Abstract:
In today&#39;s environment class-D amplifiers are used to provide an integrated solution for applications such as powered audio devices due to their advantages in power consumption and size over more traditional analog amplifiers. Due to power output requirements, the output stages of power drivers such as class-D amplifiers require a supply voltage in excess of the technologically allowed voltage for the switches in the output stage. A level shifter is used to ensure voltages supplied to the output switches do not exceed the technological limits. An ideal level shifter should provide the optimal voltage swing to output switches under all process, supply voltage and temperature (PVT) variations. The ideal level shifter should also provide fast transitions when the control signal changes from high to low and low to high.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Inventions 
     The invention relates to integrated power driver circuits and in particular to an improved level-shifter for driving the output stage of an integrated power driver. 
     2. Background Information 
     In today&#39;s environment, class-D amplifiers are used to provide an integrated solution for applications such as powering audio devices. Class-D amplifiers have an advantage in power consumption and size over more traditional analog amplifiers. Generally, they do not require bulky transformers or heat sinks making them more suitable for integrated circuits. 
     In particular, the class-D amplifier unlike a traditional amplifier produces an output comprising a sequence of pulses. Typically, these pulses vary in width or in density in methods known as pulse width modulation (PWM) or pulse density modulation (PDM). The average value of these pulses represents the instantaneous amplitude of the output signal. These pulses introduce unwanted high-frequency harmonics, which are typically removed by a low pass filter. 
       FIG. 1  is a block diagram illustrating the typical architecture of class-D amplifier  100 . The input signal is converted to pulses using modulator  102  which can be a pulse width modulator or a pulse density modulator. A common implementation of a pulse width modulator uses a high speed comparator to compare the input signal against a triangle wave. The modulated signal is then amplified by amplifier  104  and finally demodulated by low pass filter  106 . The demodulated signal can then be used, for example by speaker  108 . 
     Of particular interest in integrated power drivers such as Class-D amplifiers as described above and DC-DC converters are the output stages which typically use a complimentary MOS structure.  FIG. 2  is a circuit diagram exemplifying the output stage of a power driver. At the output is a p-type field effect transistor (PFET)  202 , also referred to as the high-side switch, and an n-type field effect transistor (NFET)  204 , also referred to as the low-side switch, in a complementary structure. Both field effect transistors operate as a switch. When high-side switch  202  is in the ON state and NFET  204  is in the OFF state, the output voltage is pulled up to the V DDH  level. When high-side switch  202  is in the OFF state and the NFET  204  is in the ON state, the output voltage is pulled down to the V SSH  level. As is typical with PFET switches, the PFET is in the ON state when the gate voltage is at least one PFET threshold below its source and is OFF when the gate-to-source voltage is zero. Similarly, typically with NFET switches, the NFET is in the ON state when the gate voltage is at least one NFET threshold above its source and is OFF when the gate-to-source voltage is zero. 
     The difficulty arises when the output requirements call for a higher voltage than is typically tolerated by the technology. For example, speaker drivers in PC audio applications must produce an average power of 2.5W on a 4-Ω speaker; therefore a 5V power supply is needed. However, most advanced integration technologies use 0.35 μm or smaller geometries that only produce 3.3V tolerant FET devices. Because the source of high-side switch  202  is tied to the power rail V DDH , the control signal supplied to the gate of high-side switch  202  can only be allowed to drop 3.3V below V DDH ; for the case where V DDH  is set to 5V, the gate cannot go below 1.7V. Similarly, the control signal supplied to the gate of low-side switch  204  can only be allowed to go 3.3V above V SSH ; for the case where V SSH  is set to ground (0V), the gate cannot go above 3.3V. 
     Therefore in order to operate this circuit in this type of environment, level shifter  206  is used to shift a modulated signal so that the swing between the power supply voltage V DDH  and the gate of high-side switch  202  never exceeds 3.3V. Additionally, level shifter  206  can also be used to supply the gate voltage to low-side switch  204  such that the swing between the gate and power supply voltage V SSH  never exceeds 3.3V. 
     The level-shift operation should be fast, in particular there should be no significant delays between the signals to high-side switch  202  and low-side switch  204 . Any delays between the signals can reduce the performance of the circuit; in class-D amplifiers a delay between the two signals in the output stage causes longer transition periods between the OFF and ON states of the output which produces a degradation in total harmonic distortion (THD). 
