Pulse-frequency mode DC-DC converter circuit

[0001] This application claims benefit of priority of U.S. provisional application Ser. No. 60/595,236 titled “Pulse-Frequency Mode DC-DC Converter Circuit”, filed Jun. 17, 2005.

[0002] 1. Field of the Invention

[0003] This invention relates to power converters in computer systems and digital systems, and more particularly to designing a DC-to-DC voltage converter.

[0004] 2. Description of the Related Art

[0005] DC-to-DC conversion is often performed by having switching power regulators, or step-down regulators, converting a higher voltage (e.g. 12V) to a lower value as required by one or more load devices. A common architecture features distribution of the higher voltage to multiple power regulators, each producing a different (or possibly the same) voltage to one or more loads. Switching power regulators often use two or more power transistors to convert energy at one voltage to another voltage. One common example of such a power regulator, commonly called a “Buck regulator”, implemented with MOS devices is shown in FIG. 1. Buck regulator 100 may be configured to operate in the PWM mode, switching a P-channel device 108 and an N-channel device 110 in order to produce a square-wave at their common node LX. The produced square-wave can be smoothed out using an LC circuit comprising inductor 112 and capacitor 114 to produce the desired voltage, Vout. A control loop, comprised of an error amplifier 116 and a Control Logic block 102 can be configured to control the duty-cycle of the output square-wave, thereby controlling switching P-channel device 108 and N-channel device 110, and hence the resulting value of Vout. In general, transistors 108 and 110 are controlled such that they do not conduct current at the same time. Typically, when transistor 108 is turned on (Vg_P is logic 0), transistor 110 is turned off (Vg_N is logic 0), and when transistor 108 is turned off (Vg_P is logic 1), transistor 100 is turned on (Vg_N is logic 1). I_L represents the load current flowing in inductor 112.

[0006] In addition to operating in PWM mode, Buck regulator 100, and other regulators, may also be configured to operate in PFM (pulse-frequency mode—also known as skip mode). One example of a synchronous buck regulator that can operate in both modes is the Micrel MIC2177. FIG. 2 shows a simplified functional diagram of the MIC2177 configured to operate in PFM. During PFM operation of regulator circuit 200 the output P-channel device 120 is turned on at a frequency and duty cycle that is a function of VIN, VOUT, and the value L1 of inductor 122. While in PFM, the N-channel device 121 coupled to P-channel device 120 is kept turned off to optimize efficiency by reducing gate charge dissipation. VOUT is regulated by skipping switching cycles that turn on P-channel device 120. Comparator 126 regulates VOUT by controlling when regulator circuit 200 skips cycles. It compares the voltage at node FB (VFB) to the reference voltage (VREF) and has 10 mV of hysteresis to prevent oscillations in the control loop. When VFB is less than VREF−5 mV, the output of comparator 126 is logic 1, allowing P-channel device 120 to turn on. Conversely, when VFB is greater than VREF+5 mV, P-channel device 120 is turned off. Although regulator circuit 200 operates in PFM, it requires external “catch” diode 124 for its operation, which increases the cost of building a regulator using regulator circuit 200.

[0007] Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.

[0008] In one set of embodiments, a switching power regulator for performing DC-to-DC (DC-DC) conversion may operate in pulse-frequency mode (PFM) based on peak current sense to regulate the load current with a variable frequency to achieve high efficiency at wide load variation without requiring an external diode component.

[0009] A PFM DC-DC voltage regulator may include an inductor and a load capacitor both coupled to a node configured as the regulator voltage output V_out. A voltage divider circuit comprising two resistors may be configured to provide an attenuated, or divided down, version of V_out at a second node (FB). The regulator may include two power transistors—a PMOS device and an NMOS device whose channels (drain terminals) may be coupled in series at a node (LX) between a supply voltage AVDD and a signal reference voltage AVSS—, an inverter configured to drive the gate of the PMOS device, an inverter configured to drive the gate of the NMOS device, and control circuitry for controlling the PMOS and NMOS devices. The control circuitry may include a zero-crossing comparator, a skip-mode comparator, a peak-current comparator, and additional logic circuitry comprising a one-shot pulse generator, a delay element, and respective SR latches for controlling the gate of each corresponding transistor.

