Abstract:
A dual mode regulator, having a high current PWM regulator mode and a low current LDO regulator mode, briefly changes the operating parameters of the PWM and LDO regulators during a transition between modes. Changes during the transition period include: raising the error amplifier reference voltage of the LDO or PWM regulator to ensure a definite handover, raising the bias current in the LDO stages during the transition to cause the LDO regulator to quickly and stably respond to voltage glitches, and augmenting the LDO regulator series pass transistors with one or more additional pass transistors during the transition to enable the LDO regulator to handle higher currents. After the transition, the operating parameters of the enabled regulator portion are reset to their nominal values. The PWM regulator is started with a soft start routine to limit current through the power transistor. If the PWM regulator uses a synchronous rectifier, a reverse current limiting circuit is preferably used to limit reverse current when the PWM regulator is starting up to avoid loading down the LDO regulator.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to voltage regulators and, in particular, to a dual mode regulator that employs a linear regulator mode at low load currents and a pulse width modulation (PWM) mode at higher currents.  
       BACKGROUND  
       [0002]     Dual mode regulators are known that use a linear regulator mode for low currents and a PWM mode for medium and high currents. A PWM regulator switches a power transistor on and off at a regulated duty cycle to maintain a constant voltage at the output of the regulator. The high conductivity of the switching transistor results in low losses across the transistor. This makes the PWM regulator mode efficient for medium to high load currents. At very low currents, although there is low loss across the switching transistor, the losses from turning the transistor on and off at the high switching frequency (typically exceeding 1 MHz) become a significant factor in the regulator&#39;s efficiency.  
         [0003]     At low currents, a linear regulator, also referred to as a low drop out (LDO) regulator, is more efficient than a PWM regulator because there are no switching losses, and the loss through the series transistor is not very significant at low currents.  
         [0004]     In a dual mode regulator, when the load is put into a low current standby mode, for example, the regulator receives a signal initiating the transition between the PWM and LDO regulator modes, and the regulator rapidly changes modes by enabling and disabling the appropriate circuitry. Such a transition causes voltage spikes to appear at the regulator&#39;s output unless a large output capacitor is used. Applicants have discovered that the reasons for the voltage glitches include: 1) a poorly controlled handover of the voltage regulation control while one mode is being disabled and the other mode is being enabled; 2) a normally slow reaction time of the LDO regulator and very little current handling capability to handle glitches during the changeover.  
       SUMMARY  
       [0005]     A dual mode regulator is disclosed that briefly changes the operation parameters of the PWM and LDO regulators during a transition period while the regulator is transitioning into a low current mode or a high current mode.  
         [0006]     In one embodiment, when the dual mode regulator is transitioning into the low current mode, the LDO regulator is enabled, and the reference voltage for the LDO error amplifier is raised so that the LDO regulator takes over the voltage regulation from the PWM regulator at a definite time to prevent both the LDO and PWM regulators from regulating at the same time. To improve the response time of the LDO regulator to variations in output voltage during the transition period and to temporarily increase its load current capability, the biasing currents in the LDO regulator are temporarily increased to shorten the response time of all pertinent transistors in the LDO regulator, and one or more additional transistors are added to the normal LDO series pass transistor to increase the current handling capability. The PWM regulator is then disabled. After a short period, the parameters of the LDO regulator are reset to their normal optimal values and the additional transistor(s) are decoupled from the series pass transistor.  
         [0007]     When the dual mode regulator is transitioning to a high current mode, the biasing currents in the LDO regulator are raised to improve its regulation response time, the series pass transistor is augmented to increase the current handling capability, and the reference voltage for the PWM error amplifier is raised so that the PWM regulator will take over the voltage regulation from the LDO regulator. The PWM regulator is then enabled. The PWM regulator is started with a soft start routine to limit current through the switching transistor. The PWM reference voltage is reset to its nominal value. The LDO regulator is disabled after a short delay, its bias currents are reset, and the additional transistor(s) are decoupled from the series pass transistor.  
         [0008]     If the PWM regulator uses a synchronous rectifier, a reverse current limiting circuit is preferably used to limit reverse current when the PWM regulator is starting up to avoid loading down the LDO regulator.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a high level schematic diagram of various functional units in a dual mode regulator in accordance with one embodiment of the invention.  
         [0010]      FIG. 2  is a detailed schematic diagram of the dual mode regulator of  FIG. 1 .  
