CN111987907A - A switch mode power supply circuit - Google Patents

Abstract

The present application discloses a switch mode power supply circuit (500) comprising a feedback terminal (502B), a control circuit (512), a comparator (514) and a switch (516). The comparator (514) includes a first input terminal (514A) coupled to a reference voltage source (526) and an output terminal (514C) coupled to an input terminal (512D) of the control circuit (512). Switch (516) includes a first terminal (516A) coupled to feedback terminal (502B), a second terminal (516B) coupled to a second input terminal (514B) of comparator (514), and a control circuit (512) the third terminal (516C) of the output terminal (512C).

Description

A switch mode power supply circuit

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims U.S. Provisional Patent Application No. 62/852,436, entitled "A DC-DC Converter with Improved Noise Immunity," filed May 24, 2019 of priority, which is hereby incorporated by reference in its entirety.

Background technique

A switched-mode power supply is an electronic circuit that converts an input direct current (DC) supply voltage into one or more DC output voltages that are higher or lower in magnitude than the input DC supply voltage. A switched mode power supply that generates an output voltage lower than the input voltage is called a buck converter or a buck converter. Switch-mode power supplies that generate an output voltage higher than the input voltage are called boost converters or boost converters.

Some switch-mode power supply topologies include drive/power transistors coupled to an energy storage inductor/transformer at the switch node. By alternately opening and closing the switch according to the switching signal, electrical energy is delivered to the load through the energy storage inductor/transformer. The power delivered to the load is a function of the on/off duty cycle of the switch and the frequency of the switching signal. Switched mode power supplies are widely used in electronic devices, especially battery powered devices such as portable cellular phones, laptop computers, and other electronic systems that require efficient use of power.

SUMMARY OF THE INVENTION

Disclosed herein is a switch-mode power supply circuit that reduces the undesired effects of switching noise during circuit operation by blanking the feedback voltage during transistor switching. In one example, a switched mode power supply circuit includes a feedback terminal, a control circuit, a comparator, and a switch. The comparator includes a first input terminal coupled to the reference voltage source and an output terminal coupled to the control circuit. The switch includes a first terminal coupled to the feedback terminal, a second terminal coupled to the second input terminal of the comparator, and a third terminal coupled to the control circuit.

In another example, a switched mode power supply circuit includes a feedback terminal, a comparator, a switch, and a control circuit. The feedback terminal is configured to receive a feedback signal. The comparator is configured to compare the feedback signal with the reference signal. A switch is coupled to the feedback terminal and the comparator, and is configured to pass a feedback signal from the feedback terminal to the comparator. The control circuit is coupled to the comparator and is configured to generate a control signal that turns on the high-side transistor to open the switch before the edge of the control signal and close the switch after the edge of the control signal.

In another example, a switch-mode power supply includes an inductor, a high-side transistor, a low-side transistor, a voltage divider, a control circuit, a comparator, and a switch. A high-side transistor, a low-side transistor, and a voltage divider are coupled to the inductor. The control circuit includes a first output terminal coupled to the high-side transistor and a second output terminal coupled to the low-side transistor. The comparator includes a first input terminal coupled to the reference voltage source and an output terminal coupled to the input terminal of the control circuit. The switch includes a first terminal coupled to the voltage divider, a second terminal coupled to the second input terminal of the comparator, and a third terminal coupled to the third output terminal of the control circuit.

In yet another example, an electricity meter includes metering circuitry and a switch mode power supply. The switch-mode power supply is coupled to the metering circuitry and includes an inductor, a high-side transistor, a low-side transistor, a voltage divider, a control circuit, a comparator, and a switch. A high-side transistor, a low-side transistor, and a voltage divider are coupled to the inductor. The control circuit includes a first output terminal coupled to the high-side transistor and a second output terminal coupled to the low-side transistor. The comparator includes a first input terminal coupled to the reference voltage source and an output terminal coupled to the input terminal of the control circuit. The switch includes a first terminal coupled to the voltage divider, a second terminal coupled to the second input terminal of the comparator, and a third terminal coupled to the third output terminal of the control circuit.

