Abstract:
A converter coupled to a DC voltage input and connectable to a load, includes a signal responsive switch coupled between a first circuit point and a second circuit point. In lieu of burst mode operation during low load conditions, the peak switch current is varied directly with load condition and a switch deactivation interval is varied inversely with load condition. The switch deactivation level is within a maximum level to avoid audio frequency band interference, while maintaining high efficiency operation throughout the load range.

Description:
TECHNICAL FIELD  
       [0001]     The disclosure relates to switched voltage regulators, more particularly to the control of peak current and switch off-time in accordance with sensed load condition.  
       BACKGROUND  
       [0002]     The use of switching regulators to control a DC output voltage at a level higher than, lower than, or the same as, an input voltage is well known. Typically, one or more switches are activated to supply current pulses via an inductor to charge an output capacitor. The output voltage level is maintained at a desired level by adjusting the on and off times of the switching pulses in accordance with output voltage and load conditions.  
         [0003]     A conventional step-up, or boost, DC/DC converter is illustrated in  FIG. 1 . Inductor  10  and diode  12  are connected in series between input (V IN ) and output (V OUT ) nodes. The input node is typically connected to a DC source, the controlled output node coupled to a load. Capacitor  14  is connected between the output node and ground. Signal responsive switch  16  and resistor  18  are connected in series between the inductor/diode junction and ground. The switch is represented by a transistor having a base connected to the output of latch  20  through switch driver circuit  22 . A set terminal of the latch is connected to the output of AND gate  24 . Delay circuit  26  has an input connected to the reset output of the latch and an output connected to a first input of the AND gate. A second input of the AND gate is connected to the output of comparator  28 . A first input of the comparator receives a feedback signal related to an output parameter. The output parameter may be the voltage at the output, the feedback signal derived through a feedback circuit  30 , the feedback appropriately scaled for comparison with a reference voltage REF 1    32  applied to a second input of the comparator. The reset terminal of the latch  20  is connected to the output of a second comparator  34 . A first input of comparator  34  is connected to the junction between switch  16  and resistor  18 . A second input of comparator  34  is connected to reference voltage circuit  36 .  
         [0004]     In operation, when switch  16  is in the on, or closed, state, current flows from source V IN  through inductor  10  and resistor  18  to ground. Resistor  18  is a sensing element that provides an indication of the current level through the switch when the switch is closed. When the current through the switch increases to the threshold level of reference voltage REF 2    36 , comparator  34  outputs a signal to reset the latch  20 , thereby turning off switch  16 . When the switch is turned off, energy stored in the inductor is transferred to the capacitor  14 . Delay circuit  26  ensures that the high latch reset output signal is not applied to the input of AND gate  24  until a minimum time interval has occurred. Turn-on of switch  16  is thus delayed accordingly. Thereafter, the switch will again be turned on when the feedback level exceeds the reference input to comparator  28 .  
         [0005]     In the particular conventional circuit illustrated, commonly known as a boost regulator, regulated voltage output V OUT  has a voltage level higher in magnitude than the voltage input V IN  and of the same polarity. Known converters, for example, are Linear Technology LT3463 and LT3464 converters. With appropriate arrangement of inductor, switch and capacitive elements, a regulator output voltage can be provided with a polarity opposite to that of the input voltage or in a buck regulator configuration in which voltage output V OUT  has a voltage level lower in magnitude than the voltage input V IN .  
         [0006]     In many portable systems, when the output load is light, the switching regulators are controlled to go into a power saving sleep mode. In the sleep mode, the regulator reduces the operating current by turning off some internal circuitry and operates intermittently in a burst mode. In a traditional “burst” mode scheme, a hysteretic comparator is used to monitor when the output voltage falls out of regulation in the sleep mode condition. Circuitry is then enabled to deliver the burst current pulses until the output voltage is brought back to within regulation level. Internal circuitry is again turned off in the sleep mode to save power consumption. With light output load, the output voltage then drifts lower to the programmed level at which the regulator “wakes up” to drive the output higher in burst cycles.  