     Also, it is desirable to control the voltage levels at the gates of both high-side switch  202  and low-side switch  204  accurately. Ideally the gate-to-source voltage (V GS ) for high-side switch  202  in the ON state should be −3.3V and V GS  for low-side switch  204  in the ON state should be 3.3V under all process, voltage, and temperature (PVT) conditions. However, it should not exceed these limits. In particular, variations in supply voltages V DDH  and V SSH  can be induced by large amounts of current flowing from and into the load which can be problematic. In the case of a class-D amplifier the current can flow in either direction resulting in a significant increase or decrease of supply potentials. For example, if the V SSH  potential increases while low-side switch  204  is driven to 3.3V, its effective V GS  is reduced resulting in increased resistance in the ON state. The increased resistance can result in requiring a larger FET, an increase in power consumption, a decrease in performance or a combination of all three. If the V SSH  potential decreases the low-side switch  204  switch V GS  is increased and may exceed the technology limitations causing reliability issues. Similar considerations apply to high-side switch  202  and V DDH  variations. 
     It is important that level shifter  206  be able to maintain a controlled and constant output swing independent of the voltage of the power rails. This is especially challenging in the case of battery-operated systems where the positive power rail voltage can vary substantially depending on battery conditions. 
     In paper M. Berkhout “An Integrated 200-W Class-D Audio Amplifier”, IEEE JSSC, vol. 38, no. 7, July 2003, an external bootstrap capacitor and an externally-decoupled power rail are used to drive the two power FETS. The bootstrap capacitor must be large compared to the capacitive load given by the high-side FET device, therefore it cannot be integrated. This is undesirable because to include this solution as part of an integrated package, additional pins must be supplied in order to use an external capacitor. 
       FIG. 3  is a circuit diagram of a known level-shifter for driving the high-side switch. The level-shifter comprises PFET  302  and NFET  308  coupled by a cascode pair, PFET  304  and NFET  306 . The cascode pair is coupled to an intermediate voltage V INT , which is typically the average of supply voltages V DDH  and V SSH . The output of the level shifter swings between V DDH  and V INT +V thp  where V thp  is the threshold voltage for PFET  304 . As an example, if V DDH  is 5V, V SSH  is 0V, and V thp  is 0.8V, V INT  is equal to 2.5V and the high-side swing is only 1.7V, which falls short of the desired 3.3V. The suboptimal voltage swing as mentioned above leads to higher power consumption and lower performance by the output stage or requires a larger high-side switch. Even if a typical V INT  was chosen to bring the voltage swing closer to the net 3.3V desired, the falling edge of the control signal is slow because for the control signal to go from high to low requires current to flow through three series pull-down elements PFET  304 , NFET  306  and NFET  308 . Finally, the net voltage swing is highly dependent on the supply voltages. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies. 
     SUMMARY OF INVENTION 
     In today&#39;s environment class-D amplifiers are used to provide an integrated solution for applications such as powered audio devices due to their advantages in power consumption and size over more traditional analog amplifiers. Due to power output requirements, the output stages of power drivers such as class-D amplifiers require a supply voltage in excess of the technologically allowed voltage for the switches in the output stage. A level shifter is used to ensure voltages supplied to the output switches do not exceed the technological limits. An ideal level shifter should provide the optimal voltage swing to output switches under all process, supply voltage and temperature (PVT) variations. The ideal level shifter should also provide fast transitions when the control signal changes from high to low and low to high. 
     An exemplary level shifter can comprise a pull-up transistor such as a PFET, where an input signal can cause it to pull up a high-side output to the high supply rail, a pull-down transistor such as an NFET, where an input signal can cause it to pull down the low-side output to the low supply rail and a floating battery element which maintains the voltage between the high-side output and the low-side output in such a way the target swing voltage is maintained even with PVT variations. The floating battery element comprises a battery transistor, bias network, a high-side clamping transistor and a low-side clamping transistor. The battery transistor can be an NFET with a high aspect ratio relative to the pull-down NFET. The level shifter can be used inside the output stage of a class-D amplifier or power driver. 
     The level shifter can comprise a means for pulling up a high-side output to a high supply rail having a high supply voltage, a means for pulling down a low-side output to a low supply rail having a low supply voltage; and a means for maintaining a voltage between the high-side output and the low-side output to insure the target swing voltage is maintained even with PVT variations. The means for maintaining the voltage can comprise a battery transistor, a means for biasing the battery transistor, a means for clamping the high-side output to a high-side clamping voltage coupled to the battery transistor, and a means for clamping the low-side output to a low-side clamping voltage coupled to the battery transistor. 