Description of drawings

For a detailed description of various examples, reference will now be made to the accompanying drawings, in which:

1 shows an isometric perspective view of an integrated circuit package suitable for use in a switched mode power supply in accordance with the present specification;

Figure 2 illustrates output switching coupled to a feedback voltage in a switched mode power supply circuit;

Figure 3 illustrates subharmonic oscillations caused by output switching coupled to a feedback voltage in a switched mode power supply circuit;

4 illustrates an exemplary low-pass filter applied to attenuate the effects of output switching coupled to a feedback voltage in a switched-mode power supply circuit;

5 illustrates a block diagram of an exemplary switch-mode power supply including blanking to eliminate coupling of output switching on the feedback voltage in accordance with the present specification;

FIG. 6 shows an example of blanking applied in the switch mode power supply of FIG. 5 when the switching signal is active longer than the blanking time;

FIG. 7 shows an example of blanking applied in the switch mode power supply of FIG. 5 when the switching signal active time is shorter than the blanking time;

FIG. 8 shows an example of blanking applied in the switch mode power supply of FIG. 5 when the switching signal inactive time is shorter than the blanking time;

FIG. 9 illustrates exemplary signals generated in the switch mode power supply of FIG. 5; and

10 shows a block diagram of an exemplary electricity meter including a switch mode power supply in accordance with the present specification.

Detailed ways

A switch-mode power supply operating in a buck mode includes a high-side transistor and a low-side transistor coupled in series between a voltage input node and a ground node and joined at the switch node. An output circuit is coupled to the switch node for generating an output voltage based on the switching signal generated at the switch node. The switch mode power supply also includes a feedback node configured to receive a feedback signal based on the output voltage. During operation, the control circuit of the switch mode power supply generates a control signal to mutually turn on and off the high-side transistor and the low-side transistor, the control signal generating a switching signal at the switch node. A ripple voltage generated based on the switching signal at the switch node is coupled to the feedback signal. The switching signal is generated based on the difference between the reference voltage and the feedback signal in combination with the ripple voltage.

As integrated circuits and integrated circuit packages get smaller, the pitch between pins shrinks, which increases the parasitic capacitance between the pins. FIG. 1 shows an isometric perspective view of an integrated circuit package 100 suitable for use in a switched mode power supply in accordance with the present specification. The integrated circuit package 100 includes pins 104 for coupling the high-side and low-side transistors to the inductor and pins 106 for receiving a feedback voltage. Parasitic capacitance 102 is shown between pin 104 and pin 106 . As package size decreases, parasitic capacitance 102 increases and the coupling between the switching signal provided on pin 104 and the feedback signal received at pin 106 increases.

FIG. 2 shows the coupling of the switching signal to the feedback voltage in the integrated circuit package 100 . The switching signal (not shown) switches at a frequency of about 500 kHz. Switching produces approximately 250 millivolts of noise on the feedback signal 202 at the edges of the switching signal. Switching noise coupled to the feedback signal 202 produces an unexpected voltage ramp at pin 106, which causes sub-harmonic oscillations and shifts in the output voltage. Figure 3 shows subharmonic oscillations caused by output switching coupled to a feedback voltage in a switched mode power supply circuit. In Figure 3, current 302 is the current flowing in the inductor of the switch mode power supply. Switching signal 304 is provided at pin 104 of integrated circuit package 100, which is coupled to the inductor. The feedback signal 306 is proportional to the output voltage of the switched mode power supply and includes switching noise at the edges of the switching signal 304 . Based on the feedback signal 306 after the low pass filter, an internal feedback signal 308 is generated at the internal feedback node of the circuitry provided in the integrated circuit package 100 . An exemplary low pass filter may include a 40 kohm resistor coupled to a 2 picofarad capacitor. Even with low pass filtering, the internal feedback signal 308 is disturbed by switching noise. Switching noise in the internal feedback signal 308 causes subharmonic oscillations in the switch mode power supply.

