Soft-start reference ramp and filter circuit

[0001] The present invention relates to voltage regulation, and, in particular, to a circuit that provides a controlled ramping and a filtering of a reference voltage.

[0002] The function of a voltage regulator is to provide a predetermined and substantially constant output voltage in response to an unregulated input voltage. Two types of voltage regulators are commonly used including linear regulators and switching regulators.

[0003] A linear regulator typically controls the output voltage by controlling a voltage drop across a power transistor, which is connected in series with a load. The power transistor is generally operated in its linear region and conducts current continuously.

[0004] A switching regulator typically controls the output voltage by using a power transistor as a switch to provide a pulsed flow of current to a network of inductive and capacitive energy storage elements, which smooth the switched current pulses into a continuous and regulated output voltage. The power transistor may be operated either in a cutoff or saturated state at a duty cycle as needed by the voltage differential between the input and output voltages. Varying the duty cycle or frequency varies the regulated output voltage of the switching regulator.

[0005] Generally, a duty cycle of a switching regulator may be controlled by monitoring the output voltage or current through a switch. The latter type of switching regulator is known as a current-mode switching regulator, and is easier to frequency stabilize and has better response to transients than does a switching regulator in which the duty cycle is controlled directly by the output voltage.

[0006] Switching regulators have at least two advantages over linear regulators. First, switching regulators typically operate with greater efficiency than linear regulators, a particularly important factor in high current regulators. Second, switching regulators are more versatile than linear regulators. Switching regulators can provide output voltages which are less than, greater than, or of opposite polarity to the input voltage, depending on the mode of operation of the switching regulator, whereas linear regulators can only provide output voltages which are less than the input voltage.

[0007] Switching regulators, although more flexible than linear regulators in circuit applications, are typically more complex than linear regulators. Frequency control and regulation in voltage control systems for sensitive applications may have less than desirable accuracy and reliability. Especially, regulation during initial power up has proven to be difficult to control, when power supply voltages have not yet stabilized under a sudden load.

[0008] Thus, it is with respect to these considerations and others that the present invention has been made.

[0016] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of an entirely hardware embodiment or an embodiment combining software and hardware aspects. The following detailed description is, therefore, not to be taken in a limiting sense.

[0017] Briefly stated, the present invention is related to a method and circuit for providing a controlled ramping and filtering to a reference voltage in a regulator circuit such that an inrush current is limited and noise coupling to an output voltage is reduced in a steady state.

[0018] In a voltage regulation circuit, a large inrush current may be employed, if a reference voltage V_ref, thereby, an output voltage V_out is to be established quickly. A soft-start and filter circuit may be inserted between a feedback amplifier and a charge transfer circuit of a voltage regulator. The soft-start and filter circuit may include a pair of transistors that are arranged to operate as a current mirror, a biasing current source that is configured to determine the current drawn from the current mirror, and a soft-start capacitor that is shunted by a third transistor during shutdown and charged by a constant charging current during a power-up. The soft-start capacitor may provide a conditioned reference voltage V_ref′. When the reference voltage V_ref rises rapidly during power-up, charging of the soft-start capacitor with the constant current provides a relatively softer slope to the conditioned reference voltage V_ref′. In steady state, the soft-start capacitor and a resistance of the transistor provide a first order low pass filter reducing noise that couples from a variety of source such as power source, reference source, and the like, to the reference voltage.

[0019] While a preferred embodiment of the present invention may be implemented in a switching voltage regulator circuit, the invention is not so limited. The described circuit may be employed as part of virtually any regulation circuit known to those skilled in the art.

[0020] FIG. 1 illustrates block diagram 100 of an embodiment of a voltage regulation circuit implementing the claimed invention. Block diagram 100 includes power source 102, voltage regulator 104 and load 106. In one embodiment, power source 102 may be a DC voltage supply such as a battery, and the like. The circuit may be terminated by capacitor C₀ and a load represented by resistor R_load.

