BACKGROUND
Voltage regulators are circuits designed to automatically maintain a constant output voltage. Some voltage regulators operate in a linear mode, and some operate in a switching mode (in which at least one element acts as an on/off switch). Typically, voltage regulators cannot respond instantaneously to a sudden change in load current. Output capacitors are commonly used to reduce ripple and to help reduce output voltage transients resulting from sudden changes in load current.
Many systems use voltage regulator output capacitors that have a ceramic dielectric. These types of capacitors, with ceramic layers and metal electrodes, are inherently piezoelectric. As a result, they may vibrate audibly if there are periodic voltage changes that are within the human audible frequency range (20 Hz-20 KHz). For example, liquid crystal displays used for computer monitors and televisions may be refreshed at 240 times per second, and 3D displays may be refreshed at twice that rate. Each refresh cycle may require a large change in power supply current, which may cause a voltage transient at the voltage regulator output capacitors. For voltage regulators driving displays or other systems with periodic load current changes, objectionable audible noise may be generated by the output capacitors. There is a general need for reducing voltage transients at the output of voltage regulators and in particular, a need for reducing audible noise from the output capacitors of voltage regulators.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of an example embodiment of a prior-art voltage regulator.
FIGS. 2A, 2B and 2C are timing diagrams illustrating current, voltage, and signal waveforms in the voltage regulator of FIG. 1.
FIG. 3 is a schematic block diagram of an example embodiment of an improved voltage regulator.
FIGS. 4A, 4B, 4C and 4D are timing diagrams illustrating signal and voltage waveforms in the voltage regulator of FIG. 3.
FIG. 5 is a schematic block diagram illustrating additional detail for an example embodiment of part of the voltage regulator of FIG. 3.
FIG. 6 is a schematic block diagram illustrating additional detail for an example embodiment of part of the voltage regulator of FIG. 3.
FIG. 7 is a flow chart of an example embodiment of a method for reducing periodic output voltage ripple in a voltage regulator.
DETAILED DESCRIPTION
FIG. 1 illustrates an example of a prior art voltage regulator 100 for providing power to a load 108. The load 108 is depicted as a resistor, but in general the load 108 may be a system with varying current demands. The illustrated voltage regulator 100 (simplified to facilitate illustration and explanation) includes a DC-DC converter 102 and feedback circuitry 104. The DC-DC converter 102 converts power at an input voltage VIN to power at an output voltage VOUT. The input voltage VIN may be greater than the output voltage VOUT or the output voltage VOUT may be greater than the input voltage VIN. The DC-DC converter 102 may be a linear circuit, or it may be a switching circuit. The feedback circuit 104 compares the output voltage VOUT to a reference voltage VREF and generates a feedback signal SFB that causes the DC-DC converter 102 to generate more or less current to maintain a constant output voltage VOUT. An output capacitor 106 is essential for switching DC-DC converters. For all types of DC-DC converters, the output capacitor 106 helps reduce ripple on the output voltage, and also helps reduce output voltage transients resulting from sudden changes in load current ILOAD.
FIGS. 2A-2C illustrate various example waveforms associated with the example voltage regulator 100 of FIG. 1. In the example of FIG. 2A, the current iLOAD to the system 108 varies periodically, suddenly increasing at times t1 and t3, and suddenly decreasing at times t2 and t4.
In FIG. 2B, the voltage regulator 100 of FIG. 1 cannot respond instantaneously to the load current changes depicted in FIG. 2A. When the load current iLOAD increases (t1, t3), the current provided by the DC-DC converter 102 (iOUT) is insufficient, and the output voltage VOUT starts to decrease as the output capacitor 106 starts to discharge. It takes a finite amount of time for the closed-loop voltage regulator system to respond and restore the output voltage VOUT back to the desired voltage, so negative voltage transients are generated on the output voltage VOUT at times t1 and t3. Likewise, when the load current iLOAD decreases (t2, t4), the DC-DC converter 102 provides more current (iOUT) than the load 110 requires, and the output voltage VOUT starts to increase as the capacitor 106 starts to charge. Again, it takes a finite amount of time for the closed-loop voltage regulator system to respond, and positive voltage transients are generated on the output voltage VOUT at times t2 and t4. If the voltage transients on VOUT are sufficiently large, and if the frequency of the voltage transients is within the human auditory band, the output capacitor 106 may generate objectionable auditory noise.
In FIG. 1, the feedback signal SFB controls the current output of the DC-DC converter 102. In FIG. 2C, at times t1 and t3, the closed-loop voltage regulator response time causes the feedback signal SFB to slowly increase to a higher level to increase iOUT (the current output of the DC-DC converter 102), and at times t2 and t4, the voltage regulator closed-loop response time causes the feedback signal SFB to slowly decrease to a lower level to decrease iOUT.