         [0007]     Inductor current and output voltage waveforms for typical burst/sleep mode operation are illustrated in  FIG. 2 . At time t 0 , the output voltage has fallen to a low regulation threshold voltage and a burst cycle has been initiated. The switched current pulses are applied to the output capacitor  14 , building up the output voltage until a high regulation threshold voltage is attained at time t 1 . Switching is then terminated until the output voltage again falls to the low threshold at time t 2 . Operation continues in this manner during light load conditions. The regulator can wait a relatively long period of time before delivering pulses of energy. While waiting, the regulator enters into a state of low quiescent current and no switching activity, whereby energy is conserved.  
         [0008]     The intermittent bursts in the switching waveforms can contain a low frequency content that forms noise in the audio band. A switching regulator operating in burst mode can readily produce switching frequencies below 40 kHz. The audio switching frequency can cause a ceramic capacitor to emit audio waves, which are undesirable to the end user of a system. A prior approach to this problem is use of a constant frequency, pulse width modulated regulator in which an internal fixed frequency oscillator gates the power switch on each cycle. Such control effects low noise in the audio frequency range. However, the efficiency at light load is poor as the switching frequency remains high during light load conditions.  
         [0009]     A need thus exists for a switching regulator that operates at high efficiency over a wide load range, including light load conditions, without having the disadvantage of producing unwanted noise.  
       DISCLOSURE  
       [0010]     The present invention fulfills the above-described needs of the prior art. In a switching regulator, a load condition is sensed. A current threshold level that is variable in accordance with the sensed load condition is set and a time interval that is variable in accordance with the sensed load condition is set. A regulator switch is activated until the switch current attains the set current threshold level and then deactivated. The switch remains deactivated for the set time interval and then reactivated. Activation and deactivation proceeds continuously while the load condition continues to be sensed and the set current threshold and the set time interval are adjusted in accordance with changes in the sensed load condition. The load condition may be load current that is sensed by detecting a voltage proportional to load voltage, the current threshold level being set to a maximum at low load voltage and to a minimum at high load voltage. The time interval is set to a maximum at high load voltage and to a minimum at low load voltage.  
         [0011]     A switch control circuit is coupled to a control input of the regulator switch to activate and deactivate the switch. A variable delay circuit is coupled to the load sensor and the switch control circuit, the variable timing circuit configured to set a time interval between successive switch activations based on sensed load condition. A maximum deactivation time interval is set for light load condition. A maximum current setting circuit is coupled to the load sensor and the switch control circuit so that the maximum switch current is varied in accordance with load.  
         [0012]     The switch control circuit may comprise a latch having a first output coupled to the control input of the switch and a second output of a state reciprocal to the state of the first output, the second output coupled to an input of the variable delay circuit. The latch is responsive to a pulse applied to a set input to activate the switch and is responsive to a pulse applied to a reset input to deactivate the switch. The output of the a variable delay circuit is coupled to the set input.  
         [0013]     A switch current sensor is coupled to a positive input of a comparator. A negative input of the comparator is coupled to the set maximum current reference level. The output of the comparator is coupled to the reset input. The comparator applies a pulse to the reset input of the latch to deactivate the switch when the sensed current reaches the maximum current reference level. An error amplifier has a first input for receiving a voltage representing the load condition, for example, a voltage proportional to the load voltage. A second input of the error amplifier is coupled to a preset reference potential. The error amplifier output represents the load condition and is applied to the negative input of the comparator to provide the set maximum current reference level. The error amplifier output may also be coupled through an inverting amplifier to the variable delay circuit.  
         [0014]     Thus the regulator switch is continuously and adaptively controlled for successive activation and deactivation over a wide range of load conditions to provide efficient operation over the wide range while preventing the deactivation time of the switch from exceeding a predetermined level. The maximum switch off-time can be limited to a value, for example 25 μsec., so that switching frequency will not fall into the audio band. Traditional burst mode operation has been eliminated.  
