System and method for providing a highly efficient wide bandwidth power supply for a power amplifier

[0001] The present invention is generally directed to the manufacture of power supplies for semiconductor circuits and, in particular, to a system and method for providing a highly efficient wide bandwidth power supply for a power amplifier.

[0002] The telecommunications industry continually attempts to improve the transmitter circuitry in wireless communication systems. Power amplifier (PA) circuitry is a major component of a transmitter of a wireless communication device. Power amplifier (PA) circuitry provides the power for transmitting a signal (including data modulated and carried by the signal) so that a base station or a receiver can receive the signal.

[0003] Power amplifier (PA) circuitry uses a large amount of power. The power amplifier (PA) module is one of the most power consuming components of a wireless communication device. Therefore it is very desirable to provide power amplifier (PA) circuitry that is power efficient.

[0004] One method for improving power amplifier (PA) efficiency is to use a drain/collector modulation technique. In the drain/collector modulation technique a non-linear high efficiency power amplifier can be used (e.g., a class C power amplifier) instead of a linear low efficiency power amplifier (e.g., a class A amplifier). The power control of the power amplifier (PA) circuitry is achieved by adjusting the power amplifier (PA) power supply V_CC. A high efficiency power supply combined with a high efficiency power amplifier (PA) (with constant bias) would be ideal.

[0005] In prior art power amplifier (PA) modules in GSM (Global System for Mobile Communications) telecommunication devices such as RF3110 (manufactured by RFMD) and TQM7M4014 (manufactured by Triquint), the power amplifier (PA) power supply V_CC is from a linear regulator or “low-drop-out” (LDO) circuit. An LDO circuit can have a high efficiency when the value of its output voltage (V_CC) is near the value of its input voltage (V_BATT). But an LDO circuit will have a very low efficiency when its output voltage (V_CC) is very low compared with its input voltage (V_BATT).

[0006] The maximum efficiency for an LDO circuit is the ratio of the output voltage V_CC to the input voltage V_BATT. That is, the maximum efficiency is given by the ratio V_CC/V_BATT. For example, the maximum efficiency for an LDO in a typical GSM handset with an output voltage of nine tenths volts (V_CC=0.9 volts) and an input voltage of three and six tenths volts (V_BATT=3.6 volts) is twenty five percent (25%).

[0007] One method for increasing the efficiency of the power amplifier (PA) power supply V_CC is to use a switching converter. Presently existing switching converters, however, are designed to provide a constant output voltage. These converters are called “DC/DC converters” because they operate with direct current (DC) in and direct current (DC) out. DC/DC converters switch from a few hundred kilohertz (kHz) to a few megahertz (MHz) with a loop unit gain bandwidth having a range of approximately one hundred kilohertz (100 kHz).

[0008] On the other hand, GSM power amplifiers (PAs) require the supply voltage V_CC to be able to follow the input voltage ramp signal (V_ramp) with very high accuracy. In a GSM system, the V_ramp signal is required to slew from zero to its maximum value in ten microseconds (10 μs) to twenty microseconds (20 μs). This means that the supply voltage V_CC must be able to slew from zero to approximately three and seven tenths volts (3.7 V) in ten microseconds (10 μs) to twenty microseconds (20 μs) and follow the V_ramp signal in the close loop fashion with the power amplifier (PA) load. There are presently no switching converters available that can provide this level of performance.

[0009] Therefore, there is a need in the art for a system and method that is capable of providing a highly efficient wide bandwidth power supply for a power amplifier.

[0010] To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a system and method for providing an improved power supply control circuit that is capable of providing a highly efficient wide bandwidth power supply for a power amplifier.

[0011] One advantageous embodiment of the power supply control circuit of the invention comprises a low drop out (LDO) circuit and a switcher circuit that are coupled through an inductor. The power supply control circuit of the invention provides to a power amplifier a power control signal that has high efficiency and a wide bandwidth. The wide bandwidth is provided by the low drop out (LDO) circuit and the high efficiency is provided by the switcher circuit.

[0012] The switcher circuit comprises a switcher control circuit that receives an I_LDO current signal from an I_LDO current probe within the low drop out (LDO) circuit. The switcher control circuit also receives an I_SWITCHER current signal from an I_SWITCHER current probe within the switcher circuit. The switcher control circuit uses the values of the I_LDO current signal and the I_SWITCHER current signal to control an amount of current that is provided by the switcher circuit. The I_LDO current from the low drop out (LDO) circuit and the I_SWITCHER current from the switcher circuit are used to control a supply voltage V_CC that is provided to a power amplifier.

