Low ripple scalable DC-to-DC converter circuit

[0002] The invention relates to a method and circuit for providing a low ripple regulated output voltage of greater magnitude than an unregulated input voltage applied to the circuit.

[0003] FIG. 1A shows a conventional charge pump 1 in which an input voltage V_INis applied between an input terminal 2 and the ground conductor 3. During a first phase, a pair of switches 4 and 8 are closed to connect the upper terminal 5 and the lower terminal 9 of a pump capacitor C_PMPto V_INand to ground, respectively. A pair of switches 6 and 10 each have a terminal connected to terminal 5 and to terminal 9, respectively. Both of switches 6 and 10 are open during the first phase. The pump capacitor C_PMPtherefore is charged to V_INduring the first phase. During a second phase, switches 4 and 8 are open, and switches 6 and 10 are closed, as shown in FIG. 1B. Switch 6 connects the upper terminal 5 of C_PMPto output conductor 7, which is connected to the upper terminal of output capacitor C_OUT. Switch 10 connects the lower terminal 9 of C_PMPto V_IN. As the first and second phases are repeated, the output voltage V_OUTon conductor 7 increases to 2 V_IN. However, conventional charge pump 1 produces a large ripple voltage, typically of approximately 50 millivolts amplitude, in V_OUT. This is because pump capacitor C_PMPis repetitively connected and disconnected between V_INand V_OUT, causing a ripple voltage proportional to the ratio between C_PMPand C_OUT.

[0004] In some applications, a much lower level of ripple voltage amplitude may be required, for example, in a DC-to-DC converter used as a power supply for a precision voltage reference, a low-noise, low-offset operational amplifier, or any application sensitive to noise or perturbations on the power supply voltage. Reducing the ripple voltage by filtering may be unduly impractical and expensive because of the size and cost of the components needed to implement the filter. Some applications of low noise DC-to-DC converter circuits include their use in cell phones, PDAs (personal digital assistants), VCO (voltage controlled oscillator) and PLL (phase locked loop) power supplies, and smart card readers.

[0005] FIG. 2 shows a conventional linear voltage regulator 12 in which an operational amplifier 13 has its (−) input connected to a reference voltage V_REFand output connected to the gate of a P-channel pass transistor 14. The source of pass transistor 14 is connected by conductor 17 to an unregulated input voltage V_IN. The drain of pass transistor 14 is connected by conductor 15 to produce a regulated output voltage V_OUTon one terminal of an output capacitor C_OUThaving to the terminal connected to ground. A feedback circuit includes a resistor R_Fand a resistor R_Sconnected in series between V_OUTand ground. The junction between resistor R_Fand resistor RS is connected by conductor 16 to the (+) of operational amplifier 13. Regulator 12 always produces a value of V_OUThaving a lower magnitude than V_IN.

[0006] Linear Technology Corporation markets a linear regulator circuit, the LTC 1682, that utilizes a charge pump connected to supply the input voltage of a linear regulator circuit similar to the regulator shown in FIG. 2. Although this linear regulator circuit is referred to in its product specification sheet as a “low noise linear regulator”, it nevertheless has the shortcoming that it produces a large ripple voltage (i.e., a large noise voltage) superimposed on the regulated output voltage produced.

[0007] Thus, there is an unmet need for an improved charge pump circuit that is capable of boosting an input voltage so as to provide a boosted output voltage having a much lower ripple voltage than conventional charge pump circuits.

[0008] There also is an unmet need for an improved DC-to-DC converter circuit that is capable of boosting an input voltage to provide a boosted output voltage having a much lower ripple voltage than conventional DC-to-DC converter circuits.

[0009] Accordingly, it is an object of the invention to provide an improved charge pump circuit that is capable of boosting an input voltage so as to provide a boosted output voltage having a much lower ripple voltage than conventional charge pump circuits.

[0010] It is another object of the invention to provide an improved charge pump circuit that is capable of boosting an input voltage so as to provide a boosted output voltage having a low ripple voltage, which charge pump circuit is conveniently scalable to boost the input voltage to produce a higher amplitude, low ripple output voltage by using multiple pump capacitors.

