TWI796077B - Method and circuit of automatic mode-switching for dc-dc converter - Google Patents

Abstract

The invention discloses an auto mode-switching method for DC-DC converter, including following steps: (a) obtaining a first width of a duty cycle of a PWM signal and a second width of a duty cycle of a PFM signal respectively, wherein the first width is related to the error voltage and the second width is related to the peak current; and (b) outputting a switch signal according to the comparison result of the first width and the second width to automatically switch the DC-DC converter to operate on the pulse width modulation mode or pulse frequency modulation mode.

Description

Automatic mode switching method and circuit for DC-DC converter

The present invention is related to DC-DC converters, in particular to an automatic mode switching method and circuit for DC-DC converters.

In general, DC-DC converters usually have two different modes of operation:

(1) Pulse width modulation mode (PWM mode): due to its large power consumption (Power consumption) and small output ripple (Ripple), it is more suitable for high current applications;

(2) Pulse frequency modulation mode (PFM mode): Due to its small power consumption and large output ripple, it is more suitable for low current applications.

In the application of high-efficiency Power IC, the pulse frequency modulation (PFM) mode will be used for light load, and the pulse width modulation (PWM) mode will be switched to realize when the load increases to heavy load, so as to At the same time, it achieves the effects of high output efficiency and low output ripple.

However, in practical applications, when the heavy load turns to light load, it is easy to cause the DC-DC converter to repeatedly switch between the pulse frequency modulation (PFM) mode and the pulse width modulation (PWM) mode, resulting in DC - The DC converter cannot switch operation modes smoothly, resulting in large output ripple, which still needs to be further resolved.

Therefore, the present invention proposes an automatic mode switching method and circuit for a DC-DC converter, so as to effectively solve the above-mentioned problems encountered in the prior art.

A preferred embodiment according to the present invention is an automatic mode switching method for a DC-DC converter. In this embodiment, the automatic mode switching method includes the following steps: (a) obtain the first width of the duty cycle of the PWM signal and the second width of the duty cycle of the PWM signal respectively, wherein the first width is is related to the error voltage and the second width is related to the peak current; and (b) outputting a switching signal to automatically switch the DC-DC converter operating in the pulse width modulation mode or the pulse frequency according to the comparison result of the first width and the second width modulation mode.

In one embodiment, the step (b) includes: (b1) if the comparison result is that the first width is greater than the second width, then automatically switch the DC-DC converter to operate in the pulse width modulation mode; and (b2) if the comparison result Because the first width is smaller than the second width, the DC-DC converter is automatically switched to operate in the pulse frequency modulation mode.

In one embodiment, step (a) is to determine the first width according to the first rising edge and the first falling edge of the pulse width modulation signal and determine the second width according to the second rising edge and the second falling edge of the pulse frequency modulation signal. Two widths.

In one embodiment, the first rising edge and the second rising edge both correspond to the first time, and the first falling edge and the second falling edge correspond to the second time and the third time respectively, if the second time is later than the first time Three times, the comparison result is that the first width is greater than the second width, and the DC-DC converter operates in the pulse width modulation mode; if the second time is earlier than the third time, the comparison result is that the first width is smaller than the second width, The DC-DC converter operates in pulse frequency modulation mode.

In one embodiment, the second time is determined by the intersection of the error voltage and the ramp signal and the third time is determined by the intersection of the peak current and the output inductor current.

In one embodiment, the error voltage is generated according to the reference voltage and the feedback voltage related to the output voltage.

Another preferred embodiment of the present invention is also an automatic mode switching method for a DC-DC converter. In this embodiment, the automatic mode switching method includes the following steps: (a) obtain the threshold voltage value of the error voltage and the pulse frequency modulation signal respectively; and (b) output the switching signal according to the comparison result of the error voltage and the threshold voltage value to Automatically switch the DC-DC converter to operate in pulse width modulation mode or pulse frequency modulation mode.

In one embodiment, the step (b) includes: (b1) if the comparison result is that the error voltage is greater than the threshold voltage value, automatically switching the DC-DC converter to operate in the pulse width modulation mode; and (b2) if the comparison result is When the error voltage is less than the critical voltage value, the DC-DC converter is automatically switched to operate in the pulse frequency modulation mode.