FIG. 3 illustrates an example embodiment of an improved voltage regulator 300, which is the voltage regulator 100 of FIG. 1 with additional feedforward circuitry added. Signal SLOAD is a signal from the system 108 being powered by the voltage regulator 300, indicating the current required by the system 108. More specifically, signal SLOAD indicates that the system 108 will require a change in current, thereby allowing the voltage regulator 300 to anticipate a sudden change in load current. The feedforward circuit 302 uses the signal SLOAD to generate a feedforward signal SFF that is coupled to the feedback signal SFB. The feedforward circuit 302 also receives the voltage regulator output voltage VOUT. The feedforward circuit 302 uses VOUT and the signal SLOAD to generate a feedforward voltage output VFF. The feedforward signal SFF has a variable amplitude, and the amplitude of the feedforward signal SFF is the amplitude of the feedforward voltage output VFF. In FIG. 3, the modified feedback signal SFB is the sum of the output of the feedback circuitry 104 (as illustrated in FIG. 2C) and the feedforward signal SFF. The signals SLOAD, SFF, the contribution of SFF to SFB, and the resulting modified SFB are illustrated in FIGS. 4A-4D.
Typically, a voltage regulator cannot anticipate that load current is about to change. The signal SLOAD and the feedforward circuit 302 enable the voltage regulator 300 to anticipate a current change slightly before there is sufficient current change to be sensed in a feedback loop, and slightly before there is any transient on the output voltage VOUT to be sensed. The feedforward signal SFF modifies the feedback signal SFB so that instead of a relatively slow closed-loop response as in FIG. 2C, the modified feedback signal SFB responds instantly (but briefly) to signal SLOAD. The feedforward Signal SFF adds a spike (a step in voltage followed by a decay) to the feedback signal SFB immediately after each step change in the signal SLOAD. In the example of FIG. 3, the feedforward signal SFF is capacitively coupled to the feedback signal SFB, but capacitive coupling is not required. The capacitive coupling is just one example of a way to provide a brief signal after each step in signal SLOAD. When the signal SLOAD is periodic, the feedforward circuit 302 can adjust the amplitude of the feedforward signal SFF (the amplitude of the feedforward signal SFF is the amplitude of the feedforward voltage output VFF) over time so that corresponding periodic voltage transients on the output voltage VOUT are essentially eliminated.
In FIG. 4A, the signal SLOAD is a signal from the system 108 being powered by the voltage regulator 300. The signal SLOAD is a signal corresponding to a changing system current, for example a signal in a controller that will change a state of the system 108. For example, SLOAD may be a display refresh signal or a signal for periodic pulse-width-modulated display back-lighting. In the example of FIG. 4A, when SLOAD is high, load current iLOAD is high, and when SLOAD is low, load current iLOAD is low.
In FIG. 4B, the feedforward signal SFF is a signal with the same timing as the load signal SLOAD but with a variable amplitude (the amplitude of the feedforward signal SFF is the amplitude of the feedforward voltage output VFF). For the example of FIG. 3, the modified feedback signal SFB is the sum of the output of the feedback circuitry 104 (as illustrated in FIG. 2C) and the capacitively coupled feedforward signal SFF.
In FIG. 4C, signal SFB(SFF) is the contribution of the feedforward signal SFF to the modified feedback signal SFB. SFB(SFF) adds a positive step to the feedback signal SFB at times t1 and t3, and each step decays as determined by a resistor 304 and capacitor 306. SFB(SFF) adds a negative step to the feedback signal SFB at times t2 and t4, and each step decays as determined by the resistor 304 and capacitor 306. Again, capacitive coupling is just one example of a way to implement a brief response to each step in VLOAD. The variable amplitude of the steps is the amplitude of the feedforward voltage output VFF from the feedforward circuit 302.
FIG. 4D illustrates the feedback signal SFB when the decaying steps from the feedforward signal SFF are added to the output of the feedback circuit 104 (as illustrated in FIG. 2C). The resulting feedback signal SFB has essentially instantaneous rise and fall times. The rapid rise and fall times of the feedback signal SFB cause the DC-DC converter 102 to respond immediately to load current changes at times t1, t2, t3, and t4, in contrast to the slower closed-loop response times illustrated in FIG. 2C. Effectively, the feedforward circuit 302 causes the DC-DC converter circuit 102 to respond to a load current change before there is any change on the output voltage VOUT, and the periodic voltage transients on VOUT resulting from the periodic load current changes are essentially eliminated. As a result of eliminating the periodic voltage transients on VOUT, audible noise from the output capacitor 106 is eliminated.
FIG. 5 illustrates additional detail for an example embodiment of the voltage regulator 300 of FIG. 3, using a switching DC-DC converter 502. In FIG. 5, the DC-DC converter 502 illustrates one possible example of the DC-DC converter 102 of FIG. 3. The DC-DC converter 502 is an example of a switching DC-DC boost converter (the output voltage VOUT is higher than the input voltage VIN). The DC-DC converter 502 comprises an inductor 504, an electronic switch (transistor) 506, and a diode 508. An output capacitor 510 is an essential energy storage component of the DC-DC conversion process, and output capacitor 510 helps reduce output voltage transients.