[0010] In one set of embodiments, the skip-mode comparator may sense V_out and control the switching of the output power transistors to ramp the output voltage between a specified value V_out, ±5%. If the value of V_out falls below the ‘high’ threshold of the skip-mode comparator, the PMOS transistor may be turned on. The PMOS transistor may remain turned on until V_out reaches the ‘high’ threshold of the skip-mode comparator or until the peak load (or inductor) current exceeds a specified current level set for PFM. Once the PMOS transistor is turned off, an SR latch whose output is configured to partially control the gate of the NMOS device may be set until the inductor current ramps to zero. The zero-crossing comparator may be configured to sense what the polarity of the voltage at node LX is with respect to AVSS. When the voltage at node LX switches from a negative value to a positive value with respect to AVSS, the NMOS transistor may be turned off to prevent simultaneous reverse current flow in the inductor. When the value of V_out falls below the ‘high’ threshold of the skip-mode comparator, the PMOS transistor may be turned on again, and the entire switching cycle (PMOS/NMOS) may be repeated—to store charge on the capacitor—until V_out reaches the desired level corresponding to the ‘high’ threshold of the skip-mode comparator. At that point the NMOS device may be briefly turned on to ramp the current in the inductor to zero, following which the PMOS transistor and the NMOS transistor may both be turned off to enter an extremely low power mode.

[0011] Should the load current increase when, as a result of operating in PFM, V_out falls below the ‘low’ threshold of the skip mode comparator, the power regulator may automatically transition into fixed-frequency PWM mode. During PFM operation, the PMOS transistor may be turned on at a frequency and duty cycle (for PWM) that is a function of the power regulator input voltage, the power regulator output voltage, and the value of the inductor, and the NMOS transistor may be kept off to optimize efficiency by reducing gate charge dissipation. The output voltage of the power regulator may therefore be regulated by skipping switching cycles that turn on the PMOS transistor.

[0021] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).”

[0022] Referring again to FIG. 1, operation of a synchronous buck converter may be further examined. FIG. 3 shows voltage waveforms Vg_P 202, Vg_N 204, and Vx 206 (voltage at node LX), as well as inductor current I_L 208 and corresponding load current I_Load as observed during high current load operation of synchronous buck converter 100. When Vg_P 202 is logic low, transistor 108 is turned on. When Vg_N is logic high, transistor 110 is turned on. The deadtime delay t_dead indicates the time period when both transistors are turned off. As shown in FIG. 3, the average inductor current (i.e. the average of I_L) is essentially equal to the DC load current provided by converter 100, indicated in FIG. 3 as I_Load. FIG. 4 shows a timing diagram of the inductor current (and correspondingly the DC load current) observed during light current load operation of converter 100. As seen in FIG. 4, when I_Load is less than half the magnitude of the peak-to-peak ripple of current I_L, the entire I_L waveform 208 may be shifted down, shown as I_L waveform 308 in FIG. 4. It should be noted that in an asynchronous buck converter, during light current load operation, inductor current I_L may go to zero due to the effects of a the rectifier diode that may be coupled across the load, resulting in inductor 112 operating in discontinuous conduction mode (DCM). In converter 100, during light current load operation, inductor 112 may operate in a continuous conduction mode (CCM) by reversing the polarity of inductor current I_L, as illustrated in I_L waveform 308 in FIG. 4.

[0023] CCM operation at light current loads may be undesirable, as the reversal of the polarity of the inductor current I_L may contribute to conduction losses—because of the inductor current flowing from capacitor 114 to ground—without contributing to the output load current (I_Load). DCM in a synchronous regulator during low current load operation may be achieved by disabling N-channel device 110 at the zero-crossing point of the inductor current I_L. The resulting inductor current I_L waveform 408 is shown in FIG. 5. As a result of N-channel device 110 being turned off at the zero-crossing point, inductor 112 does not conduct reverse current, hence I_L 408 only flows in one direction. In other words, in DCM the inductor current I_L may remain positive. However, during low current load operation in DCM, the duty cycle of the inductor current I_L may be significantly lower than during CCM, which may affect the efficiency of power regulator 100.