Some switch-mode power supplies attempt to reduce the effects of cross-coupled switching noise by lowering the corner frequency of the low-pass filter. Figure 4 shows that a low pass filter is applied to attenuate the effects of output switching coupled to the feedback voltage in a switched mode power supply circuit. However, lowering the corner frequency of the low-pass filter reduces system bandwidth and transient performance, which is unacceptable in some applications.

5 shows a block diagram of an exemplary switch mode power supply 500 that includes blanking to eliminate output switching coupled to the feedback voltage. Switch mode power supply 500 includes switch mode power supply circuit 502 , inductor 504 and voltage divider 506 . Switch terminal 502A is coupled to inductor 504 . Inductor 504 is coupled to voltage divider 506 to divide the output voltage of switched mode power supply 500 by a predetermined divisor to generate feedback signal 544 . Voltage divider 506 is coupled to feedback terminal 502B to provide output voltage feedback to switch mode power supply circuit 502 . Switched mode power supply circuit 502 may be implemented in integrated circuit package 100 or a similar package.

Switched mode power supply circuit 502 includes high side transistor 508 , low side transistor 510 , control circuit 512 , comparator 514 , switch 516 , capacitor 518 , capacitor 520 , resistor 522 , resistor 524 , voltage reference circuit 526 and clock generator 528 . Clock generator 528 is coupled to control circuit 512 and generates a clock signal. Activation of the clock signal causes control circuit 512 to deactivate control signal 538 and turn off low-side transistor 510 and activate control signal 540 and turn on high-side transistor 508 . Comparator 514 is also coupled to control circuit 512 . Comparator 514 compares the feedback from voltage divider 506 to a reference voltage generated by voltage reference circuit 526 . Activation of comparator output signal 530 causes control circuit 512 to deactivate control signal 540 and turn off high-side transistor 508 and activate control signal 538 and turn on low-side transistor 510 .

The control circuit 512 includes an output terminal 512A and an output terminal 512B. Output terminal 512A is coupled to gate terminal 508G of high-side transistor 508 to turn high-side transistor 508 on and off via control signal 540 . Output terminal 512B is coupled to gate terminal 510G of low-side transistor 510 to turn low-side transistor 510 on and off via control signal 538 . Drain terminal 508D of high-side transistor 508 is coupled to power rail 532, and source terminal 508S of high-side transistor 508 is coupled to switching terminal 502A to provide charging current to inductor 504 when high-side transistor 508 is on. The drain terminal 510D of the low-side transistor 510 is coupled to the switching terminal 502A, and the source terminal 510S of the low-side transistor 510 is coupled to a common voltage source 534 (such as ground) to discharge the inductor 504 when the low-side transistor 510 conducts .

Comparator 514 includes an output terminal 514C coupled to input terminal 512D of control circuit 512 for providing the result of comparing reference signal 552 with feedback signal 544 . Comparator 514 also includes an input terminal 514A coupled to voltage reference circuit 526 via resistor 522 to receive reference signal 552 for comparison with feedback signal 544 . Resistor 522 includes a terminal 522A coupled to voltage reference circuit 526 and a terminal 522B coupled to input terminal 514A of comparator 514 for providing reference signal 552 (derived from the output of voltage reference circuit 526 to comparator 514). Resistor 524 includes a terminal 524A coupled to input terminal 514A of comparator 514 and a terminal 524B coupled to a reference voltage source, such as ground for working with resistor 522 to set the voltage of reference signal 522 . Input terminal 514B of comparator 514 is coupled to voltage divider 506 via switch 516 . Input terminal 514B is coupled to terminal 516B of switch 516 for receiving feedback signal 544 when switch 516 is closed. Terminal 516A of switch 516 is coupled to voltage divider 506 via feedback terminal 502B for providing feedback signal 544 to switch 516 and comparator 514 . Terminal 516C of switch 516 is coupled to output terminal 512C of control circuit 512 for opening and closing switch 516 with respect to edges of control signal 538 and control signal 540 generated by control circuit 512 . Blanking control signal 536 generated by control circuit 512 controls switch 516 .

