CN103296887B - Power supply circuit, switching power supply and its control circuit and control method - Google Patents

Abstract

The invention provides a switching power supply, wherein an error amplifying circuit in a control circuit compares a feedback signal related to an output voltage with a reference signal to adjust the feedback signal to the level of the reference signal through a loop feedback balance mechanism, and when the switching power supply enters a shutdown procedure, the reference signal is reduced gradually to reduce the output voltage to a shutdown critical value.

Description

Power supply circuit, switching power supply and its control circuit and control method

technical field

The present invention relates to a switchable power supply and its control method, in particular to a switchable power supply which reduces the output voltage to a shutdown critical value according to the input voltage when the switchable power supply enters the shutdown program and the its control method.

Background technique

FIG. 1A shows a schematic diagram of a conventional power supply circuit. As shown in FIG. 1A, the power supply circuit 1 includes a filter circuit 12 and two sets of synchronous step-down switching power supplies 14 and 16, which are used to filter and convert the input voltage Vin (for example, 12V) into Vo1 (for example, 12V), Vo2 (eg 5V), and Vo3 (eg 3.3V). Please refer to FIG. 1B , which shows the signal waveforms of the input voltage Vin and the output voltages Vo1 , Vo2 , and Vo3 when the power supply circuit 1 enters the shutdown procedure. The so-called shutdown procedure means that when the input voltage Vin starts to drop below the under-voltage lockout threshold (UVLOth), then the switching power supplies 14 and 16 reduce the output voltages such as Vo2 and Vo3 to the shutdown threshold ( generally 0V), and stop converting the input voltage Vin to output voltages Vo2 and Vo3.

Referring to FIG. 1B and referring to FIG. 1A , at the time point t0 when the shutdown procedure starts, the input voltage Vin starts to drop, and at the time point t1, the input voltage Vin drops to the undervoltage lockout threshold value UVLOth, so the switch mode power supply 14 And 16 start to reduce the output voltage Vo2 and Vo3. If the output voltage Vo3 (3.3V) supplies light load or no load at this time, and Vo2 (5V) supplies heavy load, the load current I2 is greater than I3, so the charge stored in the output capacitor Co3 will discharge faster than the charge stored in The charge discharge speed of the output capacitor Co2 is slow, resulting in the situation that the output voltage Vo3 (supply 3.3V during normal operation) is greater than the output voltage Vo2 (supply 5V during normal operation) after the time point t2 during the shutdown process (the arrow in the figure As indicated by the sign), this result violates the norms of general power supply circuits.

FIG. 2A and FIG. 2B show another schematic diagram of a conventional power supply circuit for improving the aforementioned problems of the prior art. As shown in FIG. 2A , the power supply circuit 10 is coupled to the diode D1 between the output voltages Vo2 and Vo3 of the two sets of synchronous step-down switching power supplies 14 and 16, and its positive end is electrically connected to the power supply during normal operation. The lower voltage output voltage Vo3 (3.3V) is electrically connected to the negative terminal to supply the higher voltage output voltage Vo2 (5V) during normal operation. Please refer to FIG. 2B , which shows the signal waveforms of the input voltage Vin and the output voltages Vo1 , Vo2 , and Vo3 when the power supply circuit 10 enters the shutdown process. As shown in FIG. 2B , at the time point t0 when the shutdown procedure starts, the input voltage Vin starts to drop. At the time point t1', the input voltage Vin drops to the undervoltage lockout threshold value UVLOth, and the switch mode power supplies 14 and 16 start to Reduce the output voltage Vo2 and Vo3. Still assuming that the load current I2 is greater than I3, between the time point t2' and the time point t3', due to the effect of the diode D1, when the output voltage Vo3 is greater than the output voltage Vo2 plus the forward voltage of the diode D1, it is stored in the output The charge of the capacitor Co3 will flow into the output voltage Vo2 terminal through the diode D1, and will be consumed by the load current I2 of Vo2 until the time point t3', the output voltage Vo2 drops below the forward voltage of the diode D1; and the time After point t4', the output voltage Vo3 drops to 0V. Compared with the prior art shown in Figure 1A, although the prior art shown in Figure 2A can solve its problems, it still has several disadvantages:

(1) Adding a diode increases the manufacturing cost.

(2) Between the time points t2' and t4', the output voltage Vo3 is still greater than the output voltage Vo2, but the amplitude is small, but it may still cause operation errors on the load circuit.