[0011] It is another object of the invention to provide a DC-to-DC converter that is capable of boosting an input voltage so as to provide a boosted output voltage having a low ripple voltage, including a charge pump circuit that is conveniently scalable to boost the input voltage of the DC-to-DC converter to produce a higher amplitude, low ripple output voltage.

[0012] It is another object of the invention to provide an improved DC-to-DC converter circuit that is capable of boosting an input voltage so as to provide a boosted output voltage having a low ripple voltage, which DC-to-DC converter circuit is conveniently scalable to boost the input voltage to produce a higher amplitude, low-ripple output voltage by using an inductor and capacitor.

[0013] Briefly described, and in accordance with one embodiment thereof, the invention provides a DC-to-DC conversion circuit having an input conductor for receiving an input voltage (V_IN) and an output conductor (15) for conducting an output voltage (V_OUT), the DC-to-DC conversion circuit, including a pass transistor (14) having a first electrode coupled to receive the input voltage (V_IN) and a second electrode coupled to a first conductor (17). An amplifier circuit (13) has a first input coupled to receive a first reference voltage (V_REF) and an output coupled to a control electrode of the pass transistor. Level shifting circuitry (18) is coupled to the first conductor (17), the output conductor (15), the input conductor, and a conductor conducting a second reference voltage (GND), wherein the level shifting circuitry is capable of providing energy needed to boost the output voltage to a required level. A feedback circuit is coupled between the output conductor (15) and a second input of the amplifying circuit (13).

[0014] In one embodiment, the invention provides a circuit for boosting an input voltage (V_IN) to provide a low ripple output voltage (V_OUT) by regulating flow of current between a source of the input voltage (V_IN) and a circuit node (17) in response to a feedback signal (16) representative of the output voltage (V_OUT). A charge pump circuit operates to repetitively charge a pump element (C_PMPor L_PMP) to a voltage determined by the input voltage (V_IN) and redistribute charge between the pump element and a level-shifting capacitor (C_LS) coupled between the circuit node (17) and an output conductor (15) conducting the output voltage (V_OUT) so as to maintain the boosted output voltage (V_OUT).

[0015] In one embodiment a DC-to-DC conversion circuit having an input conductor receiving an input voltage (V_IN) and an output conductor (15) for conducting an output voltage (V_OUT), the DC-to-DC conversion circuit includes a pass transistor (14) having a first electrode coupled to receive the input voltage (V_IN) and the second electrode coupled to the circuit node (17). An amplifier circuit (13) driving the pass transistor has a first input coupled to receive a first reference voltage (V_REF) and second input receiving the feedback signal (16). A level-shifting isolation capacitor (C_LS) is coupled between the circuit node (17) and the output conductor (15). A charge pump circuit includes a first input terminal coupled to receive a second reference voltage (GND), a second input terminal coupled to receive the input voltage (V_IN), a first output terminal coupled to the circuit node (17), and a second output terminal coupled to the output conductor (15). The charge pump circuit includes a pump capacitor (C_PMP), a first switch (20) coupled between the circuit node (17) and a first terminal (21) of the pump capacitor, a second switch (22) coupled between the first terminal of the pump capacitor and the second reference voltage (GND), a third switch (23) coupled between the output conductor (15) and a second terminal (24) of the pump capacitor, and a fourth switch (25) coupled between the second terminal of the pump capacitor and the input voltage (V_IN). The feedback circuitry includes a first resistor coupled between the output conductor and the second input of the amplifier circuitry and a second resistor coupled between the second input of the amplifier circuitry and the second reference voltage (GND). The first switch (20) is a P-channel transistor having a source coupled to the circuit node (17) and a drain coupled to the first terminal of the pump capacitor, the second switch (22) is an N-channel transistor having a source coupled to the second reference voltage and a drain coupled to the first terminal of the pump capacitor, the third switch (23) is a P-channel transistor having a source coupled to the output conductor (15) and a drain coupled to the second terminal of the pump capacitor, and the fourth switch (25 is a P-channel transistor having a source coupled to the input voltage (V_IN) and a drain coupled to the second terminal of the pump capacitor. The capacitance of the level-shifting isolation capacitor (C_LS) is substantially greater than the capacitance of the pump capacitor, and wherein an output capacitance (C_OUT) is substantially greater than the capacitance of the level-shifting isolation capacitor.