[0013] In one embodiment of the invention the functions of the switcher control circuit are implemented using a hysteretic current mode control technique. In another embodiment of the invention the functions of the switcher control circuit are implemented using a pulse width modulation (PWM) current mode control technique. In yet another embodiment of the invention the functions of the switcher control circuit are implemented using a technique that utilizes either a “constant on” time period or a “constant off” time period.

[0014] It is an object of the present invention to provide a system and method for providing an improved power supply control circuit that is capable of providing a highly efficient wide bandwidth power supply for a power amplifier.

[0015] It is also an object of the present invention to provide a system and method for providing an improved power supply control circuit that comprises a low drop out (LDO) circuit and a switcher circuit.

[0016] It is yet another object of the present invention to provide a system and method for an improved power supply control circuit that comprises a low drop out (LDO) circuit, a switcher circuit, and a switcher control circuit for controlling an output current of the switcher circuit.

[0017] It is another object of the present invention to provide a switcher control circuit in an improved power supply control circuit that comprises a low drop out (LDO) circuit and a switcher circuit in which the switcher control circuit operates using a hysteretic current mode control technique.

[0018] It is also another object of the present invention to provide a switcher control circuit in an improved power supply control circuit that comprises a low drop out (LDO) circuit and a switcher circuit in which the switcher control circuit operates using a pulse width modulation (PWM) current mode control technique.

[0019] It is yet another object of the present invention to provide a switcher control circuit in an improved power supply control circuit that comprises a low drop out (LDO) circuit and a switcher circuit in which the switcher control circuit operates using a technique that utilizes either a “constant on” time period or a “constant off” time period.

[0020] The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.

[0021] Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as future uses, of such defined words and phrases.

[0035] FIGS. 1 through 9 and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged power amplifier circuit.

[0036] To simplify the drawings the reference numerals from previous drawings will sometimes not be repeated for structures that have already been identified.

[0037] FIG. 1 illustrates a schematic diagram of a prior art power supply control circuit 100. Power supply control circuit 100 comprises a low drop out (LDO) circuit 110. Low drop out (LDO) circuit 110 comprises an operational amplifier 120 that receives a V_ramp signal on its inverting input. A feedback voltage signal V_FB is provided to the non-inverting input of operational amplifier 120. The operating voltage for low drop out (LDO) circuit 110 is provided by V_BATT.

[0038] The output of low drop out (LDO) circuit 110 is the power amplifier (PA) power supply voltage V_CC. Power supply voltage V_CC is provided to radio frequency (RF) power amplifier (PA) 130. Radio frequency (RF) power amplifier (PA) 130 amplifies an RF input signal (RF_IN) to generate an amplified RF output signal (RF_OUT).

[0039] FIG. 2 illustrates a schematic diagram of a first embodiment of a power supply control circuit 200 that comprises a low drop out circuit 210 and a switcher circuit 220 in accordance with the principles of the present invention. As will be more fully described, the switcher circuit 220 is controlled by a current I_LDO that is present in the low drop out circuit 210 and a current I_SWITCHER that is present in the switcher circuit 220.

[0040] Power supply control circuit 200 comprises a low drop out (LDO) circuit 210. Low drop out (LDO) circuit 210 comprises an operational amplifier 230 that receives a V_ramp signal on its inverting input. A feedback voltage signal V_FB is provided to the non-inverting input of operational amplifier 230. The operating voltage for low drop out (LDO) circuit 210 is provided by V_BATT.

[0041] The output of operational amplifier 230 is provided to the gate of a PMOS transistor M₁. The drain of PMOS transistor M₁ is coupled to the operating voltage V_BATT. The source of PMOS transistor M₁ is coupled to a first end of an I_LDO current probe 240. The I_LDO current probe 240 detects and measures the current that is present in the source of PMOS transistor M₁.

[0042] The second end of I_LDO current probe 240 is coupled to a first end of a resistor R₁. The second end of resistor R₁ is coupled to a first end of a resistor R₂. The second end of resistor R₂ is coupled to ground. The feedback voltage signal V_FB that is provided to the non-inverting input of operational amplifier 230 is taken from a point located between resistor R₁ and resistor R₂.