(3) The forward voltage of the diode D1 should be as small as possible, so choosing a diode with such a very small forward voltage will increase a lot of manufacturing costs.

In view of this, the present invention aims at the deficiencies of the above-mentioned prior art, and proposes a switching power supply and its control method, which can be used without using the above-mentioned special diode D1, and in three groups of output voltages (Vo1, Vo2, and Vo3 ), regardless of whether the supplied load circuit is light-loaded or heavy-loaded, the output voltage Vo1>Vo2>Vo3 can be met in the shutdown procedure, and the switching power supply has a monotonic output voltage waveform and its control methods.

Contents of the invention

One of the objectives of the present invention is to overcome the deficiencies and defects of the prior art and provide a switching power supply.

Another object of the present invention is to provide a control circuit and method for a switching power supply.

Another object of the present invention is to provide a power supply circuit.

In order to achieve the above object, from one point of view, the present invention provides a switching power supply, comprising: a power stage circuit, receiving an input voltage and converting it into an output voltage; an error amplifier circuit, and The feedback signal related to the output voltage is compared with a reference signal to generate an error amplification signal, wherein the reference signal decreases with the decrease of the input voltage after the switch mode power supply is confirmed to enter the shutdown process; and A pulse width modulation (PWM) signal generation circuit generates a PWM signal to control the power stage circuit according to the error amplification signal.

From another point of view, the present invention also provides a switching power supply control method, the switching power supply is used to convert an input voltage into an output voltage, the switching power supply control method includes: The feedback signal related to the output voltage is compared with a reference signal to generate an error amplification signal; according to the error amplification signal, a PWM signal is generated to control the conversion between the input voltage and the output voltage; confirming that the switching power supply Whether the supplier enters the shutdown procedure; and after the shutdown procedure is confirmed, the reference signal is decreased as the input voltage drops, so that the output voltage decreases accordingly.

From another point of view, the present invention also provides a power supply circuit for converting an input voltage into a plurality of output voltages, the power supply circuit includes: a first voltage output circuit, according to the input voltage, output a first output voltage, wherein the first voltage output circuit can be, for example, a filter circuit, the level of the first output voltage is substantially the same as the input voltage; and at least one switching power supply, which converts the input voltage to At least one second output voltage, the switching power supply includes: a power stage circuit, receiving an input voltage and converting it into an output voltage; an error amplifier circuit, combining the feedback signal related to the output voltage with the A reference signal is compared to generate an error amplification signal, wherein, after the switch mode power supply is confirmed to enter the shutdown process, the reference signal decreases with the decrease of the input voltage; and a pulse width modulation (pulsewidthmodulation, PWM ) The signal generating circuit generates a PWM signal to control the power stage circuit according to the error amplification signal.

From another point of view, the present invention also provides a control circuit of a switching power supply, the switching power supply is used to receive an input voltage and convert it into an output voltage, the control circuit includes: an error amplifier A circuit for comparing a feedback signal related to the output voltage with a reference signal to generate an error amplification signal, wherein the reference signal decreases with the input voltage after the switch mode power supply is confirmed to enter a shutdown procedure and descending; and a pulse width modulation (PWM) signal generating circuit, which generates a PWM signal to control the conversion between the input voltage and the output voltage according to the error amplification signal.

In one of the implementation forms, the switching power supply or its control circuit preferably further includes a reference signal generating circuit, which generates a first reference signal according to the input voltage, and the first reference signal decreases as the input voltage decreases. drop, and wherein the error amplifier circuit uses the first reference signal as the aforementioned reference signal to compare with the feedback signal after confirming that it enters the shutdown procedure. Wherein, the error amplifier circuit uses a second reference signal as the aforementioned reference signal to compare with the feedback signal during normal operation.

In a preferred implementation form, the relationship between the first reference signal and the input voltage is:

$\mathrm{<span\; class="notranslate">\; Vref</span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; Vin</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Vofs</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}$

Wherein, Vref1 is the first reference signal, Vin is the input voltage, Vofs is a preset offset, and K is a constant or function.

Among the above-mentioned implementation types, K, for example, is preferably:

$\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; Vthh</span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Vthl</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; Vref</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}$

Wherein, Vref2 is the second reference signal, Vthh1 is a high threshold voltage, and Vthl1 is a low threshold voltage. When the input voltage drops to the high threshold voltage Vthh1, it is confirmed to enter the shutdown process and the error amplifier circuit uses the first The reference signal as the aforementioned reference signal is compared with the feedback signal, and when the input voltage drops to the low threshold voltage Vthl1, the first reference signal Vref1 drops to a shutdown threshold. Also, the preset offset Vofs can be set as the low threshold voltage Vthl1.