Capacitor 518 is coupled to comparator 514 and switch 516, and when switch 516 is closed, the voltage across capacitor 518 follows the voltage at feedback terminal 502B. The capacitor 518 includes a terminal 518B coupled to ground and a terminal 518A coupled to the input terminal 514B of the comparator 514 and the terminal 516B of the switch 516 to track the voltage of the feedback signal 544 when the switch 516 is closed, and hold when the switch 516 is open The voltage of the feedback signal 544 .

Capacitor 520 is coupled to input terminal 514A and input terminal 514B of comparator 514 and together with resistor 522 and resistor 524 form a low pass filter circuit. Terminal 520A of capacitor 520 is coupled to input terminal 514A of comparator 514 , and terminal 520B of capacitor 520 is coupled to input terminal 514B of comparator 514 to provide for a connection between input terminal 514A and input terminal 514B of comparator 514 . path for high frequency signals. The pole frequency of the low pass filter circuit is defined as:

in:

R1 is the resistance _of resistor 522;

R2 is the resistance of resistor 524 _; and

C _C is the capacitance of capacitor 520 .

The control circuit 512 deactivates the blanking control signal 536 to open the switch 516 when the high-side transistor 508 or the low-side transistor 510 transitions between the on state and the off state. For example, control circuit 512 deactivates blanking control signal 536 before deactivating control signal 540 and activates blanking control signal 536 after subsequently deactivating control signal 538 . Similarly, control circuit 512 deactivates blanking control signal 536 before deactivating control signal 538 and activates blanking control signal 536 after subsequent activation of control signal 540 . Since the switch 516 is open during the transition at the switching terminal 502A, there is no noise at the input terminal 514B of the comparator 514 induced by the transition at the feedback terminal 502B. When switch 516 is open, capacitor 518 provides a sample of feedback signal 544 at input terminal 514B of comparator 514 . In some embodiments of 502 , delays in gate driver circuit 548 and gate driver circuit 550 may ensure that switch 516 is open prior to the transition of switching signal 542 . For example, if control signal 540 is activated at the same time blanking control signal 536 is deactivated, switch 516 may be turned off before switching high-side transistor 508 due to delays in gate driver circuit 548 .

FIG. 6 shows an example of blanking applied in switch mode power supply 500 when switching signal 542 is active for longer than the blanking time. At 602 , the control circuit 512 deactivates the blanking control signal 536 to open the switch 516 . After switch 516 is opened, control circuit 512 turns off low-side transistor 510 and turns on high-side transistor 508 to activate switching signal 542 at switching terminal 502A at 610 . After a predetermined blanking time interval (Tblk) at 602 (after the noise induced at feedback terminal 502B by the transition of switching signal 542 at 610 has settled), control circuit 512 activates blanking control signal 536 at 604 to close switch 516.

At 606 , the control circuit 512 deactivates the blanking control signal 536 to open the switch 516 . After switch 516 is opened, at 612 , control circuit 512 turns off high-side transistor 508 and turns on low-side transistor 510 to deactivate switching signal 542 at switching terminal 502A. After a predetermined blanking time interval following 606 (after the noise induced at feedback terminal 502B by the transition of switching signal 542 at 612 has settled), control circuit 512 activates blanking control signal 536 at 608 to close switch 516 .

FIG. 7 shows an example of blanking applied in the switched mode power supply 500 when the active time of the switching signal 542 is shorter than the blanking time. At 702 , the control circuit 512 deactivates the blanking control signal 536 to open the switch 516 . After switch 516 is opened, control circuit 512 turns off low-side transistor 510 and turns on high-side transistor 508 to activate switching signal 542 at switching terminal 502A at 706 . At 708, control circuit 512 turns off high-side transistor 508 and turns on low-side transistor 510 to deactivate switching signal 542 at switching terminal 502A. After a predetermined blanking time interval following 708 (after the noise induced at feedback terminal 502B by the transitions of switching signal 542 at 706 and 708 has settled), control circuit 512 activates blanking control signal 536 at 704 to close the switch 516.