         [0011]      FIG. 3  illustrates one technique for changing the value of the reference voltage.  
         [0012]      FIG. 4  illustrates one technique for changing the value of a bias current.  
         [0013]      FIG. 5  illustrates one technique for augmenting the series pass transistor.  
         [0014]      FIG. 6  is a flowchart of the operation of the transitioning circuits when the dual mode regulator is switched to a low power mode.  
         [0015]      FIG. 7  is a flowchart of the operation of the transitioning circuits when the dual mode regulator is switched to a high power mode.  
         [0016]      FIG. 8  illustrates one technique for providing a soft start to the PWM unit. 
     
    
       [0017]     The same or similar elements in the figures are labeled with the same numerals.  
       DETAILED DESCRIPTION  
       [0018]      FIG. 1  illustrates one embodiment of the invention, and  FIG. 2  illustrates an embodiment in more detail.  
         [0019]     An input voltage Vin is applied to the PWM unit  10  and the LDO unit  12 . The PWM unit  10  and LDO unit  12  are shown in more detail in  FIG. 2 . An error signal from an error amplifier  14  is applied to a PWM controller  15  to adjust a switching duty cycle of a power transistor  16 . A synchronous rectifier transistor  18  conducts oppositely to the transistor  16  so that there is no direct path the ground. A diode may be used instead of a synchronous rectifier. An oscillator  20  sets the switching frequency for the PWM controller  15 . The PWM controller  15  issues switching signals to gate drive logic  24 , which ensures that the transistors  16  and  18  alternately conduct. Buffers  26  and  28  provide a suitable current source/sink to the gates of the transistors for a fast response.  
         [0020]     An inductor  30  smoothes out the switched current signal and provides a triangular current waveform, the average of which is the current to the load.  
         [0021]     An output capacitor  32  smoothes out the triangular current waveform and provides a relatively constant voltage (Vout) at the output  34 .  
         [0022]     To limit reverse current through the inductor  30  to ground, a reverse current limiting circuit, such as a differential amplifier  35 , detects a reversal of current through synchronous rectifier  18  while the synchronous rectifier  18  is conducting and overrides its control signal to shut off the synchronous rectifier  18 .  
         [0023]     A resistor divider  36  supplies a feedback voltage to the input of the error amplifier  14  (a differential amplifier or other suitable amplifier), and the regulator adjusts the switching duty cycle so that the regulated feedback voltage is equal to the reference voltage (Vref) applied to the other input of the error amplifier  14  by a reference source  37 . A compensation capacitor (not shown) is connected to the output of the error amplifier  14  to convert a current source/sink signal into a smoothed error voltage signal.  
         [0024]     The PWM controller  15  raises the duty cycle of the power transistor  16  when the output voltage Vout is below the desired voltage and lowers the duty cycle of the power transistor  16  when the output voltage Vout is above the desired voltage. The duty cycle is substantially constant for a given Vin and a desired value of Vout.  
         [0025]     The PWM unit  10  may be any type of PWM circuit, including a voltage mode, a current mode, a resonant mode, or other type. The PWM unit may instead be a pulse frequency modulation (PFM) unit or any other type of switching regulator.  
         [0026]     In a low load current mode, when the LDO regulator is enabled, the LDO unit  12  varies the conduction of a series transistor  42  connected between the input voltage Vin and the Vout terminal. An error amplifier  44  compares a reference voltage Vref, generated by a reference source  45 , to the divided output voltage to generate an error signal. A compensation capacitor (not shown) may be connected to the output of the error amplifier  44 . The error signal is received by a buffer  46 , which controls the conduction of the series transistor  42 . The conduction is increased to raise Vout and decreased to decrease Vout.  
         [0027]     During a transition between modes, discussed below with reference to  FIGS. 6 and 7 , reference voltage values are changed, bias currents are changed, and the series transistor is augmented.  FIGS. 3-5  illustrate some possible circuits for performing these functions.  
         [0028]      FIG. 3  illustrates tapped series resistors used for generating two reference voltages. A fixed voltage V supplies a current through the series resistors. A nominal reference voltage Vref(n) is tapped from the first node, and a higher reference voltage Vref(t) is tapped from the second node. A simple transistor switch  50  is controlled to select the desired reference voltage.  