The above power supply circuit may include a plurality of switching power supplies to generate multiple output voltages, and the time point when the higher output voltage generated by the switching power supply drops to the shutdown critical value shall not be later than the later The point at which the low output voltage drops below the shutdown threshold.

The following will be described in detail through specific embodiments, so that it is easier to understand the purpose, technical content, characteristics and effects of the present invention.

Description of drawings

FIG. 1A shows a schematic diagram of an existing power supply circuit;

FIG. 1B illustrates the waveforms of the signals in FIG. 1A by way of example;

FIG. 2A shows another schematic diagram of a conventional power supply circuit;

Fig. 2B shows the waveform of each signal in Fig. 2A by way of example;

Fig. 3 shows an application embodiment of the present invention;

FIG. 4A shows a more specific embodiment of the switching power supply of the present invention;

FIG. 4B is a waveform diagram, showing for example each signal waveform in the embodiment of FIG. 4A;

FIG. 4C is a waveform diagram, for example showing that the switching power supplies 24 and 26 can be properly set to ensure that the output voltage Vo1>Vo2>Vo3 requirements are met;

5A and 5B show an embodiment of a power stage circuit;

Figure 6 shows an embodiment of an error amplifier circuit;

Figures 7A-7C summarize the concept of the present invention.

Explanation of symbols in the figure

1, 10, 20 power supply circuit

12 filter circuit

14, 16, 24, 26 switching power supplies

260 control circuit

262 reference signal generation circuit

264 error amplifier circuit

266PWM signal generation circuit

268 power stage circuit

270 feedback circuit

I2, I3 load current

L inductance

SWL lower bridge switch

SWU upper bridge switch

t0, t1, t2, t1', t2', t3', t4', t1", t2", t3", t4", t5"

UVLOth voltage is too low lockout threshold

Vfb feedback signal

Vin input voltage

Vo1, Vo2, Vo3 output voltage

Vref reference signal

Vref1 first reference signal

Vref2 Second reference signal

Vthh1, Vthh2 high threshold voltage

Vthl1, Vthl2 low threshold voltage

detailed description

Please refer to Fig. 3, which shows the first embodiment of the present invention. This embodiment shows an application architecture utilizing the present invention, but the present invention is not limited to be applied to this architecture. As shown in FIG. 3, in the exemplary application framework, the power supply circuit 20 is used to convert the input voltage Vin into multiple output voltages Vo1, Vo2, and Vo3, wherein the input voltage Vin is, for example but not limited to, 12V, and the output The voltages Vo1, Vo2, and Vo3 are, for example but not limited to, 12V, 5V, and 3.3V. The power supply circuit 20 includes a filter circuit 12 , a switching power supply 24 , and a switching power supply 26 . Wherein, the filter circuit 12 converts the input voltage Vin into an output voltage Vo1 without stepping down after filtering; the switching power supply 24 and the switching power supply 26 respectively convert the input voltage Vin into output voltages Vo2 and Vo3. In the above example application structure, the level of the output voltage Vo1 and the input voltage Vin are substantially the same, and the filter circuit 12 is used to filter the noise, but if the noise is not concerned, the filter circuit 12 can also be omitted. Different from the prior art, the switch mode power supply 24 and the switch mode power supply 26 in this embodiment have the function of shutdown control. When the input voltage Vin drops and enters the shutdown procedure, according to the input voltage Vin, the The output voltages Vo2 and Vo3 drop to the shutdown critical value (for example, 0V), so that the output voltage Vo3 (3.3V) generated by the switching power supply 26 drops to the shutdown critical value (0V), no later than the output voltage The time point when Vo1 (12V) and Vo2 (5V) drop to the shutdown critical value, and the time point when the output voltage Vo2 (5V) generated by the switching power supply 24 drops to the shutdown critical value (0V), no later than the output The point in time when the voltage Vo1 (12V) drops to the shutdown critical value. That is to say, for a switching power supply that provides a smaller output voltage, in the shutdown process, the time when the output voltage drops to the shutdown critical value is no later than the time when the larger output voltage drops to the shutdown critical value, so as to meet the general The specification of the power supply circuit does not need to use the special diode D1 of the prior art, which can reduce the manufacturing cost. Moreover, if only one of the switching power supply 24 and the switching power supply 26 has the function of shutdown control, it certainly falls within the scope of the present invention.