[0043] The output of low drop out (LDO) circuit 210 is the power amplifier (PA) power supply voltage V_CC. The power supply voltage V_CC is taken from a point located between the I_LDO current probe 240 and resistor R₁. Power supply voltage V_CC is provided to a radio frequency (RF) power amplifier (PA) (not shown in FIG. 2). Capacitor C_VCC is coupled in parallel with the series combination of resistor R₁ and resistor R₂.

[0044] Power supply control circuit 200 also comprises a switcher circuit 220. Switcher circuit 220 comprises a PMOS transistor M₂, an NMOS transistor M₃, an I_SWITCHER current probe 250, a gate driver with dead time control 260, and a switcher control circuit 270. As shown in FIG. 2, the switcher circuit 220 is inductively coupled to low drop out (LDO) circuit 210 through an inductor L. Inductor L is coupled to a point that is located between I_SWITCHER current probe 250 and NMOS transistor M₃.

[0045] The drain of PMOS transistor M₂ is coupled to the operating voltage V_BATT. The gate of PMOS transistor M₂ is coupled to a first output of gate driver with dead time control 260. The source of PMOS transistor M₂ is coupled to a first end of I_SWITCHER current probe 250. The I_SWITCHER current probe 250 detects and measures the current that is present in the source of PMOS transistor M₂.

[0046] The second end of I_SWITCHER current probe 250 is coupled to the drain of NMOS transistor M₃. The gate of PMOS transistor M₃ is coupled to a second output of gate driver with dead time control 260. The source of NMOS transistor M₃ is coupled to ground.

[0047] As shown in FIG. 2, an output of switcher control circuit 270 is provided to an input of gate driver with dead time control 260. Switcher control circuit 270 receives an I_LDO signal from the I_LDO current probe 240. Switcher control circuit 270 also receives an I_SWITCHER signal from the I_SWITCHER current probe 250. Switcher control circuit 270 uses the I_LDO signal and the I_SWITCHER signal to create a voltage control signal (designated V_CON—SW) for controlling the operation of gate driver with dead time control 260. Gate driver with dead time control 260 controls the operation of PMOS transistor M₂ and NMOS transistor M₃ by applying a control voltage signal to the gate of PMOS transistor M₂ and by applying a control voltage signal to the gate of NMOS transistor M₃.

[0048] The power supply circuit 200 that is shown in FIG. 2 provides a power control signal that has high efficiency and a wide bandwidth. The wide bandwidth is provided by the low drop out circuit 210 and the high efficiency is provided by the switcher circuit 220. As previously described, the operation of the switcher control circuit 270 is regulated by the I_LDO signal from I_LDO current probe 240 (i.e., the current present in the source of PMOS transistor M₁) and the I_SWITCHER signal from I_SWITCHER current probe 250 (i.e., the current present in the source of PMOS transistor M₂).

[0049] FIG. 3 illustrates a schematic diagram of a second embodiment of a power supply control circuit 300 comprising a low drop out circuit 210 and a switcher circuit 220 in accordance with the principles of the present invention. In power supply control circuit 300 the switch control unit utilizes hysteretic current mode control. Except for the differences described below, the operation of the low drop out circuit 210 and the switcher circuit 220 of power supply control circuit 300 is the same as the operation of the first embodiment shown in FIG. 2.

[0050] The switch control unit comprises a current comparator circuit 310. Current comparator circuit 310 receives two inputs. The first input is the I_LDO signal from I_LDO current probe 240 and the second input is a scaled I_SWITCHER signal. The scaled I_SWITCHER signal is the I_SWITCHER signal from I_SWITCHER current probe 250 that has been divided by a scale factor K. The scale factor K is provided to establish the ratio of I_SWITCHER signal and the I_LDO signal in the steady state so that the switcher circuit 220 will provide K times the current provided by the I_LDO current.