[0016] In another embodiment, the charge pump circuitry includes a first pump capacitor (C_PMP1), a first switch (20) coupled between the first conductor (17) and a first terminal (21) of the first pump capacitor, a second switch (22) coupled between the first terminal of the first pump capacitor and the second reference voltage (GND), a third switch (27) coupled between a second terminal (24) of the first pump capacitor and a first terminal (31) of a second pump capacitor (C_PMP2), a fourth switch coupled between the second terminal (24) of the first pump capacitor and the input voltage (V_IN), a fifth switch (28) coupled between the first terminal (31) of the second pump capacitor (C_PMP2) and the second reference voltage (GND), between the output conductor (15) and a second terminal (24) of the pump capacitor, a sixth switch (30) coupled between a second terminal of the second pump capacitor (C_PMP2) and the input voltage (V_IN), and coupling circuitry coupled between the output conductor (15) and the second terminal of the second pump capacitor (C_PMP2). The described coupling circuitry includes a seventh switch (23) coupled between the output conductor (15) and the second terminal of the second pump capacitor (C_PMP2).

[0017] In another embodiment, an input voltage (V_IN) is boosted to provide a low ripple output voltage (V_OUT) by regulating flow of current between a source of the input voltage (V_IN) and a circuit node (17) in response to a feedback signal (16) representative of the output voltage (V_OUT), and operating DC-to-DC converter circuitry to repetitively charge a pump inductor (L_PMP) to a current supplied by a source of the input voltage (V_IN) and redistribute energy stored in the pump inductor between the pump inductor (L_PMP) and a level-shifting capacitor (C_LS) coupled between the circuit node (17) and an output conductor (15) conducting the output voltage (V_OUT) so as to maintain the output voltage (V_OUT) at a regulated level of substantially greater magnitude than the input voltage (V_IN).

[0018] In another embodiment, the invention provides a DC-to-DC conversion circuit having an input conductor for receiving a positive polarity input voltage (V_IN) and an output conductor (15) for conducting a negative polarity output voltage (V_OUT), the DC-to-DC conversion circuit including a pass transistor (14B) having a first electrode coupled to receive a first reference voltage (GND) and a second electrode coupled to a first conductor (17). An amplifier circuit (13) has a first input coupled to receive a first reference voltage (V_REF) and an output coupled to a control electrode of the pass transistor. Level shifting circuitry (18D) is coupled to the first conductor (17), the output conductor (15), the input conductor, and a conductor conducting the second reference voltage (GND), wherein the level shifting circuitry is capable of providing energy needed to boost the output voltage in the negative direction to a required level. A feedback circuit coupled between the output conductor (15) and a second input of the amplifying circuit (13).

[0019] FIG. 1A is a schematic diagram of a conventional charge pump showing its operation during a first phase.

[0020] FIG. 1B is a schematic diagram illustrating the charge pump of FIG. 1A during a second phase.

[0021] FIG. 2 is a basic schematic diagram of a conventional linear stepdown voltage regulator circuit.

[0022] FIG. 3 is a schematic diagram of a step up linear voltage regulator circuit of the present invention.

[0023] FIG. 4A is a schematic diagram of a first embodiment of a low ripple charge pump DC-to-DC converter of the present invention which, depending on the amount of feedback, can increase the output voltage to as much as double the input supply voltage.

[0024] FIG. 4B is a schematic diagram of a second embodiment of a low ripple charge pump DC-to-DC converter of the present invention which, depending on the amount of feedback, can increase the output voltage to as much as double the input supply voltage.

[0025] FIG. 5 is a schematic diagram of a low ripple charge pump DC-to-DC converter of the present invention which, depending on the amount of feedback, can increase the output voltage to as much as triple the input supply voltage.

[0026] FIG. 6 is a timing diagram showing the clock signals, the voltage of an internal circuit node, and the output voltage of the DC-to-DC converter of FIGS. 4A, 4B and 5.

[0027] FIG. 7 is a schematic diagram of a low ripple DC-to-DC converter of the present invention which uses an inductor and capacitor in a charge pump to create the level shift voltage required to increase the output voltage beyond the input voltage.