Please refer to FIG. 4A, which shows a more specific embodiment of the switching power supply 24 or 26, illustrating how the switching power supply 24 or 26 adaptively reduces the output voltage Vo2 or Vo3 to the shutdown threshold according to the input voltage Vin. value. For convenience of description, the following will take the switching power supply 26 as an example. As shown in FIG. 4A, the switching power supply 26 includes a control circuit 260, a power stage circuit 268 and a feedback circuit 270, wherein the feedback circuit 270 can be omitted depending on the circumstances. In this case, the output voltage Vo3 is directly used as the feedback signal. Vfb. The control circuit 260 includes a reference signal generating circuit 262 , an error amplifier circuit 264 , and a PWM signal generating circuit 266 . The reference signal generating circuit 262 generates a first reference signal Vref1 according to the input voltage Vin. The error amplification circuit 264 compares the feedback signal Vfb with the first reference signal Vref1 or the second reference signal Vref2 to generate an error amplification signal and output it to the PWM signal generation circuit 266 . The PWM signal generation circuit 266 generates a PWM signal to control the upper and lower switches SWU and SWL in the power stage circuit 268 according to the error amplification signal, and generates an output voltage Vo2 through the inductor L.

In normal operation, the error amplifier circuit 264 compares the feedback signal Vfb with the second reference signal Vref2, and adjusts the feedback signal Vfb to the level of the second reference signal Vref2 according to the feedback balance mechanism of the loop to control the output voltage Vo3. When the power supply circuit enters the shutdown process, the input voltage Vin drops, and the first reference signal Vref1 generated by the reference signal generating circuit 262 also gradually drops. When the first reference signal Vref1 is lower than the reference signal in normal operation, that is, the second When the reference signal Vref2 is used, the first reference signal Vref1 replaces the second reference signal Vref2 as the comparison reference of the error amplifier circuit 264, that is, the error amplifier circuit 264 compares the feedback signal Vfb with the first reference signal Vref1, and the feedback balance mechanism of the loop The feedback signal Vfb is adjusted to the level of the first reference signal Vref1, so that the output voltage Vo3 is adaptively adjusted as the input voltage Vin decreases, and gradually decreases to a shutdown critical value (for example, 0V).

In detail, please refer to FIG. 4B , which shows various signal waveforms in the embodiment of FIG. 4A as an example. As shown in FIG. 4B , when the power supply circuit enters the shutdown process, the input voltage Vin starts to decrease at time t0 , and according to the present invention, the first reference signal Vref1 also decreases along with the input voltage Vin. When the input voltage Vin drops to the preset high threshold voltage Vthh1, or when the first reference signal Vref1 drops to the second reference signal Vref2, the error amplifier circuit 264 selects the first reference signal Vref1 instead of the second reference signal Vref2 for comparison. Reference, and the feedback balance mechanism of the loop adjusts the feedback signal Vfb to the level of the first reference signal Vref1, so the output voltage Vo3 drops correspondingly, so that at the time point t3", that is, the input voltage Vin drops to the lower critical voltage When Vthl1, the output voltage Vo3 is reduced to 0V.

In FIG. 4B, the high threshold voltage Vthh1 corresponds to the input voltage Vin when the first reference signal Vref1 is equal to the second reference signal Vref2. When the voltage Vin is lower than the upper threshold voltage Vthh1, it is confirmed that the power supply circuit enters the shutdown process, and the second reference signal Vref2 is replaced by the first reference signal Vref1, but when the input voltage Vin is not lower than the upper threshold voltage Vthh1, the power supply circuit is considered to be The supply circuit is still operating normally, and the second reference signal Vref2 is still used as a comparison reference for the error amplifier circuit 264 . But of course, the above is only a preferred embodiment rather than a limitation. If the high threshold voltage Vthh1 is not set, and the second reference signal Vref2 is replaced by the first reference signal Vref1 directly from time t0, it should also belong to the present invention. range.