[0051] In this manner the majority of the current is provided by the switcher circuit 220 to achieve overall high efficiency. The output to track the V_ramp signal is provided by the low drop out (LDO) circuit 210. In the switcher circuit 220 of power supply control circuit 300 the PMOS transistor M₂ will turn “on” and the NMOS transistor M₃ Will turn “off” when the I_SWITCHER/K signal is less than the I_LDO signal. The PMOS transistor M₂ will turn “off” and the NMOS transistor M₃ Will turn “on” when the I_SWITCHER/K signal is greater than the I_LDO signal. The switching frequency of the current comparator circuit 310 is varying depending upon the current comparator hysteresis and the loop delay elements.

[0052] FIG. 4 illustrates a schematic diagram of a third embodiment of a power supply control circuit 400 comprising a low drop out circuit 210 and a switcher circuit 220 in accordance with the principles of the present invention. In power supply control circuit 400 the switch control unit utilizes pulse width modulation (PWM) current mode control. Except for the differences described below, the operation of the low drop out circuit 210 and the switcher circuit 220 of power supply control circuit 400 is the same as the operation of the first embodiment shown in FIG. 2.

[0053] The switching circuit 220 comprises a R-S flip flop circuit 410 and a current comparator circuit 420. The operation of the current comparator circuit 420 is the same as the operation that has been previously described for the current comparator circuit 310 of FIG. 3. The output of current comparator circuit 420 is provided to the reset (R) input of flip flop circuit 410. A clock signal is provided to the set (S) input of flip flop circuit 410. The switching frequency of switcher circuit 220 is set by the frequency of the clock signal.

[0054] In the switcher circuit 220 of power supply control circuit 400 the PMOS transistor M₂ is turned “on” and the NMOS transistor M₃ is turned “off” by the clock pulse. The PMOS transistor M₂ is turned “off” and the NMOS transistor M₃ is turned “on” when the I_SWITCHER/K signal is greater than the I_LDO signal.

[0055] The operation of power supply control circuit 400 may be modeled with the low drop out (LDO) circuit shown in FIG. 5A and the switcher circuitry shown in FIG. 5B. FIG. 5A and FIG. 5B are designed to be viewed together. The terminals designated A, B, and C in FIG. 5A connect to the respective terminals designated A, B, and C in FIG. 5B.

[0056] The low drop out (LDO) circuit in FIG. 5A comprises operational amplifier 510. The output of operational amplifier 510 provides the power amplifier (PA) power supply voltage V_CC to a radio frequency (RF) power amplifier (PA) (not shown in FIG. 5A).

[0057] The switcher circuitry in FIG. 5B comprises PMOS transistor M₂, NMOS transistor M₃, gate driver circuitry 520, driver timer 530, R-S flip flop circuit 540, and clock 550.

[0058] Implementation details of power supply control circuit 400 are shown in the low drop out (LDO) circuit shown in FIG. 6A and in the switcher circuitry shown in FIG. 6B. FIG. 6A and FIG. 6B are designed to be viewed together. The terminals designated A, B, C, D, E and F in FIG. 6A connect to the respective terminals designated A, B, C, D, E and F in FIG. 6B.

[0059] The low drop out (LDO) circuit in FIG. 6A comprises operational amplifier 610. The output of operational amplifier 610 provides the power amplifier (PA) power supply voltage V_CC to a radio frequency (RF) power amplifier (PA) (not shown in FIG. 6A). In this embodiment the low drop out (LDO) PMOS transistor M₁ is located at the output of the operational amplifier 610. The gate of PMOS transistor M1 is shown in FIG. 6A as “pgate” terminal 615. The low drop out (LDO) circuit in FIG. 6A also comprises a switcher tristate control unit 620.

[0060] The switcher circuitry in FIG. 6B comprises PMOS transistor M₂, NMOS transistor M₃, gate driver circuitry 630, driver timer 640, R-S flip flop circuit 650, clock 660, pulse width comparator unit 670, and comparator circuit 680.

[0061] The low drop out (LDO) current (I_LDO) is sensed by a current mirror that is formed by LDO PMOS transistor M₁ (within operational amplifier 610) and PMOS transistor M₆ (with the signal “pgate” at their respective gates). That is, the gate of PMOS transistor M₆ is coupled to the “pgate” terminal 615 that is connected to the gate of PMOS transistor M₁ within operational amplifier 610. The I_LDO current is mirrored by the current mirror formed by NMOS transistor MN₀ and NMOS transistor MN₁. The I_LDO current is converted to a voltage signal V_LDO by the switch resistance of PMOS transistor M₉. The voltage signal V_LDO is provided to an input of comparator circuit 680.