[0028] FIG. 8 is a schematic diagram of a low ripple DC-to-DC converter of the present invention which generates a negative output voltage from a positive input voltage.

[0029] Referring to FIG. 3, voltage regulator 120 is similar to the linear voltage regulator 12 of FIG. 2, except that the drain of pass transistor 14 is connected by conductor 17 to a (−) terminal of a voltage source 18 constituting a voltage level shift circuit 18 having its (+) terminal connected to output conductor 15 and producing a voltage level shift V_LSthat is added to the voltage V₁₇on a high impedance internal circuit node 17. This boosts the value of V_OUTthat would be produced by the conventional voltage regulator 12 of FIG. 2 by V_LSvolts. Pass transistor 14, in conjunction with the feedback from V_OUTprovided by means of amplifier 13 and the resistor voltage divider including resistors R_Fand R_S, regulates the flow of current between a source of V_INand circuit node 17. It should be noted that the level shift circuit 18 must be capable of storing the energy needed to boost the output voltage by V_LS. In the subsequently described embodiments, the level shift circuit 18 is a charge pump, but theoretically it could be a battery or other energy-storing voltage source.

[0030] Referring to FIG. 4A, an integrated circuit DC-to-DC converter 120A is provided by replacing the voltage level shift circuit 18 in FIG. 3 by a charge pump circuit 18A which includes level-shifting isolation capacitor C_LS. Charge pump circuit 18A includes a P-channel transistor 20 having its source connected to node 17 and its drain connected by conductor 21 to the (−) terminal of a charge pump capacitor C_PMP. Node 17 also is connected to the (−) terminal of a level shift capacitor C_LS, the (+) terminal of which is connected to output conductor 15. Conductor 21 also is connected to the drain of an N-channel transistor 22 having its source connected to ground. The source of a P-channel transistor 23 is connected to output conductor 15, and the drain of transistor 23 is connected by conductor 24 to the (+) terminal of C_PMPand to the drain of a P-channel transistor 25 having its source connected to V_IN. The gate of transistor 22 is connected to receive a clock signal Φ2. The gate of transistor 25 is connected to receive the complement of Φ2. The gates of transistors 20 and 23 are connected to receive the complement of a clock signal Φ1 which is out of phase with Φ2. Representative values of C_PMP, C_LSand C_OUTare 2.2, 2.2, and 2.2 picofarads, respectively. Ordinarily, these large capacitors would be discrete external components. (Those skilled in the art will recognize that a field effect transistor is bilateral in the sense that its source and drain are functionally interchangeable. Therefore, the arrows in the drawings indicating the source electrodes of the MOSFETs (metal oxide semiconductor field effect transistors) are intended to identify the electrodes, not to limit their functions to being source electrodes. Similarly, in the claims the terms “source” and “drain” may be used to identify a particular physical terminal of a field effect transistor but do not necessarily limit the terminal to functioning only as a source or only as a drain.)

[0031] In operation, transistors 22 and 25 are both turned on and transistors 20 and 23 are turned off during 42. This causes C_PMPto be charged to V_IN. Then during Φ transistors 20 and 23 are turned on and transistors 22 and 25 are turned off. This causes a transfer of charge between C_PMPand C_LSso as to tend to equalize the voltage across them. The repetitive cycles of Φ1 and Φ2 cause the voltage across C_LSto gradually increase to and be maintained at approximately +V_INvolts as charge pumped into C_LSis continually redistributed through output conductor 15 to C_OUTSo as to maintain V_OUTa level such that the voltage of the (+) input of operational amplifier 13 is kept equal to V_REFThus, the charge pump circuitry shown in FIG. 4A performs the function of repetitively charging pump capacitor C_PMPto a voltage equal to the input voltage V_INand redistributing charge between pump capacitor C_PMPand the level-shifting capacitor C_LScoupled between circuit node 17 and output conductor 15 so as to maintain V_OUTat a regulated level of substantially greater magnitude than V_IN. Since C_OUTis much larger than C_LS, the ripple in V_OUTis very small, typically less than 5 millivolts. In the embodiment of FIG. 4A, the high impedance of the drain electrode of pass transistor 14 allows it to function as a controlled current source so that the voltage V₁₇on the internal node 17 can swing up and down as needed to enable the operation of transistors of 20, 22, 23 and 25 and charge pump capacitor C_PMPto keep C_LScharged up to an average value of V_INvolts without causing a large ripple component in V_OUT.