In one embodiment, the error voltage is generated according to the reference voltage and the feedback voltage related to the output voltage.

In one embodiment, when the DC-DC converter operates in the PWM mode, the falling edge of the switching signal is determined by the intersection of the error voltage and the ramp signal; when the DC-DC converter operates in the PWM mode mode, the falling edge of the switching signal is determined by the intersection of the peak current and the output inductor current.

Another preferred embodiment according to the present invention is an automatic mode switching circuit for a DC-DC converter. In this embodiment, the automatic mode switching circuit includes: a first comparator for providing a first width of the duty cycle of the PWM signal, wherein the first width is related to the error voltage; a second comparator for providing a second width of the duty cycle of the pulse frequency modulation signal, wherein the second width is related to the peak current; The comparison result of the width outputs a switching signal to automatically switch the DC-DC converter to operate in the PWM mode or the PWM mode.

In one embodiment, if the comparison result is that the first width is greater than the second width, the DC-DC converter operates in the PWM mode; if the comparison result is that the first width is smaller than the second width, the DC-DC converter Operates in PWM mode.

In one embodiment, the two input terminals of the first comparator respectively receive the error voltage and the ramp signal and the two input terminals of the second comparator respectively receive the voltage generated by the output inductor current flowing through the resistor and the reference voltage, the error voltage is according to The reference voltage and the feedback voltage related to the output voltage are generated.

In one embodiment, the first width is determined by the first rising edge and the first falling edge of the PWM signal and the second width is determined by the second rising edge and the second falling edge of the PWM signal.

In one embodiment, the first rising edge and the second rising edge both correspond to the first time, and the first falling edge and the second falling edge correspond to the second time and the third time respectively, if the second time is later than the first time Three times, the comparison result is that the first width is greater than the second width, and the DC-DC converter operates in the pulse width modulation mode; if the second time is earlier than the third time, the comparison result is that the first width is smaller than the second width, The DC-DC converter operates in pulse frequency modulation mode.

In one embodiment, the second time is determined by the intersection of the error voltage and the ramp signal and the third time is determined by the intersection of the peak current and the output inductor current.

Another preferred embodiment according to the present invention is an automatic mode switching circuit for a DC-DC converter. In this embodiment, the automatic mode switching circuit includes: an error amplifier, used to provide an error voltage; a PWM reference generator, used to provide a critical voltage value of a PWM signal; and a comparator, respectively coupled to the error The amplifier and the pulse frequency modulation reference generator are used to output switching signals according to the comparison result of the error voltage and the critical voltage value to automatically switch the DC-DC converter to operate in the pulse width modulation mode or the pulse frequency modulation mode.

In one embodiment, if the comparison result is that the error voltage is greater than the critical voltage value, the DC-DC converter operates in the PWM mode; if the comparison result is that the error voltage is smaller than the critical voltage value, the DC-DC converter operates in the PWM mode. Pulse frequency modulation mode.

In one embodiment, two input terminals of the error amplifier respectively receive a reference voltage and a feedback voltage related to the output voltage to generate an error voltage.

In one embodiment, when the DC-DC converter operates in the PWM mode, the falling edge of the switching signal is determined by the intersection of the error voltage and the ramp signal; when the DC-DC converter operates in the PWM mode mode, the falling edge of the switching signal is determined by the intersection of the peak current and the output inductor current.

Compared with the prior art, the automatic mode switching method and circuit for the DC-DC converter proposed by the present invention can realize linear and smooth switching of the pulse frequency modulation (PFM) mode/pulse width modulation (PWM) mode, so It can effectively solve the problem that the traditional DC-DC converter is easy to switch repeatedly between the pulse frequency modulation (PFM) mode and the pulse width modulation (PWM) mode when it is from heavy load to light load, so as to make the output ripple smaller .

A preferred embodiment according to the present invention is an automatic mode switching method for a DC-DC converter.