[0062] The voltage that is present at the switcher switching node “vswt” when the switcher PMOS transistor M2 is “on” (and after a certain blanking time has elapsed to prevent false triggering) is also provided to an input of comparator circuit 680. Comparator 680 compares the voltage signals in order to determine whether the I_SWITCHER/K current is greater than or less than the I_LDO current. The factor K is achieved by appropriately selecting the device size ratio of switcher PMOS transistor M₂ to PMOS transistor M₉, and the device size ratio of NMOS transistor MN₀ to NMOS transistor MN₁, and the device size ratio of LDO PMOS transistor M₁ to PMOS transistor M₆.

[0063] Another feature of the embodiment of the invention shown in FIG. 6A is the switcher tristate control unit 620. The output of switcher tristate control unit 620 is coupled to an input of the driver timer 640 (through terminal E). The switcher tristate control unit 620 operates when the LDO loop is open during the V_CC ramp down. During the V_CC ramp down the current in the LDO PMOS transistor M₁ quickly becomes zero. Then the switcher NMOS transistor M₃ will be “on” to ramp down the inductor current.

[0064] The inductor current would decrease more quickly if NMOS diode is “on” instead of the channel because of the forward diode voltage drop. The V_CC ramp down in this case is sensed by a slightly different tap from a V_CC feedback voltage network that indicates that V_CC is above the desired value.

[0065] For a case in which the V_CC ramp down is required to track V_ramp to a high degree of accuracy, the switcher tristate control unit 620 may not be sufficient. The embodiment of the invention illustrated in FIG. 7A and in FIG. 7B shows one exemplary method for controlling the V_CC ramp down actively with a class-AB amplifier. FIG. 7A and FIG. 7B are designed to be viewed together. The terminals designated A, B, C, D and F in FIG. 7A connect to the respective terminals designated A, B, C, D and F in FIG. 7B.

[0066] The low drop out (LDO) circuit in FIG. 7A comprises class AB operational amplifier 710. The output of operational amplifier 710 provides the power amplifier (PA) power supply voltage V_CC to a radio frequency (RF) power amplifier (PA) (not shown in FIG. 7A). In this embodiment the low drop out (LDO) PMOS transistor M₁ is located at the output of the operational amplifier 710. The gate of PMOS transistor M1 is shown in FIG. 7A as “pgate” terminal 715. In this embodiment a low drop out (LDO) NMOS transistor is also located at the output of the operational amplifier 710. The gate of the low drop out (LDO) NMOS transistor is shown in FIG. 7A as “ngate” terminal 718. The low drop out (LDO) circuit in FIG. 7A also comprises a switcher tristate control unit 720 and a R-S flip flop circuit 730.

[0067] The switcher circuitry in FIG. 7B comprises PMOS transistor M₂, NMOS transistor M₃, gate driver circuitry 740, driver timer 750, R-S flip flop circuit 760, clock 770, pulse width comparator unit 780, and comparator circuit 790.

[0068] The operation of the embodiment shown in FIG. 7A and in FIG. 7B is the same as the operation of the embodiment shown in FIG. 6A and in FIG. 6B except for the modifications described below. Unlike the switcher tristate control unit 620 of FIG. 6A, the output of switcher tristate control unit 720 is coupled to an S input of an R-S flip flop circuit 730. An enable signal is coupled to the R input of the flip flop circuit 730. The output of flip flop circuit 730 is coupled to an input of the class AB operational amplifier 710.

[0069] The LDO NMOS transistor that has its gate coupled to the “ngate” output terminal 718 of the class AB operational amplifier 710 will be active only when necessary to reduce the current consumption. During normal operation this NMOS transistor will be turned “off” by the signal “pd_nmos” from the R-S flip flop circuit 730 and the class AB operational amplifier 710 will operate as an LDO. The NMOS transistor that has its gate coupled to the “ngate” output terminal of the class AB operational amplifier 710 will be activated (i.e., turned “on”) if the V_CC voltage signal is not tracking the V_ramp voltage.

[0070] The embodiment shown in FIG. 7A and in FIG. 7B only demonstrates the case in which the value of voltage V_CC is too high. The circuitry of the present invention can be modified to handle the case in which the value of voltage V_CC is too low. For example, another tap can be added and multiplexed to the comparator circuit 790.