[0032] FIG. 4B shows another DC-to-DC converter 120B which is essentially identical to the one shown in FIG. 4A except that P-channel pass transistor 14 in FIG. 4A has been replaced by the combination of N-channel pass transistor 14A, and a low noise charge pump 33 connected between V_INand the voltage supply terminal of amplifier 13. Low noise charge pump 33 can be a conventional circuit, such as the low noise charge pump shown in FIG. 4 of the product specification publication for the Texas Instruments REG 101 low-dropout regulator. The operation of DC-to-DC converter 120B is essentially the same as the operation of DC-to-DC converter 120A. The low noise charge pump 33 operates to control the gain of the N-channel pass transistor 14A, which is connected in a source follower configuration. Charge pump 33 produces a higher supply voltage than the input voltage V_INand allows the output of amplifier 13 to swing higher than V_IN. This allows amplifier 13 to control transistor 14A given when the voltage on conductor 17 approaches V_IN. Although node 17 in FIG. 4B has lower impedance than in FIG. 4A, the feedback path in FIG. 4B provides enough gain to cause the voltage V₁₇of node 17 in FIG. 4B to have a waveform similar to the corresponding waveform in FIG. 4A, so as to “absorb” the ripple caused by the charge pump operation of charge pump circuit 18A and thereby provide a very low ripple voltage component in V_OUTon output conductor 15.

[0033] The timing diagram of FIG. 6 shows the waveforms of the out-of-phase clock signals Φ1 and Φ2 and also shows the waveforms under steady-state conditions of the voltage V₁₇on node 17 due to the above described charge pumping of transistors 20, 22, 23 and 25 in FIGS. 4A and 4B in response to Φ1 and Φ2. The charge pumping causes V₁₇to have an amplitude of approximately 50 millivolts, which is large enough to “absorb” nearly all of the voltage generated across capacitor C_LSby the charge pumping, leaving a much lower amplitude “ripple” voltage component of only approximately 5 millivolts in V_OUT, as shown in the bottom waveform of FIG. 6.

[0034] FIG. 5 shows a DC-to-DC converter 120C which is a variation of the circuit of FIG. 4A, wherein the charge pump circuitry 18C keeps C_LScharged up to a voltage which is 2 times V_IN(rather than 1 times V_IN). Referring to FIG. 5, the feedback circuit, including resistors R_Fand R_S, operational amplifier 13, pass transistor 14, C_LS, and transistors 20 and 22, is the same as in FIG. 4. However, the (+) terminal of a first charge pump capacitor C_PMP1is connected by conductor 24 to the drain of a P-channel transistor 25 having its source connected to V_INand to the source of a P-channel transistor 27. The drain of transistor 27 is connected by conductor 31 to a drain of an N-channel transistor 28 having its source connected to ground. Conductor 31 also is connected to the (−) terminal of a second charge pump capacitor C_PMP2having its (+) terminal connected by conductor 29 to the drain of a P-channel transistor 23 and to the drain of a P-channel transistor 30. The source of transistor 23 is connected to output conductor 15, and the source of transistor 30 is connected to V_IN. The operation of the charge pump circuitry is similar to that in FIG. 4. During 42, transistors 22 and 25 are closed to charge C_PMPup to +V_INvolts, and similarly, transistors 28 and 30 are closed in order to charge C_PMP2up to +V_INvolts. During Φ1, transistors 20, 27, and 23 are closed to connect C_PMP1and C_PMP2in series across level shift capacitor C_LSto thereby charge its stored voltage to 2 times V_IN. This allows a larger value of the regulated output voltage V_OUTto be produced, with a very low magnitude ripple voltage thereon.

[0035] The above described waveforms in FIG. 6 also are applicable to the DC-to-DC converter 120C of FIG. 5.