FIG. 8 shows an example of blanking applied in the switched mode power supply 500 when the inactive time of the switching signal 542 is shorter than the blanking time. At 802 , the control circuit 512 deactivates the blanking control signal 536 to open the switch 516 . After switch 516 is opened, control circuit 512 turns off high-side transistor 508 and turns on low-side transistor 510 to deactivate switching signal 542 at switching terminal 502A at 806 . At 808, control circuit 512 turns off low-side transistor 510 and turns on high-side transistor 508 to activate switching signal 542 at switching terminal 502A. After a predetermined blanking time interval following 808 (after the noise induced at feedback terminal 502B by the transitions of switching signal 542 at 806 and 808 has settled), control circuit 512 activates blanking control signal 536 at 804 to close the switch 516.

In the example of FIGS. 6-8, the length of the blanking time interval is determined based on the settling time of the feedback signal at feedback terminal 502B. For example, the blanking time interval can be longer than the settling time of the feedback signal at the feedback terminal 502B, and short enough to ensure that the feedback signal can be sensed in a single cycle of the switched mode power supply 500 .

FIG. 9 shows example signals generated in switch mode power supply 500 . At 902 , the control circuit 512 deactivates the blanking control signal 536 to open the switch 516 . After switch 516 is opened, control circuit 512 turns off low-side transistor 510 and turns on high-side transistor 508 at 904 to activate switching signal 542 at switching terminal 502A. When high-side transistor 508 is on (in interval 922), current 902 in inductor 504 increases. After a predetermined blanking time interval following 904 (after the noise transient 914 induced in the feedback signal 544 at the feedback terminal 502B by the transition of the switching signal 542 at 904 has settled), the control circuit 512 activates blanking at 906 Control signal 536 to close switch 516 .

At 908 , the control circuit 512 deactivates the blanking control signal 536 to open the switch 516 . After switch 516 is opened, control circuit 512 turns off high-side transistor 508 and turns on low-side transistor 510 at 910 to deactivate switching signal 542 at switching terminal 502A. After a predetermined blanking time interval after 910 (after the noise transient 916 induced at the feedback terminal 502B by the transition of the switching signal 542 at 910 has settled), the control circuit 512 activates the blanking control signal 536 at 912 to close switch 516.

Because switch 516 is open when noise transient 914 and noise transient 916 are present at feedback terminal 502B, there is no noise transient 914 and noise transient on internal feedback voltage 546 at input terminal 514B of comparator 514 State 916. Difference signal 924 represents the difference between reference signal 552 at input terminal 514A of comparator 514 and internal feedback voltage 546 at input terminal 514B of comparator 514 . In difference signal 924, in interval 918, noise transient 914 is absent, and in interval 920, noise transient 916 is absent. Therefore, noise transient 914 and noise transient 916 do not cause subharmonic oscillations in switched mode power supply 500 . In addition, parasitic inductance caused by ground noise between the printed circuit board and the ground on which the switch mode power supply circuit 502 is mounted is attenuated.

The switch-mode power supply control circuitry described herein offers many advantages over other solutions. The blanking and filtering circuitry of the switched mode power supply circuit 502 is relatively simple to implement and effectively filters disturbances in the feedback signal 544 caused by transitions in the switching signal 542 and senses undisturbed in the absence of transient noise feedback signal 544. The switched mode power supply circuit 502 does not sacrifice transient performance by reducing the corner frequency of the low pass filter applied to the feedback signal. Jitter performance is improved because the internal feedback voltage 546 is static when the switch 516 is open (during the blanking time). The embodiment of 500 is stable even in embodiments that are not optimal printed circuit board (PCB) routing and employ small integrated circuit packages, which allows for flexible packaging and PCB routing.

10 shows a block diagram of an exemplary electricity meter 1000 including a switch mode power supply in accordance with the present specification. The electricity meter 1000 is a smart electricity meter that measures the alternating current drawn from the mains power supply and communicates the measurement to the electricity provider. Electricity meter 1000 includes AC/DC converter 1002 , battery circuitry 1004 , DC/DC converter 1006 , metering circuitry 1008 , processor 1010 , user interface circuitry 1012 , and communication circuitry 1014 . AC/DC converter 1002 includes circuitry to convert AC voltage received from a mains power source to DC voltage. The battery circuitry 1004 includes a battery and switching circuitry to switch the output of the battery or AC/DC converter 1002 to the DC/DC converter 1006 .