Please refer to FIG. 1 . FIG. 1 shows a flow chart of an automatic mode switching method for a DC-DC converter in this embodiment. As shown in FIG. 1, the automatic mode switching method for the DC-DC converter in this embodiment may include the following steps:

Step S10: obtaining the first width of the duty cycle of the PWM signal and the second width of the duty cycle of the PWM signal respectively, wherein the first width is related to the error voltage and the second width is related to the peak current;

Step S12: comparing the first width and the second width;

Step S14: If the comparison result of step S12 is that the first width is greater than the second width, then automatically switch the DC-DC converter to operate in the pulse width modulation mode; and

Step S16: If the comparison result of step S12 is that the first width is smaller than the second width, automatically switch the DC-DC converter to operate in the pulse frequency modulation mode.

It should be noted that in step S10, the first width can be determined according to the first rising edge and the first falling edge of the PWM signal, and the second width can be determined according to the second rising edge and the second falling edge of the PWM signal. , but not limited to this.

In one embodiment, it is assumed that the first rising edge and the second rising edge both correspond to the first time, and the first falling edge and the second falling edge correspond to the second time and the third time, respectively. If the second time is later than the third time, the comparison result of step S12 is that the first width is greater than the second width, and then the method will execute step S14 to automatically switch the DC-DC converter to operate in the PWM mode. If the second time is earlier than the third time, the comparison result of step S12 is that the first width is smaller than the second width, and then the method will execute step S16 to automatically switch the DC-DC converter to operate in the PWM mode.

In practical applications, the second time can be determined by the intersection of the error voltage and the ramp signal and the third time can be determined by the intersection of the peak current and the output inductor current, but not limited thereto. In addition, the error voltage can be generated according to the reference voltage and the feedback voltage related to the output voltage, but it is not limited thereto.

Another preferred embodiment according to the present invention is an automatic mode switching circuit for a DC-DC converter.

Please refer to FIG. 2 . FIG. 2 is a schematic diagram of an automatic mode switching circuit for a DC-DC converter in this embodiment. As shown in FIG. 2 , the automatic mode switching circuit 2 for a DC-DC converter includes an error amplifier 20 , a current source 22 , a compensation network 24 , a comparator 26 , a comparator 28 , a logic circuit 30 and a resistor R. The output terminal of the error amplifier 20 is coupled to the positive input terminal + of the comparator 28 . The current source 22 is respectively coupled to the resistor R and the positive input terminal + of the comparator 26 . The resistor R is coupled between the current source 22 and the ground terminal GND. The compensation network 24 is coupled to the ground terminal GND. The output terminals of the comparator 26 and the comparator 28 are both coupled to the logic circuit 30 .

The positive input terminal + and the negative input terminal - of the error amplifier 20 respectively receive the reference voltage VREF and the feedback voltage FB related to the output voltage of the DC-DC converter and generate an error voltage Verr accordingly to the positive input terminal + of the comparator 28 . The positive input terminal + and the negative input terminal - of the comparator 26 respectively receive the voltage generated by the output inductor current ILX provided by the current source 22 flowing through the resistor R and the reference voltage VREF to generate the pulse frequency modulation signal PFM_Ipeak to the logic circuit 30 , wherein the second width of the duty cycle of the pulse frequency modulation signal PFM_Ipeak is related to the peak current Ipeak. The positive input terminal + and the negative input terminal - of the comparator 28 receive the error voltage Verr and the ramp signal SAW respectively and generate a pulse width modulation signal PWM_duty to the logic circuit 30 accordingly, wherein the duty cycle of the pulse width modulation signal PWM_duty is the first The width is related to the error voltage Verr.

The logic circuit 30 is respectively coupled to the output terminals of the comparator 26 and the comparator 28 for receiving the pulse width modulation signal PWM_duty and the pulse frequency modulation signal PFM_Ipeak respectively and according to the first width and the duty cycle of the pulse width modulation signal PWM_duty The comparison result of the second width of the duty cycle of the pulse frequency modulation signal PFM_Ipeak outputs the switching signal S1 to automatically switch the DC-DC converter to operate in the PWM mode or the PWM mode.

In practical applications, if the above comparison result is that the first width is greater than the second width, the switching signal S1 is used to automatically switch the DC-DC converter to operate in the pulse width modulation mode; if the above comparison result is that the first width is smaller than the second width, the switching signal S1 is used to automatically switch the DC-DC converter to operate in the pulse frequency modulation mode.

Please refer to FIG. 3 . FIG. 3 shows a timing diagram of the output inductor current ILX, the ramp signal SAW, the pulse width modulation signal PWM_duty, the pulse frequency modulation signal PFM_Ipeak, and the switching signal S1 respectively.