[0036] FIG. 7 shows another DC-to-DC converter 120D which is the same as the one shown in FIG. 4A except that the charge pump circuitry 18A in FIG. 4A is replaced by the charge pump circuitry 18C shown in FIG. 7. In charge pump circuitry 18C, the (−) terminal of level shift capacitor C_LSis connected to node 17 and the (+) terminal of level shift capacitor C_LSis connected to output conductor 15, as in FIG. 4A. A steering diode 35 has its anode connected to node 17 and its cathode connected by conductor 36 to the drain of a P-channel transistor 37 and to one terminal of a “pump inductor” L_PMP. The source of transistor 37 is connected to V_IN, and its gate is connected to Φ1, which is the complement of Φ1. The other terminal of pump inductor L_PMPis connected by conductor 39 to the drain of an N-channel transistor 41 and to the anode of a steering diode 40. The cathode of steering to 40 is connected to output conductor 15. The source of transistor 41 is connected to ground, and its gate is connected to Φ1. In operation, during Φ1 transistors 37 and 41 are both on and steering diodes 35 and 40 are off, and the current flowing from V_INthrough transistor 37, pump inductor L_PMP, and transistor 41 to ground is charged up to a value I. When transistors 37 and 41 are turned off, the current I then flows through pass transistor 14 and steering diodes 35 and 40. The voltage across pump inductor L_PMPthen causes the voltage V_LSacross level shift capacitor C_LSto be charged toward the voltage across pump inductor L_PMPminus the forward voltage drops across steering diodes 35 and 40. As this process is repeated, V_OUTgradually reaches a low-ripple steady state value determined by V_REFand the values of the feedback circuit resistors R_Fand R_S. Ordinarily, pump inductor L_PMPand capacitors C_LSand C_OUTwould be external components.

[0037] FIG. 8 shows another DC-to-DC converter 120E that provides a negative value of V_OUTfrom a positive value of V_IN. The connections and operation of DC-to-DC converter 120E are similar to the connections and operation of the circuit shown in FIG. 4A. However, in FIG. 8, the output of amplifier 13 is connected to the gate of an N-channel pass transistor 14B having its source connected to ground rather than V_IN. The (+) terminal of level shifting capacitor C_LSis connected by conductor 17 to the drain of pass transistor 14B, and the (−) of level shifting capacitor C_LSis connected to V_OUT. Charge pump circuit 18D includes P-channel transistor 20 having its source connected by conductor 21 to V_INand its drain connected to both the (+) terminal of pump capacitor C_PMPand drain of N-channel transistor 22 period transistor 22 has its source connected to conductor 17 and its gate connected to Φ2. The gate of transistor 20 is connected to Φ1. The (−) terminal of C_PMPis connected by conductor 24 to the anode of diode 23A and the cathode of diode 25A. The cathode of diode 23Ace connected to ground, and the anode of diode 25A is connected by conductor 15 to V_OUT. Resistor R_Sis referenced to V_REF2rather than to V_REF.

[0038] When transistor 20 is on and transistor 22 is off, C_PMPis charged to V_INvolts minus the forward voltage drop of diode 23A. When transistor 22 is on and transistor 20 is off, the charge on C_PMPis redistributed with the charge on capacitor C_LS, which causes V_OUTto be of negative polarity, with amplitude determined by the values of R_F, R_S, V_REFand V_REF2.

[0039] By way of definition, the term “charge pump” as used herein is intended to include both capacitive charge pumps, for example as shown in FIGS. 4A and 5 and inductive charge pumps, for example as shown in FIG. 7.

[0040] While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make the various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. It is intended that all elements or steps which are insubstantially different or perform substantially the same function in substantially the same way to achieve the same result as what is claimed are within the scope of the invention.

[0041] For example, additional pump capacitors and associated charging/discharging switches could be coupled between the right terminal of the second pump capacitor C_PMP2shown in FIG. 5 and V_OUTto produce the level shift voltage as a higher integral multiple of V_INto maintain a higher average voltage across level shift capacitor C_LS. Other types of transistors than the disclosed MOSFETs could be utilized for the pass transistor and/or the switches in the charge pump circuitry. Also, the circuits disclosed could be easily modified to boost a negative polarity input voltage to a larger magnitude negative polarity output voltage. The pump capacitor could be replaced with an inductor or transformer or other device to transfer charge to the level shift capacitor. The level shift capacitor could be replaced with a photovoltaic diode or other device to create the level shift voltage.