In addition, the purpose of setting the low threshold voltage Vthl1 is to control the time point of t3", so that the time for the output voltage Vo3 to drop to 0V is earlier than the time for the input voltage Vin to reach or be lower than the low voltage lock threshold value UVLOth, because when the input When the voltage Vin reaches or is lower than the low voltage locking threshold UVLOth, the switching power supply 26 will stop the control operation, so setting Vthl1 higher than UVLOth can ensure that the output voltage Vo3 is reduced to 0V under control, and It must also be earlier than the time when the output voltage Vo1 drops to 0V. But of course, the above is only a preferred embodiment rather than a limitation, such as not setting the low threshold voltage Vthl1, or setting the low threshold voltage Vthl1 to be lower than UVLOth Any value should also belong to the scope of the present invention.

In addition, the figure shows that the second reference signal Vref2 is a fixed value, which is also an example and not a limitation; depending on different applications, the second reference signal Vref2 can also be a variable value. For example, in some application environments, the circuit connected to the output terminal will generate a signal, which is fed back to the power supply circuit to dynamically change the output voltage. In this case, the second reference signal Vref2 will change dynamically.

According to the above control mechanism, the first reference signal Vref1 is set as, for example but not limited to:

$\mathrm{<span\; class="notranslate">\; Vref</span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; Vin</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Vofs</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}$

Wherein, Vofs is a preset offset, and K is a constant or a function, which can be designed according to needs; in the embodiment of FIG. 4B, Vofs is set as the low threshold voltage Vthl1, which is to determine the time point t3", and the constant K determines Vref1 The rate of decline, that is, the time t1" that determines the interleaving of Vref1 and Vref2, for example, in the embodiment of Figure 4B, K is set to:

$\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; Vthh</span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Vthl</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; Vref</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}$

But of course, as mentioned above, both Vofs and K can be set to other values, for example, Vofs can be equal to UVLOth, or any value greater than zero, or zero; K can be set to any rate of decline, etc.

In the example application architecture shown in FIG. 3 , two switching power supplies 24 and 26 are included, and it is hoped that the output voltage Vo1>Vo2>Vo3 can be achieved during the shutdown procedure. In this case, please refer to FIG. 4C. For example, in the switching power supply 26, Vofs can be set to Vthl1, K can be set to (Vthh1-Vthl1)/Vref2, and in the switching power supply 24, Vofs Set to Vthl2, set K to (Vthh2-Vthl2)/(Vref2'), and Vthh1>Vthh2>Vthl1>Vthl2>UVLOth, where Vref2' is the switching power supply 24 in normal operation. The reference for the error amplifier circuit in the supply 24. Since Vthh1, Vthh2, Vthl1, and Vthl2 respectively determine the time points t1", t2", t3", and t4", and the output voltage Vo1 must drop to 0 after the input voltage Vin is lower than UVLOth, this arrangement can ensure During the procedure, the requirement of output voltage Vo1>Vo2>Vo3 is achieved.

5A and 5B show two more specific embodiments of a power stage circuit (such as the power stage circuit 268 in the switching mode power supply 26). As shown in FIGS. 5A and 5B , the power stage circuit 268 is, for example but not limited to, a buck switching power supply or a buck-boost switching power supply.

FIG. 6 shows an embodiment of the error amplifier circuit 264 . As shown in the figure, the error amplifier circuit 264 can have three input terminals, for example, and compares the highest negative input signal with the positive input signal, in other words, compares the absolute values of the feedback signal Vfb and (the reference signal Vref1 and the reference signal Vref2 the lower one) for comparison. In addition, the error amplifier circuit 264 can also be implemented in other ways, and is not limited to the illustrated embodiment.

Figures 7A and 7B summarize the concept of the present invention. In the switching power supply, the error amplifier circuit (such as the error amplifier circuit 264 in the switching power supply 26) compares the feedback signal Vfb with the reference signal Vref, and the feedback balance mechanism of the loop will adjust the feedback signal Vfb to the level of the reference signal Vref to control the output voltage. The concept of the present invention is that after confirming that the shutdown procedure is entered (the confirmed time point can be the start of the shutdown procedure, such as t0 in Figure 4B, or after a safe interval, such as t1" in Figure 4B), make the reference signal Vref Decrease gradually to control the output voltage to gradually drop to 0. The falling rate of the reference signal Vref and the time point of falling to 0V can be set as required. As for the method of generating the waveform of the reference signal Vref in Figure 7B, the aforementioned implementations The example is only an example and not a limitation. After confirming that the shutdown procedure is entered, any method of decreasing the waveform of the reference signal Vref belongs to the concept of the present invention. In addition, the method of decreasing the reference signal Vref is not limited to a slope that must be decreased. For example, as shown in Fig. 7C descending in steps, or other regular or irregular descending manners also belong to the concept of the present invention.