As shown in FIG. 3 , at time t1, the ramp signal SAW rises to be equal to the error voltage Verr, the pulse width modulation signal PWM_duty output by the comparator 28 changes from a high level to a low level, and the output inductor current ILX is still smaller than the peak current Ipeak, the pulse frequency modulation signal PFM_Ipeak output by the comparator 26 is still at a high level. At time t2, the ramp signal SAW continues to rise and is greater than the error voltage Verr, the pulse width modulation signal PWM_duty output by the comparator 28 remains at a low level, and the output inductor current ILX rises to be equal to the peak current Ipeak, the pulse output by the comparator 26 The frequency modulation signal PFM_Ipeak changes from a high level to a low level.

Since the first rising edge of the pulse width modulation signal PWM_duty and the second rising edge of the pulse frequency modulation signal PFM_Ipeak both correspond to the same time, but the time t1 corresponding to the first falling edge of the pulse width modulation signal PWM_duty is earlier than the pulse frequency The time t2 corresponding to the second falling edge of the modulation signal PFM_Ipeak, that is, the first width of the duty cycle of the pulse width modulation signal PWM_duty is smaller than the second width of the duty cycle of the pulse frequency modulation signal PFM_Ipeak, therefore, the logic circuit 30 According to the comparison result, a switching signal S1 in phase with the pulse frequency modulation signal PFM_Ipeak is output to automatically switch the DC-DC converter to operate in the pulse frequency modulation mode. The situation at time t3 and t4 is similar to the situation at time t1 and t2, and the DC-DC converter is still operating in the PWM mode.

At time t5, the output inductor current ILX rises to be equal to the peak current Ipeak, the pulse frequency modulation signal PFM_Ipeak output by the comparator 26 changes from a high level to a low level, and the ramp signal SAW is still lower than the error voltage Verr, the output of the comparator 28 The pulse width modulation signal PWM_duty remains at a high level. At time t6, the output inductor current ILX continues to rise and is greater than the peak current Ipeak, the pulse frequency modulation signal PFM_Ipeak output by the comparator 26 remains at a low level, and the ramp signal SAW rises to be equal to the error voltage Verr, and the comparator 28 outputs The pulse width modulation signal PWM_duty changes from a high level to a low level.

Since the first rising edge of the pulse width modulation signal PWM_duty and the second rising edge of the pulse frequency modulation signal PFM_Ipeak both correspond to the same time, but the time t1 corresponding to the first falling edge of the pulse width modulation signal PWM_duty is later than the pulse frequency The time t2 corresponding to the second falling edge of the modulation signal PFM_Ipeak, that is, the first width of the duty cycle of the pulse width modulation signal PWM_duty is greater than the second width of the duty cycle of the pulse frequency modulation signal PFM_Ipeak, therefore, the logic circuit 30 According to the comparison result, a switching signal S1 in phase with the PWM_duty is output to automatically switch the DC-DC converter to operate in the PWM mode. The situation at time t7 and t8 is similar to the situation at time t5 and t6, and the DC-DC converter continues to operate in the PWM mode. The rest can be deduced in the same way, and will not be repeated here.

It should be noted that the times t1, t3, t6, and t8 in Figure 3 are determined by the intersection of the ramp signal SAW and the error voltage Verr, and the times t2, t4, t5, and t7 in Figure 3 are determined by the output inductor current ILX and The intersection point of the peak current Ipeak is determined, but not limited thereto.

As shown in Figure 4, the DC-DC converter is operated in the pulse width modulation mode PWM or the pulse frequency modulation mode PFM under different load currents, when the pulse frequency modulation (PFM) mode and pulse width modulation (PWM ) mode switching, the automatic mode switching circuit 2 of the present invention can effectively make the efficiency change of the DC-DC converter relatively linear and smooth, so as to avoid large output ripples.

Another preferred embodiment of the present invention is also an automatic mode switching method for a DC-DC converter.