         [0015]     Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which:  
         [0017]      FIG. 1  is a diagram of a known step-up DC/DC converter.  
         [0018]      FIG. 2  is a diagram of inductor current and output voltage waveforms for burst mode operation of the converter of  FIG. 1 .  
         [0019]      FIG. 3  is a block diagram of a converter in accordance with the present invention.  
         [0020]      FIG. 4   a  is a more detailed diagram of a timing and peak current control circuit of  FIG. 3 .  
         [0021]      FIG. 4   b  is a variation of the diagram of  FIG. 4   a.    
         [0022]      FIGS. 5   a  and  5   b  are alternative transfer function diagrams for the timing and peak current control circuit of  FIG. 4   a.    
         [0023]      FIG. 6  is a circuit diagram of a variable delay circuit of  FIG. 3 .  
         [0024]      FIG. 7  is a timing diagram for operation of the variable delay circuit of  FIG. 5 .  
         [0025]      FIGS. 8   a  and  8   b  are waveform diagrams of load current for light load and heavy load conditions for operation of the converter of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0026]      FIG. 3  is a block diagram of a boost switching regulator in accordance with an embodiment of the present invention. It should be understood that the principles described herein are applicable to boost, buck, and buck-boost current mode switching converters that can provide a regulated output voltage, at either polarity, of any particular desired level from any given DC source. Coupled in series between input node V IN  and output node V OUT  are inductor  110  and diode  112 . Capacitor  114  is connected between the output node and ground. Signal responsive switch  116 , shown as a transistor, and sensing resistor  118  are coupled in series between the junction of inductor  110  and diode  112 , and ground. Coupled in series between the voltage output node and ground are resistors  129  and  130 .  
         [0027]     The base of transistor  116  is coupled to the Q output of latch  120  through switch driver circuit  122 . Variable delay circuit  126  is coupled between the/Q output of latch  120  and the set input of the latch. The junction of resistors  129  and  130  represents the output load and is fed back to a negative input of error amplifier  128 . The positive input of the error amplifier is supplied by preset reference potential V REF3 . The output of error amplifier  128  is coupled to timing and peak control circuit  136 . Variable delay circuit  126  is coupled to the V 1  output of circuit  136 . The junction of transistor  116  and resistor  118  is coupled to the positive input of comparator  134 . The negative input of comparator  134  is coupled to V 2  output of circuit  136 .  
         [0028]     Transistor  116  is activated to conduct current, applied to inductor  110  by the input voltage source, in response to a pulse applied to the set input of latch  120 . A high Q output is applied to the switch driver  122 . Switch  116  is deactivated in response to a pulse applied by comparator  134  to the reset input of the latch. A reset pulse is applied when the switch current, sensed by the voltage level at resistor  118  increases to the threshold voltage level of the comparator. The reset pulse changes the states of the Q and/Q latch outputs. The high level/Q output is applied by the variable delay circuit  126  to the set input after the set time delay.  
         [0029]     The output VC of error amplifier  128  represents load condition. At light load, capacitor  114  is charged to provide a high V OUT  voltage level. The output VC of error amplifier, correspondingly, is low. The peak switch current threshold of comparator  134  is set by the V 2  output of circuit  136  to a low value. The switch deactivation interval, set by variable delay circuit  126  is at a maximum level, in correspondence with a high V 1  output of circuit  136 . As load increases, capacitor  114  discharges to lower V OUT , and thus the feedback voltage applied to the negative input of error amplifier  128 . VC, thus rises accordingly. Circuit  136  is responsive to changes in VC to vary the V 1  and V 2  outputs. When VC rises, V 2  increases to increase the threshold level at the negative input of comparator  134 . V 1  decreases to shorten the delay set by the variable delay circuit  126 . The energy transferred to capacitor  114  is correspondingly increased.  