     Finally, a level shifting method can comprise pulling up the high-side output to a high supply rail having a high supply voltage when the input is high, pulling down a low-side output to a low supply rail having a low supply voltage when the input is low, maintaining a voltage between the high-side output and the low-side output substantially equal to the high supply voltage minus the low supply voltage and a target swing voltage, with the voltage tracking changes in the high supply voltage and the low supply voltage. The method for maintaining a voltage comprises biasing a battery transistor, clamping the high-side output when the high-side output goes low, and clamping the low-side output when the low-side output goes high. 
     Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram illustrating the typical architecture of a class-D amplifier; 
         FIG. 2  is a circuit diagram the output stage of a power driver; 
         FIG. 3  is a circuit diagram of a known level-shifter for driving the high-side switch; 
         FIG. 4  is a circuit diagram of an embodiment of a level-shifter; 
         FIG. 5  is a circuit diagram of an embodiment of a floating battery element that can be used to dynamically adjust VBAT to match variations in the supply and ground voltages; and 
         FIG. 6  is a circuit diagram of an exemplary embodiment of a bias network. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure as defined by the appended claims. 
       FIG. 4  is a circuit diagram of an embodiment of a level-shifter. The level shifter comprises pull-up PFET  402 , pull-down NFET  406  and floating battery element  404 . PFET  402  when switched ON pulls V gp  up to the voltage supplied by the high supply rail which has a high supply voltage denoted by V DDH . NFET  406  when switched ON pulls V gn  down to the voltage supplied by the low supply rail which has a low supply voltage denoted by V SSH  and is sometimes set to the ground potential. Floating battery element  404  maintains its voltage V BAT  across its two terminals regardless of the current that is drawn through it. The advantage to this design is that the level-shifter is suitable to drive either high-side switch  202  or low-side switch  204  or both. Because there is only one pull-up transistor and one pull-down transistor, both transitions in the control signal, i.e., high-to-low and low-to-high are fast. The potential across floating battery element  404  can be chosen to maximize the swing across both high-side switch  202  and low-side switch  204 . In particular, the voltage V gp  supplied to high-side switch  202  swings between V DDH  and V SSH +V BAT  and the voltage V gn  supplied to low-side switch  204  swings between V SSH  and V DDH −V BAT , so a nominal choice of 1.7V for V BAT  optimizes the voltage swing across both switches to 3.3V for the case of V DDH =5.0V and V SSH =0.0V. 
     Referring still to  FIG. 4 , the level-shifter satisfies two of the objectives sought, that the net swing voltage be optimal and the transitions in the control signal be fast. The third objective, that the net swing voltages be optimal regardless of variations in the supply and ground voltages, can be met if V BAT  is dynamically adjusted as a function of the supply voltages V DDH  and V SSH  such that the output swing becomes independent of them. An optimal swing voltage allows for optimal sizing of the output stage switches; that is if the swing can be maximized within technological limits, the switches in the output stages can be made smaller. If V swing  is the optimal swing voltage, it is desirable to set V BAT  equal to V DDH −V swing −V SSH . With this setting for V BAT , V gp  swings between V DDH −V swing  and V DDH , and V gn  swings between V SSH  and V SSH +V swing . Thus the voltage swing on both outputs V gp  and V gn  is equal to V swing  independent of the potentials of V DDH  and V SSH . One of ordinary skill will recognize that though V BAT  should be set equal to V DDH −V swing −V SSH  some potential variation may exist so the V BAT  is set to a voltage substantially equal to V DDH −V swing −V SSH . 
       FIG. 5  is a circuit diagram of an embodiment of a floating battery element that can be used to dynamically adjust V BAT  to match variations in the supply voltages. The floating battery element  404  comprises “battery” NFET  502 , bias network  504 , high-side clamping NFET  506 , and low-side clamping PFET  508 . Bias network  504  provides a voltage of V BAT −V thn  between the drain and gate of NFET  502 , where V thn  is the threshold of NFET  502 . When NFET  502  has a drain-to-source voltage greater than V BAT  NFET  502  turns ON. When NFET  502  has a drain-to-source voltage less than V BAT  NFET  502  turns OFF. 