         [0029]      FIG. 4  illustrates a technique for changing bias currents. A differential amplifier  54  may be the error amplifier  44  for the LDO unit  12 . The reference voltage Vref is applied to one input, and the feedback voltage Vfb is applied to the other input. The voltage at node  56  is an error signal whose magnitude indicates the mismatch between the reference voltage and the feedback voltage. The magnitude is used to control the duty cycle of the PWM unit  10 . The error signal controls the conductivity of transistors in a buffer  60 . The output of the buffer  46  is applied to the gate of the LDO regulator series transistor  42  ( FIG. 2 ). Current sources I 1  and I 2  provide bias currents for the differential amplifier  54 . One technique for changing the bias current is to switch in and out the current source I 2  by means of a transistor switch  62 . By increasing the bias current for the differential amplifier and/or buffer, higher control currents can be applied to the various transistors in the LDO regulator to cause the LDO regulator to react more quickly to regulate the output voltage Vout and remain stable (avoid oscillation).  
         [0030]      FIG. 5  illustrates a technique for augmenting the series transistor  42  of  FIG. 2  with one or more additional series transistors  65  to increase the current handling capability of the LDO during a transition to quickly compensate for voltage glitches. It is desirable to have a small transistor  42  during low current modes (e.g., 50 mA) to minimize losses from controlling the transistor. However, to quickly correct large voltage glitches, a larger series transistor is needed. By temporarily coupling two or more additional transistors  65  in parallel with the series transistor  42  via a switch  66 , such extra current handling capability (e.g., 500 mA) is made available during the transition. When the switch  66  couples the gate of PMOS transistor  65  to the error signal, the transistors&#39;  42 / 65  conduction is controlled to quickly compensate for any voltage glitch. After the transition period, the gate of the transistor  65  is coupled to its source to turn it off.  
         [0031]      FIG. 6  is a flowchart of one embodiment of a technique to provide an improved transition from a high current mode to a low current mode, such as a standby mode. It is assumed that the PWM regulator has been operating normally and the LDO regulator has been disabled.  
         [0032]     In step  70 , a mode select signal is generated, such as a low signal for entering the low load current mode. The mode select signal may be generated externally such as by a microprocessor that generates a low signal after the powered equipment (e.g., a cell phone) is not used for a period of time. The mode select signal may also be generated by detecting the actual load current (e.g., by detecting the voltage across a series resistor) and comparing the load current to a threshold. When the load current goes below a threshold, the mode select signal will automatically go low. The threshold may have hysteresis to avoid oscillation between modes.  
         [0033]     In step  72 , a timer  76  issues a PWM-to-LDO transition signal to a transition logic circuit  78 . The timer  76  may be a charged capacitor that is discharged at a rate determined by a resistor. The discharging may be by actuation of a transistor switch that is turned on when the mode select signal changes state. The end of the timed period may be the time when a certain capacitor voltage threshold (detected by a comparator) is met. The transition logic circuit  78  may consist of simple circuitry that controls various switches in a particular sequence at particular intervals. Designing such circuitry is well within the skills of those of ordinary skill in the art.  
         [0034]     In step  74 , concurrently with step  72 , the LDO unit  12  is enabled by applying power to the various LDO regulator components, such as the error amplifier  44 , voltage reference source  45 , and buffer  46 . The LDO unit  12  starts up quickly (e.g., 2 micro seconds).  
         [0035]     In step  80 , the bias levels of all the relevant LDO unit circuits are raised to quicken the regulation response speed of the LDO unit  12 . For example, the transition logic circuit  78  closes switch  62  in  FIG. 4  and a switch in buffer  46  to increase the current bias. As an example, the Ibias in  FIG. 2  may be raised from 8 microamps to 30 microamps. Such an increase in the bias current allows the LDO unit to regulate higher load currents (e.g., max load current raised from 50 mA to 500 mA) without becoming unstable.  
         [0036]     In step  81 , preferably concurrently with step  80 , one or more additional transistors  65  are enabled (or switched in) to augment the series transistor  42  so that the LDO regulator can handle higher currents during the transition.  
         [0037]     In step  82 , which may be concurrent with step  80 , the reference voltage Vref for error amplifier  44  is increased by 2% (or other suitable amount) to cause the LDO unit  12  to immediately take over the voltage regulation from the PWM unit  10 . Increasing the reference voltage causes the LDO unit  12  to believe that the output voltage is too low. The LDO unit  12  regulates the output voltage by changing the conductance of the series transistor  42 .  