         [0030]     The timing and peak current control circuit is shown in more detail in  FIG. 4   a . VC, the output of error amplifier  128 , is applied to the positive input of error amplifier  140 . Amplifier  140  is configured as an operational amplifier, with its output fed to its negative input. In this configuration, the output V 2  will follow the input VC within limits set by bias voltage V 2 MAX and ground. V 2  is applied to the negative input of comparator  134  of  FIG. 3 . V 2  is also applied to negative input of operational amplifier  142  via series resistor  144 . The output of amplifier  142  is coupled to its negative input by series resistor  146 . A reference voltage V REF4  is applied to the positive input of amplifier  142 . In this configuration, amplifier  142  functions as an inverting amplifier that inversely follows V 2  within limits set by bias voltage V 1 MAX and ground. The V 1  output is applied to variable delay circuit  126  of  FIG. 3 .  
         [0031]     V 2 MAX, V 1 MAX, V REF , and resistors  144  and  146  can be adjusted to obtain different VC/V 1  transfer functions and VC/V 2  transfer functions. Two representative transfer function relationships are illustrated in  FIGS. 5   a  and  5   b , VC represented by the abscissa. Peak switch current (ISW-PEAK) and switch deactivation interval (T off ) are shown as a function of VC in each figure. T off  is set to a maximum level at light load currents until VC increases to point A. As VC continues to increase, the change in V 1  applied to the variable delay circuit  126  produces a corresponding decrease in T off  until VC has increased to point B. At point B and thereafter, T off  is set to a minimum. ISW-PEAK is set to a minimum level at light loads until VC increases to point C. As VC continues to increase, the change in V 2  applied to the threshold input of comparator  134  produces a corresponding increase in ISW-PEAK until VC has increased to point D. At point D and thereafter, ISW-PEAK is set to a maximum.  
         [0032]     In  FIG. 5   a , as example parameters, minimum ISW-PEAK may be set to 40 ma, maximum ISW-PEAK may be set to 200 ma, minimum T off  may be set to 300 ns and maximum T off  may be set to 15 μs. The break points A and C occur at the same load (VC) and the break points B and D occur at the same load. With this transfer function, the switching frequency will be relatively constant over the load range. In  FIG. 5   b , minimum ISW-PEAK may be set to 40 ma, maximum ISW-PEAK may be set to 200 ma, minimum T off  may be set to 200 ns and maximum T off  may be set to 25 μs. The break points A and C do not occur at coincident load levels nor do the break points B and D. With A and D occurring at the same load, efficiency at light load will be higher, although frequency is less constant. The slope of the change of V 1  with respect to change of VC is related to the ratio of the values of resistors  144  and  146 . Adjustment of these values can be used to change the load break point relationships.  
         [0033]     As a further variation, with appropriate control circuit adjustment, ISW-PEAK may be held to a constant level while T off  is made to vary inversely with load over a substantial range of load current.  FIG. 4   b  illustrates a variation of the timing and peak current control circuit of  FIG. 4   a  for implementing such functionality. In this circuit, error amplifier  140  has been deleted. V 2  is supplied, instead, by a reference voltage V REF5 . As this reference voltage is applied to the negative input of error amplifier  134  of  FIG. 3 , the peak current drawn through switch  116  will be constant. The VC signal is applied to the resistor  144 , coupled in series with the negative input of error amplifier  142 . The values of resistance of resistors  144  and  146  and the voltage values of V 1   MAX  and V REF4  can be adjusted to set the maximum and minimum values and the slope and breakpoint values of T off  to provide appropriate regulation over the entire load range for a maximum peak current level set by adjustment of the value of V REF5 .  