[0024] In contrast to PWM mode, the PFM low power mode is based on the idea of reducing the switching frequency proportionally to the load current I_Load. By making the switching frequency proportional to the load current, high efficiency may be maintained over a wide range of load currents. FIG. 6 shows the PFM waveforms as relating, for example, to the operation of regulator circuit 200 shown in FIG. 2. Waveforms 500 and 502 indicate when P-channel device 120 and N-channel device 121 are turned on, respectively. Waveform 504 is the inductor current that may be conducted by inductor 122. If t_p and t_n represent the on times for P-channel device 120 and N-channel device 121, respectively, then:

[0025] $\begin{array}{cc}{t}_p=\frac{{\mathrm{LI}}_{\mathrm{peak}}}{V_{in}-V_{\mathrm{out}}},\mathrm{and}& \left(1\right)\\ t_n=\frac{{\mathrm{LI}}_{\mathrm{peak}}}{V_{\mathrm{out}}},& \left(2\right)\end{array}$
where L is the inductance of inductor 122, V_in is the input voltage to regulator circuit 200 (referring to FIG. 2), V_out is the output voltage of regulator circuit 200 (again, referring to FIG. 2), and I_peak is the peak value of current I_L conducted by inductor 122. Consequently, I_Load may be expressed as:

[0026] $\begin{array}{cc}{I}_{\mathrm{Load}}=\frac{1}{2}{I}_{\mathrm{peak}}\left(t_p+t_n\right)·f_s,& \left(3\right)\end{array}$
where f_s is the switching frequency defined as:

[0027] $\begin{array}{cc}{f}_s=\frac{1}{t_p+t_n}.& \left(4\right)\end{array}$
Combining equations 1-4, the switching frequency may be written as:

[0028] $\begin{array}{cc}{f}_s=\frac{2V_o{I}_{\mathrm{Load}}}{{\mathrm{LI}}_{\mathrm{peak}}^2}\left(1-\frac{V_o}{V_i}\right).& \left(5\right)\end{array}$
As equation 5 indicates, the switching frequency is proportional to the load current. The output voltage ripple in PFM may therefore be expressed as:

[0029] $\begin{array}{cc}\left(2\Delta V\right)\approx \frac{1}{2C}{I}_{\mathrm{peak}}\left(t_p+t_n\right)=\frac{{\mathrm{LI}}_{\mathrm{peak}}^2}{2C}·\frac{V_{in}}{V_{\mathrm{out}}\left(V_{\mathrm{out}}-V_{in}\right)},& \left(6\right)\end{array}$
indicating a higher voltage ripple than what may be observed in PWM mode.

[0030] Turning now to FIG. 7, the PFM waveforms as relating, for example, to the operation of regulator circuit 200 during maximum load current/frequency operation are shown. Waveforms 600 and 602 indicate when P-channel device 120 and N-channel device 121 are turned on, respectively. Waveform 604 is the inductor current that may be conducted by inductor 122. The maximum load current may be expressed as:

[0031] $\begin{array}{cc}{I}_{\mathrm{Load\_max},\mathrm{PFM}}=\frac{1}{2}{I}_{\mathrm{peak}},& \left(7\right)\end{array}$
with the corresponding switching frequency:

[0032] $\begin{array}{cc}{f}_s=\frac{2V_{\mathrm{out}}}{{\mathrm{LI}}_{\mathrm{peak}}}\left(1-\frac{V_{\mathrm{out}}}{V_{in}}\right).& \left(8\right)\end{array}$
The conduction losses in PMOS device 120, NMOS device 121 and inductor 122 during PFM operation may be expressed as:
P_Cond—PFM=R_on—pi_p—RMS²+R_on—ni_n—RMS²+R_LI_L—RMS²,  (9)
where R_on—p and R_on—n correspond to the on resistances of PMOS device 120 and NMOS device, respectively, i_p—RMS and i_n—RMS correspond to the RMS currents conducted by PMOS device 120 and NMOS device, respectively, R_L corresponds to the impedance of inductor 122, and I_L—RMS corresponds to the RMS current conducted by inductor 122. Equation 9 may be re-written in terms of V_in, V_out, and I_peak:

[0033] $\begin{array}{cc}{P}_{\mathrm{Cond\_PFM}}=\frac{{\mathrm{LI}}_{\mathrm{peak}}^3}{3}\left(\frac{R_{\mathrm{on\_p}}}{V_{\mathrm{in}}-V_{\mathrm{out}}}+\frac{R_{\mathrm{on\_n}}}{V_{\mathrm{out}}}+\frac{R_L{V}_{in}}{\left(V_{in}-V_{\mathrm{out}}\right)V_{\mathrm{out}}}\right)·f_s.& \left(10\right)\end{array}$
Combining equations 10 and 5, the conduction losses may be expressed as:

[0034] $\begin{array}{cc}{P}_{\mathrm{Cond},\mathrm{PFM}}=\frac{2}{3}{I}_{\mathrm{Load}}{I}_{\mathrm{peak}}\left[R_{\mathrm{on\_p}}·\frac{V_{\mathrm{out}}}{V_{in}}+R_{\mathrm{on\_n}}\left(1-\frac{V_{\mathrm{out}}}{V_{in}}\right)+R_L\right],& \left(11\right)\end{array}$
indicating that the conduction losses during PFM operation may be directly proportional to the load current.