The present invention has been described above with reference to preferred embodiments, but the above description is only for those skilled in the art to easily understand the content of the present invention, and is not intended to limit the scope of rights of the present invention. Under the same spirit of the present invention, various equivalent changes can be conceived by those skilled in the art. For example, in the circuits of the various embodiments shown, components that do not affect the main meaning of the signal can be inserted, such as other switches; and for example, the positive and negative input terminals of the error amplifier or comparator can be interchanged, and only need to correspond to the signal processing method of the correction circuit That's it. As another example, the undervoltage lockout threshold UVLOth is not a necessary restriction, and the present invention can also be applied to power supply circuits that do not set the undervoltage lockout threshold UVLOth, such as but not limited to dualpower power supply circuit. For another example, the present invention can also be applied to the situation where only a single output voltage is generated, and is not limited to the situation that must be applied to two or more output voltages. All of these can be deduced according to the teaching of the present invention, therefore, the scope of the present invention should cover the above and all other equivalent changes.

Claims (8)

1. a switched power supply, is characterized in that, comprises:

One power stage circuit, receives an input voltage and is converted into an output voltage;

One error amplifying circuit, by the feedback signal relevant with this output voltage compared with a reference signal, amplify signal to produce an error, wherein, in this switched power supply after confirming to enter shutdown programm, this reference signal falls progressively with the decline of input voltage; And

One pulse width modulation signal produces circuit, amplifies signal according to this error, produces pulse width modulation signal and controls this power stage circuit; And;

One reference signal produces circuit, it produces the first reference signal according to this input voltage, this first reference signal declines with input voltage and declines, and wherein this error amplifying circuit compares with this feedback signal using this first reference signal as above-mentioned reference signal after confirmation enters shutdown programm;

Wherein, the relational expression of this first reference signal and this input voltage is:

$Vref1=\frac{Vin-Vofs}{K}$

Wherein, Vref1 is this first reference signal, and Vin is this input voltage, and Vofs is a default bias, and K is a constant or function.

2. switched power supply as claimed in claim 1, wherein, this error amplifying circuit compares with this feedback signal using one second reference signal as above-mentioned reference signal when normal running, and K is:

$K=\frac{Vthh1-Vthl1}{Vref2}$

Wherein, Vref2 is this second reference signal, Vthh1 is a high critical voltage, Vthl1 is a low critical voltage, when this input voltage is reduced to this high critical voltage Vthh1, confirmation enters shutdown programm and this error amplifying circuit compares with this feedback signal using this first reference signal as above-mentioned reference signal, and when this input voltage is reduced to this low critical voltage Vthl1, this first reference signal Vref1 is reduced to a shutdown critical value.

3. a switched power supply control method, this switched power supply, in order to an input voltage is converted to an output voltage, is characterized in that, described switched power supply control method comprises:

By the feedback signal relevant with this output voltage compared with a reference signal, amplify signal to produce an error;

Amplify signal according to this error, produce PWM signal to control the conversion between this input voltage and this output voltage;

Confirm whether this switched power supply enters shutdown programm; And

Through confirm enter shutdown programm after, this reference signal is fallen progressively with the decline of input voltage, with make this output voltage with and fall progressively;

The step that wherein this reference signal falls progressively with the decline of input voltage comprises:

Produce the first reference signal according to this input voltage, this first reference signal declines with input voltage and declines; And

After confirming to enter shutdown programm, compare with this feedback signal using this first reference signal as above-mentioned reference signal; Wherein, the relational expression of this first reference signal and this input voltage is:

$Vref1=\frac{Vin-Vofs}{K}$

Wherein, Vref1 is the first reference signal, and Vin is this input voltage, and Vofs is a default bias, and K is a constant or function.