         [0034]      FIG. 6  is a circuit diagram of a variable delay circuit of  FIG. 3 . Coupled in series between a voltage supply and ground are resistor  150  and transistor  152 , current source  154  and transistor  156 , and resistor  158  and transistor  160 . The base of transistor  152  is coupled to the/Q output of latch  120 , indicated as “IN”, via resistor  162 . The collector of transistor  152  is coupled to the base of transistor  156  through resistor  164 . The collector of transistor  156  is coupled to the base of transistor  160 . Capacitor  166  is coupled across the base and emitter of transistor  160 . The collector of transistor  160  is coupled to the collector of transistor  168 , whose emitter is coupled to ground. The collector of transistor  152  is coupled to the base of transistor  168  via resistor  170 .  
         [0035]     The collector of transistor  168  is coupled to one-shot  172 , which produces an output pulse “OUT” that is applied to the set input of the latch  120 . The one-shot is triggered by the negative edge of a pulse at the collector of transistor  168 . Coupled in series between the V 1  input from circuit  136  and ground are resistor  174  and transistor  176 . The collector and base of transistor  176  are coupled together. Transistor  178  is coupled between the base of transistor  160  and ground. The bases of transistors  176  and  178  are coupled together to form a current mirror.  
         [0036]     In response to a transition of the “IN” input signal from low to high, a low to high pulse is produced with delay at the “OUT” signal output. Reference is made to the waveforms shown in  FIG. 7 . At time t 0 , IN is low. As  152  is rendered non-conductive, the voltage level at its collector and the base of transistor  156  is high. Transistor  156  is conductive, thereby forcing the voltage level at capacitor  166  and the base of transistor  160  to be low, rendering transistor  160  non-conductive. At this time, current source  154  is coupled to ground through transistor  156 . Transistor  168  is conductive by virtue of the high voltage level at the collector of transistor  152 . The level of the collector of transistor  168  and that of the OUT signal is low.  
         [0037]     At t 1 , the latch is reset in response to a signal received from comparator  134  to produce a low to high transition at/Q and IN. Transistor  152  is rendered conductive to force the voltage level at its collector and the base of transistor  156  low. Transistors  156  and  168  are turned off. The collector voltage of transistor  168  goes high. Charge is applied to the capacitor  166  by the current source  154 . The capacitor voltage increases at a rate commensurate with its charge rate, which in turn is affected by current mirrored to transistor  178  by the conductive path including transistor  176 . Transistor  160  remains non-conductive until the capacitor voltage at its base reaches its trigger level, typically 0.7 v. At t 2 , transistor  160  is rendered conductive, pulling the voltage level at the collector of transistor  168  low. This high to low transition initiates a one-shot pulse to set the latch  120 .  
         [0038]     Reactivation of switch  116  has been delayed by the period between t 1 , the time of the reset pulse IN, and t 2 . This period is shortened or lengthened in accordance with changes in V 1 . The current through transistor  178  shunts current of the current source  154  away from application to capacitor  166  by an amount mirrored by transistor  176 . An increased current in transistor  178  produces a decrease in the rate at which the capacitor voltage ramps, transistor  160  will be rendered conductive later, and the delay period between t 1  and t 2  is increased. A decreased current in transistor  178  produces a decreased delay period. The current in transistor  178  mirrors the current in transistor  176 , which varies directly with changes in V 1 . Thus an increase in load produces a decrease in the deactivation period and vice versa.  
         [0039]     Waveforms of the inductor current I L  for light load current and heavy load current conditions are shown in  FIGS. 8   a  and  8   b , respectively.  FIG. 8   a  can be compared with the inductor current waveform for traditional light load burst mode operation, shown in  FIG. 2 . At a given light load current, triangular shaped current pulses at low peak level are produced with a set maximum period therebetween. The maximum period assures that there will be no audio band interference created. For a heavy load current condition, a higher frequency, continuous current triangular waveform is produced, the peak load current level being significantly higher than that at low load current condition.  
         [0040]     In this disclosure there are shown and described only preferred embodiments of the invention and but a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, instead of fixing a minimum current level in accordance with load, a minimum switch on time can be set. The concepts expressed herein with respect to the illustrated regulator circuit are equally applicable to other well known regulators configurations.