Please refer to FIG. 5 . FIG. 5 is a flow chart of an automatic mode switching method for a DC-DC converter in this embodiment. As shown in FIG. 5, the automatic mode switching method for the DC-DC converter in this embodiment may include the following steps:

Step 50: obtain the error voltage and the critical voltage value of the pulse frequency modulation signal respectively;

Step 52: Comparing the error voltage and the critical voltage value;

Step 54: If the comparison result of step 52 is that the error voltage is greater than the critical voltage value, then the method automatically switches the DC-DC converter to operate in the PWM mode; and

Step 56: If the comparison result of step 52 is that the error voltage is less than the critical voltage value, then the method automatically switches the DC-DC converter to operate in the pulse frequency modulation mode.

In practical applications, the error voltage can be generated according to the reference voltage and the feedback voltage related to the output voltage of the DC-DC converter, but not limited thereto. When the method outputs the switching signal to automatically switch the DC-DC converter to operate in the pulse width modulation mode, the falling edge of the switching signal can be determined by the intersection of the error voltage and the ramp signal, but not limited thereto. When the method outputs a switching signal to automatically switch the DC-DC converter in the pulse frequency modulation mode, the falling edge of the switching signal can be determined by the intersection of the peak current and the output inductor current, but not limited thereto.

Another preferred embodiment according to the present invention is an automatic mode switching circuit for a DC-DC converter.

Please refer to FIG. 6 , which is a schematic diagram of an automatic mode switching circuit for a DC-DC converter in this embodiment. As shown in FIG. 6 , the automatic mode switching circuit 6 for a DC-DC converter includes an error amplifier 60 , a pulse frequency modulation (PFM) reference generator 62 , a compensation network 64 and a comparator 66 . The output terminal of the error amplifier 60 is coupled to a positive input terminal + of a comparator 66 . The pulse frequency modulation (PFM) reference generator 62 is coupled to the other positive input terminal + of the comparator 66 . The compensation network 64 is coupled to the ground terminal GND.

The positive input terminal + and the negative input terminal - of the error amplifier 60 respectively receive the reference voltage VREF and the feedback voltage FB related to the output voltage of the DC-DC converter, and accordingly generate the error voltage Verr to a positive input terminal of the comparator 66 +. The pulse frequency modulation (PFM) reference generator 62 is used for providing the threshold voltage value PFM_TH of the pulse frequency modulation signal to the other positive input terminal + of the comparator 66 . The two positive input terminals of the comparator 66 receive the error voltage Verr and the threshold voltage value PFM_TH of the pulse frequency modulation signal respectively, and its negative input terminal - receives the ramp signal SAW and according to the error voltage Verr and the threshold voltage value of the pulse frequency modulation signal The comparison result of the voltage value PFM_TH outputs a switching signal S1 to automatically switch the DC-DC converter to operate in the PWM mode or the PWM mode.

In practical applications, if the result of the above comparison is that the error voltage Verr is greater than the threshold voltage value PFM_TH of the pulse frequency modulation signal, the switching signal S1 is used to automatically switch the DC-DC converter to operate in the pulse width modulation mode; if the above comparison result Since the error voltage Verr is smaller than the threshold voltage value PFM_TH of the pulse frequency modulation signal, the switching signal S1 is used to automatically switch the DC-DC converter to operate in the pulse frequency modulation mode.

Please refer to FIG. 7 . FIG. 7 shows a timing diagram of the output inductor current ILX, the ramp signal SAW and the switching signal S1 respectively. As shown in Figure 7, at time t1, the output inductor current ILX rises to be equal to the peak current Ipeak, and the error voltage Verr is smaller than the threshold voltage value PFM_TH of the pulse frequency modulation signal, and the switching signal S1 is used to automatically switch the DC-DC converter It operates in the pulse frequency modulation mode, and the falling edge of the switching signal S1 is determined by the intersection point of the peak current Ipeak and the output inductor current ILX. The situation at time t2 is similar to the situation at time t1, so the DC-DC converter continues to operate in the PWM mode.

At time t3, the output inductor current ILX rises to be equal to the peak current Ipeak, and the error voltage Verr is greater than the threshold voltage value PFM_TH of the pulse frequency modulation signal, and the switching signal S1 is used to automatically switch the DC-DC converter to operate in pulse width modulation mode, and the falling edge of the switching signal S1 is determined by the intersection of the error voltage Verr and the ramp signal SAW (ie time t4). The situation at time t5 and t6 is similar to the situation at time t3 and t4, so the DC-DC converter continues to operate in the PWM mode. The rest can be deduced in the same way, and will not be repeated here.