     High-side clamping NFET  506  is connected between V DDH  and V gp  and is designed to turn on when V gp  drops below the target voltage V DDH −V swing  and thus prevents V gp  from dropping further below the target voltage. This is achieved by setting V clamp-N =V DDH −V swing +V thn . Low-side clamping PFET  508  is connected between V SSH  and V gn , and is designed to turn on when V gn  rises above its target voltage V SSH +V swing , in effect preventing V gn  from further exceeding the target voltage. This is achieved by setting V clamp-P =V SSH +V swing −V thp . Again, in practicality the voltages described for V clamp-N  and V clamp-P  are set to voltages substantially equal to V DDH −V swing +V thn  and V SSH +V swing −V thp  respectively. 
     When the control signal received by NFET  406  and PFET  402  transitions from low to high, NFET  406  turns ON and at the same time PFET  402  turns OFF. NFET  406  pulls down the voltage V gn  towards V SSH . This causes the voltage across NFET  502  to exceed V BAT  so NFET  502  turns ON and pulls down the voltage V gp . If the size of NFET  502  is large relative to NFET  406  (referred to as the aspect ratio), the delay between the pull down of V gn  and V gp  is negligible. V gn  is pulled all the way down to V SSH  and V gp  keeps dropping until NFET  502  turns OFF, which occurs when V gp =V SSH +V BAT =V DDH −V swing . When NFET  502  is OFF, high-side clamping NFET  506  prevents V gp  from falling further. 
     Similarly, when the control signal received by NFET  406  and PFET  402  transitions from high to low, NFET  406  turns OFF and at the same time PFET  402  turns ON. PFET  402  pulls up the voltage V gp  towards V DDH . This causes the voltage across NFET  502  to exceed V BAT  so NFET  502  turns ON and pulls up the voltage V gn . If the aspect ratio of NFET  502  is large relative to PFET  402 , the delay between the pull up of V gp  and V gn  is negligible. V gp  is pulled all the way up to V DDH  and V gn  keeps rising until the NFET  502  turns OFF, which occurs when V gn =V DDH −V BAT =V SSH +V swing . When NFET  502  is OFF, low-side clamping PFET  508  prevents V gn  from rising further. Therefore if bias network  504  can maintain a V BAT  that adjusts with variations in supply or ground voltages, the level shifter can maintain an optimal swing voltage to high-side switch  202  and low-side switch  204 . 
       FIG. 6  is a circuit diagram of an exemplary embodiment of a bias network. The bias network comprises bias resistor  602 , NFET  606 , voltage source  604 , resistor  608  and capacitor  610 . The gate voltage for NFET  606  is V DDH −V swing , so the source voltage of NFET  606  is V DDH −V swing −V thn , leaving a net voltage across resistor  602  of V DDH −V swing −V thn −V SSH . Bias resistor  602  and resistor  608  have the same resistance R b  and since the same current I b  is drawn through both resistors, resistor  608  also has a net voltage of V DDH −V swing −V thn −V SSH  across it. This insures that NFET  502  only switches ON when the drain to source voltage exceeds V DDH −V swing −V SSH , the desired V BAT  voltage. Capacitor  610  is added to maintain a constant voltage on resistor  608  during signal transitions. 
     The level shifter described above provides optimal swing voltages for V gp  and V gn  under all PVT conditions as proven by circuit simulations. Empirical results illustrating the performance of this level shifter circuit were obtained using transistor level simulations with Spectre tools by Cadence. Variations in process, voltage and temperature were part of this simulation. 
     The level shifter is capable of tracking large variations on the V DDH  and the V SSH  supply voltages which makes it suitable for battery operated power amplifiers. Compared to previous systems, the increase in voltage swing allows a reduction in size of the power switches without affecting their on-state resistance. In addition, voltage supply  404  does not cause a significant increase in the rise and fall time of signals V gp  and V gn , making this level shifter ideal for use in high-performance class-D amplifiers. Furthermore, this level shifter does not require any external components, hence does not add to the system cost or pin count. 
     It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. For example, the bias network could be based on switched-capacitor techniques. Also, it should be noted that the output voltage of 5V and the CMOS technology voltage limit of 3.3V serves as an example and this level shifter is applicable to other voltages such as the 2.5V or 1.8V CMOS technologies. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.