4. a switched power supply control method, this switched power supply, in order to an input voltage is converted to an output voltage, is characterized in that, described switched power supply control method comprises:

By the feedback signal relevant with this output voltage compared with a reference signal, amplify signal to produce an error;

Amplify signal according to this error, produce PWM signal to control the conversion between this input voltage and this output voltage;

Confirm whether this switched power supply enters shutdown programm; And

Through confirm enter shutdown programm after, this reference signal is fallen progressively with the decline of input voltage, with make this output voltage with and fall progressively;

The step that wherein this reference signal falls progressively with the decline of input voltage comprises:

Produce the first reference signal according to this input voltage, this first reference signal declines with input voltage and declines; And

After confirming to enter shutdown programm, compare with this feedback signal using this first reference signal as above-mentioned reference signal;

Wherein, compare with this feedback signal using one second reference signal as above-mentioned reference signal before entering shutdown programm, and K is:

$K=\frac{Vthh1-Vthl1}{Vref2}$

Wherein, Vref2 is this second reference signal, Vthh1 is a high critical voltage, Vthl1 is a low critical voltage, when this input voltage is reduced to this high critical voltage Vthh1, confirm enter shutdown programm and compare with this feedback signal as above-mentioned reference signal using this first reference signal, when this input voltage is reduced to this low critical voltage Vthl1, this first reference signal Vref1 is reduced to a shutdown critical value.

5. a power supply circuit, in order to an input voltage is converted to multiple output voltage, is characterized in that, described power supply circuit comprises:

One first voltage follower circuit, according to this input voltage, exports one first output voltage; And

At least one switched power supply, this input voltage is converted at least one second output voltage, and described switched power supply comprises:

One power stage circuit, receives an input voltage and is converted into an output voltage;

One error amplifying circuit, by the feedback signal relevant with this output voltage compared with a reference signal, amplify signal to produce an error, wherein, in this switched power supply after confirming to enter shutdown programm, this reference signal falls progressively with the decline of input voltage; And

One pulse width modulation signal produces circuit, amplifies signal according to this error, produces pulse width modulation signal and controls this power stage circuit; And

One reference signal produces circuit, it produces the first reference signal according to this input voltage, this first reference signal declines with input voltage and declines, and wherein this error amplifying circuit compares with this feedback signal using this first reference signal as above-mentioned reference signal after confirmation enters shutdown programm;

Wherein, the relational expression of this first reference signal and this input voltage is:

$Vref1=\frac{Vin-Vofs}{K}$

Wherein, Vref1 is this first reference signal, and Vin is this input voltage, and Vofs is a default bias, and K is a constant or function.

6. power supply circuit as claimed in claim 5, wherein, this error amplifying circuit compares with this feedback signal using one second reference signal as above-mentioned reference signal when normal running, and K is:

$K=\frac{Vthh1-Vthl1}{Vref2}$

Wherein, Vref2 is this second reference signal, Vthh1 is a high critical voltage, Vthl1 is a low critical voltage, when this input voltage is reduced to this high critical voltage Vthh1, confirmation enters shutdown programm and this error amplifying circuit compares with this feedback signal using this first reference signal as above-mentioned reference signal, and when this input voltage is reduced to this low critical voltage Vthl1, this first reference signal Vref1 is reduced to a shutdown critical value.

7. a control circuit for switched power supply, this switched power supply is in order to receive an input voltage and to be converted into an output voltage, and this control circuit comprises:

One error amplifying circuit, by the feedback signal relevant with this output voltage compared with a reference signal, amplify signal to produce an error, wherein, in this switched power supply after confirming to enter shutdown programm, this reference signal falls progressively;

One pulse width modulation signal produces circuit, amplifies signal according to this error, and generation PWM signal controls the conversion between this input voltage and this output voltage; And

One reference signal produces circuit, it produces the first reference signal according to this input voltage, this first reference signal declines with input voltage and declines, and wherein this error amplifying circuit compares with this feedback signal using this first reference signal as above-mentioned reference signal after confirmation enters shutdown programm; Wherein, the relational expression of this first reference signal and this input voltage is:

$Vref1=\frac{Vin-Vofs}{K}$

Wherein, Vref1 is this first reference signal, and Vin is this input voltage, and Vofs is a default bias, and K is a constant or function.

8. the control circuit of switched power supply as claimed in claim 7, wherein, this error amplifying circuit compares with this feedback signal using one second reference signal as above-mentioned reference signal when normal running, and K is:

$K=\frac{Vthh1-Vthl1}{Vref2}$

Wherein, Vref2 is this second reference signal, Vthh1 is a high critical voltage, Vthl1 is a low critical voltage, when this input voltage is reduced to this high critical voltage Vthh1, confirmation enters shutdown programm and this error amplifying circuit compares with this feedback signal using this first reference signal as above-mentioned reference signal, and when this input voltage is reduced to this low critical voltage Vthl1, this first reference signal Vref1 is reduced to a shutdown critical value.