Compared with the prior art, the automatic mode switching method and circuit for the DC-DC converter proposed by the present invention can realize linear and smooth switching of the pulse frequency modulation (PFM) mode/pulse width modulation (PWM) mode, so It can effectively solve the problem that the traditional DC-DC converter is easy to switch repeatedly between the pulse frequency modulation (PFM) mode and the pulse width modulation (PWM) mode when it is from heavy load to light load, so as to make the output ripple smaller .

S10~S16: Steps S50~S56: steps 2: Automatic mode switching circuit for DC-DC converter 20: Error amplifier 22: Current source 24: Compensation network 26: Comparator 28: Comparator 30: Logic circuit R: resistance +: Positive input -: Negative input terminal VREF: reference voltage FB: feedback voltage Verr: error voltage ILX: output inductor current PFM_Ipeak: pulse frequency modulation signal Ipeak: peak current SAW: ramp signal PWM_duty: pulse width modulation signal S1: switch signal GND: ground terminal t1~t8: time PFM: pulse frequency modulation mode PWM: pulse width modulation mode 6: Automatic mode switching circuit for DC-DC converter 60: Error amplifier 62:Pulse frequency modulation (PFM) reference generator 64: Compensation network 66: Comparator PFM_TH: Threshold voltage value of pulse frequency modulation signal

FIG. 1 is a flow chart of an automatic mode switching method for a DC-DC converter in an embodiment of the present invention.

FIG. 2 is a schematic diagram of an automatic mode switching circuit for a DC-DC converter in another embodiment of the present invention.

FIG. 3 shows the timing diagrams of the output inductor current, the ramp signal, the pulse width modulation signal, the pulse frequency modulation signal and the switching signal respectively.

FIG. 4 shows the efficiency diagram when the DC-DC converter operates in PWM mode or PWM mode under different load currents.

FIG. 5 is a flow chart of an automatic mode switching method for a DC-DC converter according to another embodiment of the present invention.

FIG. 6 is a schematic diagram of an automatic mode switching circuit for a DC-DC converter in another embodiment of the present invention.

FIG. 7 shows the timing diagrams of the output inductor current, the ramp signal and the switching signal respectively.

S10~S16: Steps

Claims (18)