[0021] Voltage regulator 104 is arranged to receive an input voltage V_in, and/or an input current I_in and provide an output voltage V_out and/or output current I_out to load resistor R_load. Voltage regulator 104 may include charge transfer circuit 108, feedback device 110, reference source 112, soft-start and filter circuit 114, and differential amplifier 116.

[0022] Charge transfer circuit 108 is arranged to provide a regulated output signal and may include a power pass transistor, a capacitive or inductive switching circuit, and the like, as mentioned above. Feedback device 110 may include a voltage divider in one embodiment. Feedback device 110 may be arranged to provide feedback voltage V_fb to an input of differential amplifier 116 for feedback to charge transfer circuit 108. V_fb may be a portion of output voltage V_out. In one embodiment, a gain of feedback device 110 may be expressed as B and have a value less than about one. Reference source 112 is arranged to provide a reference to charge transfer circuit 108. As described above, V_ref may be derived from V_in during an initial power up. Later, in the regulation state, V_ref may be derived from V_in, V_out, an independent source, and the like.

[0023] Soft-start and filter circuit 114 is arranged to provide V_ref′ from V_ref, and to control a ramp rate of V_ref′ for limiting an inrush current during start-up, and is further arranged to provide low pass filtering of the reference voltage to reduce noise coupling from a variety of sources. A construction and an operation of soft-start and filter circuit 114 is discussed below in conjunction with FIG. 3.

[0024] Voltage regulator 104 may start up from an initial inactive state and transition to an active state such as power-up, employment of an enable signal EN, and the like. Voltage regulator 104 then transitions to a steady state, and performs voltage regulation. A load for such a regulation circuit may be characterized as a combination of capacitance and resistance. During the initial unpowered state, the capacitance may be normally discharged. When the voltage regulator is powered up and in steady state, output voltage V_out may be a scaled value of a maximum reference voltage V_refmax. Reference voltage V_ref may typically be derived from an input voltage V_in during power up. In steady state, V_ref may reach V_refmax and be derived from V_in or V_out depending on a regulator design.

[0025] If reference voltage is to be established quickly during power up, a large amount of input current may be necessary. The start-up input (inrush) current may be expressed by:

[0026] $I_{\mathrm{IN\_startup}}{\mathrm{GI}}_{\mathrm{OUT\_startup}}={\mathrm{GC}}_o\frac{d{V}_o}{dt}={\mathrm{GC}}_o\frac{1}{B}\frac{d{V}_{\mathrm{ref}}}{dt},$
where G is a gain of the charge transfer circuit, C₀ is a load capacitance, and B is a gain of the feedback circuit that provides the combination of a portion of output voltage V_out and reference voltage V_ref to the charge transfer circuit.

[0027] Furthermore, noise caused by a switching circuit, power supply transients, load transients, and the like, may feed through to the reference voltage and couple via a feedback loop to output voltage V_out.

[0028] FIG. 1 shows a particular arrangement of inputs and outputs of the various components. In one embodiment, all of the components of voltage regulator 104 may be included in the same chip. Alternatively, one or more of the components of circuit 104 may be off-chip.

[0029] FIG. 2 illustrates an embodiment of subcircuit 200 of a voltage regulator such as voltage regulator 104 of FIG. 1 along with waveforms of voltages at each circuit block. subcircuit 200 includes reference source 212, soft-start and filter circuit 214, and differential amplifier 216. Represented waveforms include reference voltage 222 at an output of reference source 212, softened reference voltage 224 at soft-start and filter circuit 214, low-pass filter characteristic 226 of soft-start and filter circuit 214, and conditioned reference voltage 228 at an input of differential amplifier 216.

[0030] As shown in waveform 222, upon start-up the reference voltage V_ref may transition from about 0 V to V_refmax in a relatively short time that may be shorter than desired to sufficiently limit an inrush current. However, soft-start and filter circuit 214 may condition the reference voltage to generate a linear ramp with a substantially greater rise time T_ramp. In addition to the rise time, transients from a variety of sources such as power source, load, reference voltage source, and the like, may result in noise coupling to the reference voltage. A second function of soft-start and filter circuit 214 may include a low pass filter with a characteristic similar to that shown in filter characteristic 226. This in return may result in conditioned reference voltage V_ref′ having a shape similar to that shown in voltage representation 228 with a linear, soft rise time and reduced noise.