         [0038]     In step  84 , the PWM unit  12  is disabled by removing power from its various components (e.g., oscillator, buffers, error amplifier, logic, comparators, switching transistors, etc.).  
         [0039]     In step  86 , the timer  76  expires and issues a signal to the transition logic circuit  78 . The timer  76  may set a period on the order of 100 microseconds.  
         [0040]     In step  88 , transition logic circuit  78  resets the LDO reference voltage and bias levels to their nominal values and disables the additional series transistor(s)  65 . At this time, the LDO unit  12  uses very little power, due to the low bias currents, and regulates the output voltage for low current loads (e.g., 50 mA max).  
         [0041]      FIG. 7  is a flowchart of the transition technique when the regulator transitions from the LDO regulator mode to the PWM regulator mode.  
         [0042]     In step  90 , when the powered equipment is to come out of its standby mode, the mode select signal goes high.  
         [0043]     In step  92 , the timer  76  starts upon receiving the high mode select signal.  
         [0044]     In step  94 , the bias currents for the various LDO regulator circuits are increased (as before) to shorten the LDO regulator reaction time and allow the LDO regulator to handle the worst case anticipated voltage glitches during the transition and remain stable.  
         [0045]     In step  95 , preferably concurrently with step  94 , one or more additional transistors  65  are enabled (or switched in) to augment the series transistor  42  so that the LDO regulator can handle higher currents during the transition.  
         [0046]     In step  96 , the reference voltage for the PWM error amplifier  14  is increased by 2% (or other suitable value) to cause the PWM unit  10  to take over regulation from the LDO unit once the PWM unit  10  is enabled.  
         [0047]     In step  98 , the PWM unit  10  is enabled by applying power to the various PWM components. A typical PWM regulator begins regulating on the order of 60 microseconds after being powered up. Since the inductor  30  is completely deenergized at start up, a soft start routine is begun to limit the peak current through the power transistor  16 . A soft start routine ramps the duty cycle of the PWM unit  10  until the steady state duty cycle is reached. One simple type of soft start circuit is shown in  FIG. 8 . The PWM comparator  100  (within the PWM controller  15  in  FIG. 2 ) compares the error voltage to a sawtooth oscillator signal. The power transistor  16  stays on until the sawtooth level crosses the error voltage level. The output of the comparator  100  controls the gate drive logic  24  for turning off the power transistor  16  and turning on the synchronous rectifier  18 . The gate drive logic  24  is reset each oscillator cycle, which turns on the power transistor  16  and turns off the synchronous rectifier  18 .  
         [0048]     A soft start ramped signal is generated upon PWM unit start up, such as from a charging capacitor whose ramped voltage is determined by the size of the capacitor and its charging source. The ramped voltage controls a variable clamping circuit  104  to limit the error signal so that the error signal rises gradually. The clamping circuit  104  forces the duty cycle to increase slowly and linearly until there is no more clamping, at which time the soft start circuit has no further effect. There are various type of soft start circuits, and any of them may be used.  
         [0049]     During the soft start time, the LDO unit  12  is still regulating the output voltage. To prevent the synchronous rectifier  18  from staying on too long and drawing an undesirable reverse current through the inductor  30  during the soft start time (loading down the LDO regulator), a reverse current limiting circuit is employed (such as the zero crossing detector  35  in  FIG. 2 ) to force the synchronous rectifier  18  off during the remainder of the switching cycle.  
         [0050]     Referring back to  FIG. 7 , in step  110  the timer  76  expires.  
         [0051]     In step  112 , the transition logic circuit  78  controls various switches (e.g., switch  62  in  FIG. 4 ) to reset the LDO unit&#39;s bias currents, disable the additional series transistor(s)  65 , and disable the LDO unit  12  by removing power to its components.  
         [0052]     In step  114 , the transition logic circuit resets the reference voltage for the PWM error amplifier  14  to its nominal value. The dual mode regulator is now operating in its normal PWM regulator mode.  
         [0053]     The above-described circuitry is only one of many implementation of a dual mode regulator that can practice the invention. Although various circuits are shown directly coupled to other components, such circuits may be coupled to other components through other circuitry, such as resistors, transistors, buffers, diodes, transformers, capacitors, inductors, etc. Any component may be connected in parallel with a similar component for increased current handling. Such parallel components are still referred to herein as a single component.  
         [0054]     Having described the invention in detail, those skilled in the art will appreciate that given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.