An automatic mode switching method for a DC-DC converter, comprising the following steps: (a) separately obtaining the first width of the duty cycle of the pulse width modulation signal and the second width of the duty cycle of the pulse frequency modulation signal, wherein The first width is related to the error voltage and the second width is related to the peak current; and (b) outputting a switching signal to automatically switch the operation of the DC-DC converter according to a comparison result of the first width and the second width In PWM mode or PWM mode. The automatic mode switching method as described in claim item 1, wherein step (b) includes: (b1) if the comparison result is that the first width is greater than the second width, then automatically switch the DC-DC converter to operate on the pulse wide modulation mode; and (b2) automatically switching the DC-DC converter to operate in the pulse frequency modulation mode if the comparison result is that the first width is smaller than the second width. The automatic mode switching method as described in claim 1, wherein step (a) is to determine the first width according to the first rising edge and the first falling edge of the pulse width modulation signal and according to the first pulse frequency modulation signal The second rising edge and the second falling edge determine the second width. The automatic mode switching method as described in claim 3, wherein the first rising edge and the second rising edge both correspond to the first time, and the first falling edge and the second falling edge correspond to the second time and the second falling edge respectively. At a third time, if the second time is later than the third time, the comparison result is that the first width is greater than the second width, and the DC-DC converter operates in the pulse width modulation mode; if the second time is earlier than the third time, the comparison ends If the first width is smaller than the second width, the DC-DC converter operates in the PWM mode. The automatic mode switching method as claimed in claim 4, wherein the second time is determined by the intersection of the error voltage and the ramp signal and the third time is determined by the intersection of the peak current and the output inductor current. The automatic mode switching method according to claim 1, wherein the error voltage is generated according to a reference voltage and a feedback voltage related to the output voltage. An automatic mode switching method for a DC-DC converter, comprising the following steps: (a) separately obtaining the error voltage and the critical voltage value of the pulse frequency modulation signal; and (b) according to the difference between the error voltage and the critical voltage value The comparison result outputs a switching signal to automatically switch the DC-DC converter to operate in the pulse width modulation mode or the pulse frequency modulation mode; wherein, when the DC-DC converter operates in the pulse width modulation mode, the switching signal The falling edge of the switching signal is determined by the intersection of the error voltage and the ramp signal; when the DC-DC converter operates in the pulse frequency modulation mode, the falling edge of the switching signal is determined by the intersection of the peak current and the output inductor current Decide. The automatic mode switching method as described in claim item 7, wherein step (b) includes: (b1) if the comparison result is that the error voltage is greater than the critical voltage value, automatically switching the DC-DC converter to operate at the pulse width a modulation mode; and (b2) automatically switching the DC-DC converter to operate in the pulse frequency modulation mode if the comparison result is that the error voltage is less than the critical voltage value. The automatic mode switching method as described in claim item 7, wherein the error voltage is It is generated according to the reference voltage and the feedback voltage related to the output voltage. An automatic mode switching circuit for a DC-DC converter, comprising: a first comparator for providing a first width of a duty cycle of a pulse width modulation signal, wherein the first width is related to an error voltage; a second a comparator, used to provide a second width of the duty cycle of the pulse frequency modulation signal, wherein the second width is related to the peak current; and a logic circuit, respectively coupled to the first comparator and the second comparator, for Outputting a switch signal according to the comparison result between the first width and the second width to automatically switch the DC-DC converter to operate in a pulse width modulation mode or a pulse frequency modulation mode. The automatic mode switching circuit according to claim 10, wherein if the comparison result is that the first width is greater than the second width, the DC-DC converter operates in the pulse width modulation mode; if the comparison result is the If the first width is smaller than the second width, the DC-DC converter operates in the PWM mode. The automatic mode switching circuit as described in claim 10, wherein the two input terminals of the first comparator receive the error voltage and the ramp signal respectively, and the two input terminals of the second comparator respectively receive the output generated by the inductor current flowing through the resistance The voltage and the reference voltage, the error voltage is generated according to the reference voltage and the feedback voltage related to the output voltage. The automatic mode switching circuit according to claim 10, wherein the first width is determined by the first rising edge and the first falling edge of the pulse width modulation signal and the second width is determined by the pulse frequency modulation signal The second rising edge and the second falling edge. The automatic mode switching circuit according to claim 13, wherein the first rising edge and the second rising edge both correspond to the first time, and the first falling edge and the second falling edge correspond to the second time and the second falling edge respectively At a third time, if the second time is later than the third time, the comparison result is that the first width is greater than the second width, and the DC-DC converter operates in the pulse width modulation mode; if the second If the time is earlier than the third time, the comparison result is that the first width is smaller than the second width, and the DC-DC converter operates in the PWM mode. The automatic mode switching circuit as claimed in claim 14, wherein the second time is determined by the intersection of the error voltage and the ramp signal and the third time is determined by the intersection of the peak current and the output inductor current. An automatic mode switching circuit for a DC-DC converter, comprising: an error amplifier for providing an error voltage; a pulse frequency modulation reference generator for providing a threshold voltage value of a pulse frequency modulation signal; and a comparator, respectively coupled to the error amplifier and the pulse frequency modulation reference generator for outputting switching signals according to the comparison result between the error voltage and the critical voltage value to automatically switch the DC-DC converter to operate in the pulse width modulation mode or Pulse frequency modulation mode; wherein, when the DC-DC converter operates in the pulse width modulation mode, the falling edge of the switching signal is determined by the intersection of the error voltage and the ramp signal; when the DC-DC converter When the device operates in the pulse frequency modulation mode, the falling edge of the switching signal is determined by the intersection point of the peak current and the output inductor current. The automatic mode switching circuit according to claim 16, wherein if the comparison result is that the error voltage is greater than the threshold voltage value, the DC-DC converter operates in the pulse width modulation mode; if the comparison result is the error If the voltage is lower than the critical voltage value, the DC-DC converter operates in the pulse frequency modulation mode. The automatic mode switching circuit according to claim 16, wherein the two input terminals of the error amplifier respectively receive a reference voltage and a feedback voltage related to the output voltage to generate the error voltage.

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