When the switch 506 is open, current flows from the input VIN and from stored energy in the inductor 504 into the load 512 and into the output capacitor 510. When the switch 506 is closed, energy is stored in the inductor 504, and current to the load 512 flows from stored energy in the output capacitor 510. The example voltage regulator 500 uses feedback from the output voltage VOUT to regulate the output voltage. An amplifier 514 compares the output voltage VOUT to a reference voltage VREF. A compensation network 516 ensures stability for the closed-loop voltage regulation system. Note that many commercially available voltage regulators have an external connection (called VCOMP) for a compensation network. Therefore, the circuits of FIG. 3 or 5 may be implemented as an external circuit for commercially available integrated circuits, or the circuits of FIG. 3 or 5 may be integrated. A pulse-width-modulation (PWM) circuit 518 controls the duty cycle of the switch 506 to maintain a constant output voltage. When the duty cycle increases (that is, when the switch 506 is closed for a longer portion of each switching cycle), then more current is provided to the load 512 during each switching cycle. When the duty cycle decreases, then less current is provided to the load 512 during each switching cycle. If the current required by the load changes, it takes a finite amount of time for the output voltage VOUT to change, and for the amplifier 514 to sense the output voltage change, and for the PWM circuit 518 to change the duty cycle of the switch 506 to restore the output voltage VOUT to the target voltage.
There are many variations in switching DC-DC converter designs. The diode 506 may be replaced by another electronic switch. For step-down DC-DC converters, the input voltage VIN is higher than the output voltage VOUT, and the switch(s) are between the input voltage and the inductor. The on-time or off-time of the switch(s) may be constant and the switching frequency may be varied. Both the duty cycle and the switching frequency of the switch(s) may be varied. There may be multiple feedback loops. The particular circuit of FIG. 5 is just one example of a voltage regulator using just one example of a switching DC-DC converter for purposes of illustration. The invention applies equally to voltage regulators using other types of DC-DC converters and the invention applies equally to linear voltage regulators.
FIG. 6 illustrates additional detail of an example embodiment of the feedforward circuit 302 illustrated in FIG. 3. First, note that at the output, the feedforward signal SFF, the feedforward voltage output VFF, the resistor 304, and the capacitor 306 are as illustrated in FIG. 3. In FIG. 6, an inverter 624 inverts the load signal SLOAD. A transistor switch 622, controlled by the signal inverted SLOAD, generates a step in the feedforward signal SFF at each transition of the signal inverted SLOAD, and each step in the feedforward signal SFF decays as determined by resistance 304 and capacitance 306. The magnitude of each step in the feedforward signal SFF is the magnitude of the feedforward voltage output VFF, as controlled by an amplifier 616. At the input stage, a capacitor 602 and a resistor 604 form a high-pass filter. The output of the high-pass filter is gated by a transistor 610. Transistor 610 grounds the output of the high-pass filter except for a brief time after a high-to-low transition of signal inverted SLOAD. That is, with a capacitor 606 and a resistor 608, a high-to-low transition of signal inverted SLOAD generates a spike (a step in voltage followed by a decay) that causes transistor 610 to momentarily turn off, which allows the output of the high-pass filter to pass to a resistor 612. The amplifier 616, a capacitor 614, and the resistor 612 form an integrator. In essence, the integrator integrates AC transitions of VOUT that occur just after each high-to-low transition of signal inverted SLOAD (FIG. 4A, t1, t3).The integrator adjusts the feedforward voltage output VFF to drive the AC transitions of VOUT (occurring just after each high-to-low transition of inverted SLOAD) to zero. Note that the circuit as illustrated adjusts the feedforward voltage output VFF only in response to high-to-low transitions of the signal inverted SLOAD, assuming that the voltage transients on VOUT (FIG. 2B) are symmetrical. If for some reason the rises and drops in signal inverted SLOAD are asymmetrical, then a second feedforward circuit as in the feedforward circuit 302 may be added for the low-to-high transitions. Amplifier 618 and diode 620 are optional depending on the types of amplifiers being used. In one specific embodiment, bipolar amplifiers are used, so amplifier 618 and diode 620 are needed to prevent negative values of V2.
Note that the circuit 302 has both feedforward and feedback characteristics. Typically, a feedforward control circuit controls an output independently from changes in the output. The circuit 302 has a feedforward characteristic in that it controls the output VOUT in response to an external signal SLOAD that is independent of changes in the output VOUT. The circuit 302 also includes a feedback characteristic in that changes in the output VOUT are used to adjust the amplitude of the feedforward signal SFF.
FIG. 7 illustrates an example method 700. At step 702, a voltage regulator receives a load signal from a load being powered by the voltage regulator. At step 704, the voltage regulator controls the output current of the voltage regulator in response to the output voltage of the voltage regulator and in response to the load signal.
Patent Application
20140292293