[0035] FIG. 8 is a partial circuit diagram of a PFM DC-to-DC voltage regulator 800 configured according to one embodiment of the present invention. Regulator 800 may include an inductor 806 and a load capacitor 808 both coupled to node 860, which may be configured as the output of regulator 800, generating output voltage V_out. A voltage divider circuit comprising resistors 810 and 812 may be configured to provide an attenuated, or divided down, version of V_out at node 842. Regulator 800 may also include two power transistors, PMOS device 802 and NMOS device 804, whose channels (drain terminals) may be coupled in series between supply voltage AVDD and signal reference voltage AVSS (which may be ground in one set of embodiments), inverter 862 configured to drive the gate of PMOS device 802, inverter 864 configured to drive the gate of NMOS device 804, and control circuitry for controlling PMOS device 802 and NMOS device 804. The control circuitry for PMOS device 802 and NMOS device 804 may include comparator 818 (zero-crossing comparator), comparator 820 (skip-mode comparator), comparator 816 (I_peak comparator), and additional logic circuitry comprising one-shot pulse generator 826, delay element 828, and control latches 822 and 824, as well as logic gates 832, 830, and 834. Alternate embodiments using transistor devices other than PMOS device 802 and NMOS device 804 are possible and are contemplated.

[0036] According to one mode of operation of regulator 800, comparator 820 may sense V_out (via node 842), and control the switching of output devices 802 and 804 to ramp the output voltage between specified value V_out±a specified margin, which may be 5% in one set of embodiments. If V_out falls below the “high” threshold of comparator 820, PMOS device 802 may be turned on. PMOS device 802 may remain turned on until V_out reaches the “high” threshold of comparator 820 or until the peak current I_peak exceeds a specified current level set for PFM (I_PFM). In one set of embodiments, I_PFM may be set to equal I_Load+I_PK. Once PMOS device 802 is turned off, SR latch 824 may be set (turned on) until the inductor current (conducted by inductor 806) ramps to zero. Comparator 818 may be configured to sense the polarity of the voltage developed at node 866, in effect detecting an NMOS zero-current condition. When the voltage at node 866 switches from a negative value to a positive value, NMOS device 804 may be turned off to prevent simultaneous reverse current flow in inductor 806. When the value V_out falls below the “high” threshold of comparator 820, PMOS device 802 may be turned on again, and the entire switching cycle (PMOS/NMOS) may be repeated—to store charge on capacitor 808—until V_out reaches the desired “high” level corresponding to the “high” threshold of comparator 820. At that point NMOS device 804 may be briefly turned on to ramp the current in inductor 806 to zero, following which PMOS device 802 and NMOS device 804 may both be turned off to enter an extremely low power mode.

[0037] Should the load current increase when, as a result of operating in PFM, V_out falls below the “low” threshold of comparator 820—about 50 mV below the nominal voltage in one set of embodiments—regulator 800 may automatically transition into fixed-frequency PWM mode. It should be noted that additional circuit elements may be required for the fixed-frequency PWM operating mode of regulator 800, and that such additional components are not shown in FIG. 8 in order to better highlight the PFM operation of regulator 800. Such additional circuit components and their operation in the context of the PWM operating mode of power regulators is well known to those skilled in the art. As previously mentioned, during PFM operation PMOS device 802 may be turned on at a frequency and duty cycle (for PWM) that is a function of V_in, V_out, and the value of inductor 806, and NMOS device 804 may be kept off to optimize efficiency by reducing gate charge dissipation. V_out may therefore be regulated by skipping switching cycles that turn on PMOS device 802.

[0038] The PFM operation of regulator 800 according to one embodiment will now be discussed in more detail. Beginning with an initial state in which PMOS device 802 and NMOS device 804 are turned off, the voltage at node 866, which may be roughly equivalent to V_out, will be greater than zero, resulting in output 846 of comparator 818 being asserted (that is, being logic high). Consequently, capacitor 808 may discharge, and V_out may drop down from its nominal value. The divided voltage at node 842 may fall below the value of reference voltage 848, resulting in output 844 of comparator 820 being logic high. With outputs 844 and 846 both being logic high, the output of NAND gate 830 will be logic high, enabling one-shot pulse generator 826 to generate a trigger pulse at output 850, which may reset output Q1 of SR latch 822 to logic low, thereby turning on PMOS device 802 and causing current I_L in inductor 806 to increase linearly until it reaches a pre-set current limit (for example, I_PFM as previously shown, which may be 80 mA in one set of embodiments). Current-sense block 814—coupled to node 866—may operate to sense I_L, and generate a corresponding voltage value at output 854.