The DC/DC converter 1006 generates one or more DC voltages from the DC voltage received by the battery circuitry 1004 that power the metering circuitry 1008 , the processor 1010 , the user interface circuitry 1012 , and the communication circuitry 1014 . Metering circuitry 1008 includes circuitry such as current sensors and analog-to-digital converters that measure AC current drawn from the AC mains by electrical devices coupled to electricity meter 1000 . The metering circuitry 1008 communicates the current measurement to the processor 1010 . Processor 1010 includes a microcontroller or other instruction execution device that stores and/or communicates measurements via communication circuitry 1014 and/or user interface circuitry 1012 . User interface circuitry 1012 may include a display device such as a liquid crystal display and an input device such as a keyboard. Communication circuitry 1014 includes wired (Universal Serial Bus, RS-232, etc.) and/or wireless communication circuitry that allows processor 1010 to communicate measurements and other information to the power provider.

DC/DC converter 1006 includes an embodiment of switch mode power supply 500 . The feedback blanking circuitry included in the switch mode power supply circuit 502 allows for a reduction in the size of the integrated circuit package, which reduces the size and cost of the power meter 1000 while eliminating subharmonic oscillations without sacrificing transient performance. Therefore, the DC/DC converter 1006 can quickly respond to changes in the load current. Because the DC/DC converter 1006 is not affected by transients coupled from the switching signal 542 to the feedback signal 544, PCB routing in the DC/DC converter 1006 and packaging of the electricity meter 1000 can be simplified.

The term "coupled" is used throughout the specification. The term may encompass the connections, communications, or signal paths that achieve functional relationships consistent with the description of the present disclosure. For example, if device A generates a signal to control device B to perform an action, then in the first example, device A is coupled to device B by a direct connection, or in the second example, if intermediate component C does not alter the relationship between device A and device B The functional relationship between the two is such that device A controls device B via a control signal generated by device A, and device A is coupled to device B through intermediate component C.

Modifications may be made in the described embodiments, and other embodiments are possible, within the scope of the claims.

Claims (21)

1. A switched mode power supply circuit, comprising:

a feedback terminal;

a control circuit;

a comparator, comprising:

a first input terminal coupled to a reference voltage source; and

an output terminal coupled to an input terminal of the control circuit; and

a switch, comprising:

a first terminal coupled to the feedback terminal;

a second terminal coupled to a second input terminal of the comparator; and

a third terminal coupled to an output terminal of the control circuit.

2. The switched mode power supply circuit of claim 1, further comprising: a capacitor, comprising:

a first terminal coupled to the first input terminal of the comparator; and

a second terminal coupled to the second input terminal of the comparator.

3. The switched mode power supply circuit of claim 2, further comprising: a resistor, comprising:

a first terminal coupled to the reference voltage source; and

a second terminal coupled to the first terminal of the comparator.

4. The switched mode power supply circuit of claim 3, wherein:

the resistor is a first resistor; and is

The switched mode power supply circuit further comprises:

a second resistor, comprising:

a first terminal coupled to the first terminal of the comparator; and

a second terminal coupled to a common voltage source.

5. The switched mode power supply circuit of claim 1, further comprising: a capacitor, comprising:

a first terminal coupled to the second input terminal of the comparator; and

a second terminal coupled to a common voltage source.

6. The switched mode power supply circuit of claim 1, further comprising:

a switching terminal;

a high-side transistor, comprising:

a first terminal coupled to the control circuit;

a second terminal coupled to a power rail; and

a third terminal coupled to the switching terminal; and

a low-side transistor comprising:

a first terminal coupled to the control circuit;

a second terminal coupled to the switching terminal; and

a third terminal coupled to a common voltage source.

7. The switched mode power supply circuit of claim 6, wherein the control circuit is configured to:

generating a control signal to control the high-side transistor;

opening the switch before an edge of the control signal; and is

Closing the switch after the edge of the control signal.

