reached or has approximately reached the operating voltage, and the mode select signal MODE_SEL changes state and causes the SM-PSM controller 306 to enter the post-start-up mode 504 (FIG. 5). Thus, the predetermined threshold value derivative dFBAUX/dt represents one embodiment of the energy-indicating threshold. Because the predetermined threshold value derivative dFBAUX/dt is not dependent on absolute voltages, in at least one embodiment, the mode select signal MODE_SEL properly operates regardless of the value input voltage V_IN and the operating voltage. The relationship between times t₁, t₂, and t₃ are illustrative and are not necessarily depicted to scale.

[0044] FIG. 8 depicts an exemplary mode select signal MODE_SEL generator 800. The mode select signal MODE_SEL generator 800 includes a multiplexer 802 that selects a start-up threshold value SU_TH or an over voltage protection threshold value OVP_TH depending on the state of the threshold voltage select signal TH_SEL. When the input voltage V_IN is in a start-up condition, e.g. 0V for the equivalent of multiple cycles of an active input voltage V_IN, the SM-PSM controller 306 sets the state of the threshold voltage select signal TH_SEL so that the multiplexer 802 passes the start-up threshold value SU_TH to the non-inverting input of the multiplexer 802. The comparator 804 compares the start-up threshold value SU_TH to the auxiliary feedback voltage FBAUX on the inverting terminal The start-up threshold value SU_TH is set so that the mode select signal MODE_SEL output of the comparator 804 is a logical one until the auxiliary feedback voltage FBAUX represents an approximate operating value of the load voltage V_LD. Once the auxiliary feedback voltage FBAUX reaches the start-up threshold value SU_TH at time t₃, the mode select signal MODE_SEL transitions to a logical zero, and the SM-PSM controller 306 enters the post-start-up mode operation 408 and functions as previously described. The start-up threshold value SU_TH represents a scaled version of the approximate operating value of the load voltage V_LD. The scaling of the start-up threshold value SU_TH is same as in Equation 5 for the auxiliary feedback voltage FBAUX. In at least one embodiment, the mode select signal MODE_SEL generator 800 is not as flexible as the mode select signal MODE_SEL generator 600 because the start-up threshold value SU_TH depends on the value of the operating value of the load voltage V_LD. Thus, the start-up threshold value SU_TH represents another embodiment of the energy-indicating threshold of FIG. 4.

[0045] FIG. 9 depicts exemplary waveforms 900, which are associated with the electronic system 300 and the mode select signal MODE_SEL generator 700. As previously described, when the load voltage V_LD reaches the scaled version of the start-up threshold value SU_TH, the load voltage V_LD has reached an operating value, and the SM-PSM controller 306 enters the post-start-up mode operation 408.

[0046] FIG. 10 depicts an exemplary mode select signal MODE_SEL generator 1000. FIG. 11 depicts exemplary waveforms 1100, which are associated with the electronic system 300 and the mode select signal MODE_SEL generator 1000. As the load voltage V_LD approaches an operating value, less charge is transferred to the load capacitor 207. As indicated by Equation 5, as the off time T₂ of switch 304 decreases, less charged is transferred to the load capacitor 207. Thus, the duration of the off time T2 correlates to the operating value of the load voltage V_LD. Mode select signal MODE_SEL generator 1000 utilizes this insight when comparator 1002 compares the off time T2 against an off time threshold value T2_TH for switch 304 . When the off time T2 decreases to the off time threshold value T2_TH, the load voltage V_LD has reached an approximate operating value, and the SM-PSM controller 306 enters the post-start-up mode operation 408. Thus, the threshold value T2_TH represents another embodiment of the energy-indicating threshold of FIG. 4.

[0047] In at least one embodiment, the energy-indicating threshold is a predetermined time. When the energy-indicating threshold is a predetermined time, SM-PSM process 400 transitions from operation 406 to the post-start-up mode operation 408 after a predetermined time. In at least one embodiment, the predetermined time is any value within the range of 0.5-3 seconds.

[0048] Thus, an electronic system and method include a controller to operate in a start-up mode to accelerate driving a load to an operating voltage and then operates in a post-start-up mode.

[0049] Although embodiments have been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.