[0071] The PMOS transistor M₁ within the class AB operational amplifier 710 (that has its gate coupled to the “pgate” output terminal 715) could also be turned “off” in a similar way as the NMOS transistor that has its gate coupled to the “ngate” output terminal 718 of the class AB operational amplifier 710 during the V_CC ramp down after the NMOS transistor has been activated to sink the current. This approach will save unnecessary quiescent current.

[0072] Although the embodiment of the invention shown in FIG. 7A and in FIG. 7B is primarily intended for use with a GSM power amplifier (PA), it is understood that the various embodiments of the invention could also be applied to other applications that demand wide bandwidth or signal tracking (as long as the current in the LDO PMOS transistor M₁ is not zero to keep the LDO loop closed).

[0073] Another embodiment of the concept shown in FIG. 2 that combines an LDO and a switcher circuit involves using a switcher control circuit 270 that operates with a “constant on” time period or with a “constant off” time period. In the “constant on” time current control mode, the LDO PMOS transistor M₁ is turned on when the value of I_SWITCHER/K is less than the value of I_LDO and the LDO PMOS transistor M₁ is then kept “on” for a designated constant time period. After the designated constant time period has expired, the LDO PMOS transistor M₁ is kept “off” until the value of I_SWITCHER/K is less than the value I_LDO again.

[0074] In a similar fashion, in the “constant off” time current control mode, the LDO PMOS transistor M₁ is kept “on” until the value of I_SWITCHER/K is greater than the value of I_LDO. Then the LDO PMOS transistor M₁ is kept “off” for a designated constant time period. After the designated constant time period has expired, the LDO PMOS transistor M₁ is kept “on” until the value of I_SWITCHER/K is greater than the value I_LDO again.

[0075] FIG. 8 illustrates a graph showing waveforms of some of the signals that are present in the embodiment of the invention that is shown in FIG. 7A and in FIG. 7B. The top graph shows a waveform showing the value of the supply voltage V_CC over time. The middle graph shows a waveform showing the value of the V_ramp voltage over time. The lower graph shows a waveform showing the value of the I_SWITCHER current and the I_LDO current over time.

[0076] The V_ramp signal is ramped up and down in ten microseconds (10 μs) with steady state values of one and three tenths volts (1.3 V), five tenths of a volt (0.5 V) and one tenth of a volt (0.1 V) to represent the different power requirements. It can be seen from the graphs that the power supply VCC is able to tract the V_ramp signal very well.

[0077] An important feature that also may be seen from the graphs is that the I_SWITCHER current provides most of the current for the load. For the case of the light load condition (where V_ramp equals one tenth of a volt (0.1 V)) the power supply V_CC has a relatively larger ripple because the LDO is open loop once its PMOS transistor current reaches zero. In this case, the class AB amplifier approach illustrated in FIG. 7A and in FIG. 7B will do a better job.

[0078] The values of the V_ramp voltage signal and the load current (I_LOAD) are compared in Table One below.

[0079]

TABLE ONE
			LDO + Switcher
Vramp (Volts)	ILOAD (mA)	LDO efficiency	efficiency
1.3	2255	91%	93%
0.5	777	31%	74%
0.1	156	6%	38%

[0080] It may be seen from Table One that the combination of the LDO and the switcher circuit of the present invention has much better efficiency than that of the LDO alone.

[0081] FIG. 9 illustrates a flow chart showing the steps 900 of an advantageous embodiment of the method of the present invention. In the first step a low drop out (LDO) circuit 210 is provided that is capable of providing high bandwidth (step 910). Then a high efficiency switcher circuit 220 having a switcher control circuit 270 is coupled to the low drop out (LDO) circuit 210 (step 920). Then a value of I_LDO current from the low drop out (LDO) circuit 210 is provided to the switcher control circuit 270 (step 930). Then a value of I_SWITCHER current from the switcher circuit 220 is provided to the switcher control circuit 270 (step 940).

[0082] The value of the I_LDO current and the value of the I_SWITCHER current that are provided to the switcher control circuit 270 are the used to control a value of current that is provided by the switcher circuit 220 (step 950). Then the current from the low drop out (LDO) circuit 210 and the current from the switcher circuit 220 are then used to control a supply voltage V_CC to a power amplifier (step 960).

[0083] Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.