[0031] FIG. 3 schematically illustrates an embodiment of soft-start and filter circuit 314 that may be employed in a voltage regulation circuit. Soft-start and filter circuit 314 is arranged to receive reference voltage V_ref from a reference source such as reference source 112 of FIG. 1, and provide conditioned reference voltage V_ref′ as its output signal. Soft-start and filter circuit 314 may include transistors M331 and M332 that are arranged to operate as a current mirror. Junction areas of transistors M331 and M332 may have a predetermined ratio of

[0032] $\frac{W_1{L}_1}{W_2{L}_2}.$

[0033] Current source 330 may be coupled to a drain of transistor M331 and arranged to draw biasing current I_B. Transistor M333 may be coupled to a drain of transistor M332 and arranged to receive an enable signal EN at its gate. A soft-start capacitor C_SS may be coupled between a source and a drain of transistor M333. In one embodiment, transistors M331 and M332 may be PMOS type transistors, and M333 NMOS type transistor. Sources of transistors M331 and M332 are arranged to receive reference voltage V_ref.

[0034] When power is applied to the reference source, its output voltage V_ref may rise rapidly to V_refmax providing a supply voltage to soft-start and filter circuit 314. Transistor M333, which is arranged to discharge soft-start capacitor C_SS during shutdown, also ceases conduction when a circuit-wide enabling signal EN is applied along with V_ref. The biasing current I_B may be mirrored and scaled through transistors M331 and M332, and employed to charge soft-start capacitor C_SS. Charging C_SS with a constant current may result in a linearly ramped, conditioned reference voltage V_ref′, where a slope of V_ref′ may be expressed as:

[0035] $\frac{W_2{L}_1}{W_1{L}_2}\frac{I_B}{C_{\mathrm{SS}}}.$

[0036] V_ref′ may have this linear slope in the period between t=0 and t=t₁, where t₁ is a time point when V_ref′ reaches V_ref−(V_SG−V_tp). At about t₁, transistor M332 leaves a saturation region of operation and enters a linear region of operation, where M332 operates as a resistor with an approximate value:

[0037] $\frac{1}{K\frac{W_2}{L_2}\left(V_{\mathrm{SG}}-V_{\mathrm{tp}}\right)},$
where K is a transconductance parameter for M332.

[0038] Time point t₂ may be defined as the time point when V_ref′ exceeds 99.3% of V_ref. If time periods T₁ and T₂ are defined as T₁=t₁−0=t₁ and T₂=t₂−t₁, a total time T_ramp for V_ref′ to transition from about 0V to about V_refmax may be expressed as:

[0039] $T_{\mathrm{ramp}}\approx T_1+T_2=\frac{C_{\mathrm{SS}}\left(V_{\mathrm{ref}}-\left(V_{\mathrm{SG}}-V_{\mathrm{tp}}\right)\right)}{\frac{W_2{L}_1}{W_1{L}_2}\frac{I_B}{C_{\mathrm{SS}}}}+\frac{5C_{\mathrm{SS}}}{K_p\frac{W_2}{L_2}\left(V_{\mathrm{SG}}-V_{\mathrm{tp}}\right)}.$

[0040] Time points t₁, t₂, and time periods T₁, T₂ are illustrated in FIG. 4 and discussed below in conjunction with that figure. A maximum inrush current during a power-up of the regulator circuit may be directly proportional to the slope of reference voltage V_ref′. As described previously, the slope of V_ref′ in the linear region of operation (during T₁) may be expressed as:

[0041] $\frac{d{V}_{\mathrm{ref}}^{\prime }}{dt}=\frac{W_2{L}_1}{W_1{L}_2}\frac{I_B}{C_{\mathrm{SS}}}.$

[0042] Because, V_ref′ replaces V_ref as the regulator circuit's reference voltage at the input of a next stage differential amplifier, a maximum start-up input current may be expressed as:

[0043] $I_{I\mathrm{N\_startup}\mathrm{\_max}}={\mathrm{GC}}_o\frac{1}{B}\frac{d{V}_{\mathrm{ref}}^{\prime }}{dt}={\mathrm{GC}}_o\frac{1}{B}\frac{W_2{L}_1}{W_1{L}_2}\frac{I_B}{C_{\mathrm{SS}}},$
when there is no load coupled to the output (I_load=0).