8. The switched mode power supply circuit of claim 7, wherein:

the control signal is a first control signal; and is

The control circuit is configured to:

generating a second control signal that controls the low-side transistor;

opening the switch before an edge of the second control signal; and is

Closing the switch after the edge of the second control signal.

9. The switched mode power supply circuit of claim 8, wherein the control circuit is configured to:

opening the switch before the edge of the first control signal; and is

Closing the switch after the edge of the second control signal.

10. A switched mode power supply circuit, comprising:

a feedback terminal configured to receive a feedback signal;

a comparator configured to compare the feedback signal with a reference signal;

a switch coupled to the feedback terminal and the comparator and configured to pass the feedback signal from the feedback terminal to the comparator; and

a control circuit coupled to the comparator and configured to:

generating a control signal to turn on the high-side transistor; and is

Opening the switch before an edge of the control signal; and is

Closing the switch after the edge of the control signal.

11. The switched mode power supply circuit of claim 10, wherein:

the control signal is a first control signal; and is

The control circuit is configured to:

generating a second control signal that turns on a low-side transistor; and is

Opening the switch before an edge of the second control signal; and is

Closing the switch after the edge of the second control signal.

12. The switched mode power supply circuit of claim 11, wherein the control circuit is configured to:

opening the switch before the edge of the first control signal; and is

Closing the switch after the edge of the second control signal.

13. The switched mode power supply circuit of claim 10, further comprising a filter circuit configured to attenuate noise on the feedback signal.

14. The switched mode power supply circuit of claim 13, wherein the filter circuit comprises:

a capacitor coupled to a first input of the comparator and a second input of the comparator;

a first resistor coupled to the first input of the comparator and a reference voltage source; and

a second resistor coupled to the first input of the comparator and a common voltage source.

15. The switched mode power supply circuit of claim 10, further comprising a capacitor coupled to the comparator and a reference voltage source and configured to provide a sample of the feedback signal to the comparator when the switch is open.

16. A switched mode power supply, comprising:

an inductor;

a high-side transistor coupled to the inductor;

a low side transistor coupled to the inductor;

a voltage divider coupled to the inductor;

a control circuit, comprising:

a first output terminal coupled to the high-side transistor; and

a second output terminal coupled to the low-side transistor;

a comparator, comprising:

a first input terminal coupled to a reference voltage source; and

an output terminal coupled to an input terminal of the control circuit; and

a switch, comprising:

a first terminal coupled to the voltage divider;

a second terminal coupled to a second input terminal of the comparator; and

a third terminal coupled to a third output terminal of the control circuit.

17. The switched mode power supply of claim 16, further comprising:

a capacitor, comprising:

a first terminal coupled to the second input terminal of the comparator; and

a second terminal coupled to a common voltage source.

18. The switched mode power supply of claim 16, further comprising:

a capacitor, comprising:

a first terminal coupled to the first input terminal of the comparator; and

a second terminal coupled to the second input terminal of the comparator.

19. The switched mode power supply of claim 18, further comprising:

a resistor, comprising:

a first terminal coupled to the reference voltage source; and

a second terminal coupled to the first terminal of the comparator.

20. The switched mode power supply of claim 19, wherein:

the resistor is a first resistor; and is

The switched mode power supply further comprises:

a second resistor, comprising:

a first terminal coupled to the first terminal of the comparator; and

a second terminal coupled to a common voltage source.

21. An electricity meter, comprising:

a metering circuitry; and

a switched mode power supply coupled to the metering circuitry and comprising:

an inductor;

a high-side transistor coupled to the inductor;

a low side transistor coupled to the inductor;

a voltage divider coupled to the inductor;

a control circuit, comprising:

a first output terminal coupled to the high-side transistor; and

a second output terminal coupled to the low-side transistor;

a comparator, comprising:

a first input terminal coupled to a reference voltage source; and

an output terminal coupled to an input terminal of the control circuit; and a switch, comprising:

a first terminal coupled to the voltage divider;

a second terminal coupled to a second input terminal of the comparator; and

a third terminal coupled to a third output terminal of the control circuit.

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