[0039] When current I_L in inductor 806 increases and reaches the pre-set current limit, output 840 of comparator 816 may assert in response to output 854 of current-sense block 814, setting output Q1 of SR latch 822 to logic high, thereby turning off PMOS device 802. There is a deadtime during which both PMOS device 802 and NMOS device 804 are turned off, forcing the body-diode of NMOS device 804 to turn on (forward-biased) and pick up current I_L from inductor 806. Accordingly, the voltage at node 866 may swing below ground (−0.6V in one set of embodiments), resulting in output 846 of comparator 818 going to logic low, providing a logic low input to input S2 of SR latch 824. The inverted output of SR latch 822 may also be coupled to delay element 828, which may operate to keep input S2 of SR latch 824 at logic high for a specified additional delay period, such that input S2 of SR latch 824 is still high for a brief period after the voltage at node 866 falls below ground, even though output Q1 of SR latch 822 is now at logic high. The high S2 input and low R2 input of SR latch 824 may combine to set output Q2 of SR latch 824 to logic high. With output Q1 of SR latch 822 and output Q2 of SR latch 824 both being logic high, NMOS device 804 may be turned on to ramp down inductor current I_L to zero. When I_L crosses zero, reversing its direction, the voltage at node 866 may switch from a negative value to a positive value, setting output 846 of comparator 818 to logic high, which in turn may reset output Q2 of SR latch 824 to logic low. In addition, when the voltage at node 842 is still lower than reference voltage 848, output signal 844 of comparator 820 will be at logic high, which, combined with input R2 of SR latch 824 going to logic high, will result in the output of AND gate 830 triggering one-shot pulse generator 826, thereby resetting output Q1 of SR latch 822 to logic low. This may result in PMOS device 802 being turned on, beginning another switching cycle.

[0040] When inductor current I_L reaches ground, comparator 818 may operate to turn off NMOS device 804. There may be some noise present at ground as the comparator switches NMOS device 804 on and off. SR latch 824 may be used to control the gate of NMOS device 804 to turn NMOS device 804 off. As the voltage at node 856 goes to logic high to turn off PMOS device 802, the output of delay element 828 may remain at logic high for a specified delay period before going to logic low. Once both inputs S2 and R2 of SR latch 824 at logic low, output Q2 of SR latch 824 may still hold its current value, leaving NMOS device turned on. If the voltage at node 842 is higher than reference voltage 848, (that is, capacitor 808 is fully charged), then PMOS reset 844 may be logic low, and when NMOS device 804 is turned off because input R2 of SR latch 824 is logic high, the combination of PMOS reset 844 being logic low and input R2 of SR latch 824 being logic high results in the input of one-shot pulse generator 826 remaining logic low and not resetting SR latch 822, thereby not turning on PMOS device 802. Therefore, when V_out is fully restored (that is, the voltage at node 842 is greater than reference voltage 848), PMOS device 802 and NMOS device 804 may both be turned off.

[0041] In general, comparator 820 may regulate V_out by controlling when regulator 800 skips cycles, comparing the voltage at node 842 to reference voltage 848, with built in hysteresis, which may be 20 mV in one set of embodiments, to prevent oscillation in the feedback control loop. When the voltage at node 842 is less than reference voltage 848 minus a preset value (10 mV in one set of embodiments), output 844 of comparator 820 (in essence, PMOS reset signal 844) may be at logic high, turning on PMOS device 802 as discussed above. Conversely, when the voltage at node 842 is greater than reference voltage 848, PMOS device 802 may be turned off.

[0042] The control of PMOS device 802 may be thus be summarized as follows. Through OR gate 832, PMOS device 802 may be turned off if either or both of two conditions are met. The first condition is met when the current (I_L) sensed by current-sense block 814 reaches the I_peak limit, and the second condition is met when the voltage at node 842 exceeds the value of reference voltage 848. Through AND gate 830, PMOS device 802 may be turned on if both of two conditions are met. The first condition is met when the voltage at node 842 falls below the value of reference voltage 848, and the second condition is met when the voltage at node 866 exceeds ground level.

[0043] Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.