[0044] Once V_ref′ reaches V_refmax, soft-start and filter circuit 314 may provide steady state voltage V_refmax to the next stage differential amplifier such as differential amplifier 116, and operate as a first order low pass filter. A resistance of transistor M332 in the linear region of operation coupled in series with soft-start capacitor C_SS may operate as the first order low pass filter with a roll-off frequency:

[0045] $F_{\mathrm{LPF}}=\frac{K\frac{W_2}{L_2}\left(V_{\mathrm{SG}}-V_{\mathrm{tp}}\right)}{2\pi C_{\mathrm{SS}}}.$

[0046] Thus, the roll-off frequency of the low-pass filter can be rewritten as

[0047] $\frac{K\frac{W_2}{L_2}{V}_{\mathrm{SAT}}}{2\pi C_{\mathrm{SS}}},$
where V_SAT is a saturation voltage of the second transistor.

[0048] FIG. 4 illustrates voltage diagram 400 comparing reference voltage V_ref with output voltage V_out and conditioned reference voltage V_ref′ of soft-start and filter circuit 314 of FIG. 3.

[0049] A vertical axis of voltage diagram 400 represents voltage (V). A horizontal axis represents time (t). Voltage diagram 400 illustrates three waveforms including waveform 442 representing reference voltage V_ref, waveform 444 representing conditioned reference voltage V_ref′, and waveform 446 representing output voltage V_out.

[0050] All three voltages begin at t=0. As described previously, reference voltage V_ref rises rapidly reaching steady state value V_refmax in a relatively short time that may be shorter than desired to sufficiently limit an inrush current. Waveform 444 representing conditioned reference voltage V_ref′, on the other hand, may rise with a substantially softer slope in a linear region of operation. The linear region of operation may be a time period between t=0 and t=t₁, when V_ref′ reaches V_ref−(V_SG−V_tp). A second region for curve 444 may be defined between t₁ and t₂, which represents a time point when V_ref′ exceeds 99.3% of V_ref.

[0051] Waveform 446 representing V_out may be derived from V_ref′ employing a linear relationship based, in part, on a gain of a differential amplifier and a feedback circuit of the voltage regulator. In one embodiment, the feedback circuit may have a gain of B. In this case, regulated output voltage V_out may approach

[0052] $\frac{V_{\mathrm{refmax}}}{B}$
in steady state.

[0053] FIG. 5 schematically illustrates another embodiment of soft-start and filter circuit 500 that may be employed in a voltage regulation circuit.

[0054] Soft-start and filter circuit 500 includes transistors M531 and M532 that are configured to operate as a current mirror, current source 530, transistor M533, soft-start capacitor C_SS, and filter resistor R_f. Soft-start and filter circuit 500 is arranged to operate essentially the same way as soft-start and filter circuit 314 of FIG. 3. Reference voltage V_ref is supplied to the circuit at sources of current mirror transistors M531 and M532. Conditioned reference voltage V_ref′ is provided at node N535 between R_f and a drain of transistor M533.

[0055] One difference between soft-start and filter circuit 314 and 500 is the addition of filter resistor R_f. R_f is configured to operate as an additional parameter in determining a roll-off frequency of the low-pass filter. Particularly, if M533 is selected to be a PMOS type transistor, a resistance of R_f may be desired for a reasonable frequency range. In one embodiment, the resistance of R_f may range from about 1 kiloohms to about 10 megaohms.

[0056] The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.