CN108155627B - An Auxiliary Control Circuit for Low Dropout Buck Converter - Google Patents

Abstract

An auxiliary control circuit for a low-dropout BUCK converter belongs to the technical field of analog circuit power management. Including a first comparator, a second comparator, an RS latch and a NOR gate, the first input terminal and the second input terminal of the first comparator are respectively connected to the input terminal and the output terminal of the BUCK converter; the second comparator The first input end and the second input end of the BUCK converter are respectively connected to the input end of the BUCK converter and the switch node, and its control signal is the inversion signal of the output signal of the first comparator; the S input end of the RS latch is connected to the first The output terminal of the comparator, its R input terminal is connected to the output terminal of the second comparator; the two input terminals of the NOR gate are respectively connected to the S input terminal and Q output terminal of the RS latch, and its output terminal outputs the control signal BUCK conversion device. The auxiliary control circuit provided by the invention can prevent the BUCK converter from entering the failure state when the difference between the input voltage and the output voltage is small, thereby ensuring the stability of the output voltage of the BUCK converter.

Description

An Auxiliary Control Circuit for Low Dropout Buck Converter

technical field

The invention belongs to the technical field of analog circuit power management, and in particular relates to an auxiliary control circuit for a low-dropout BUCK converter, which is used for auxiliary control of the BUCK loop after it fails.

Background technique

In the field of power management circuits, the BUCK converter is a very commonly used DC-DC converter. Compared with low-dropout linear regulators (LDOs), BUCK converters are more efficient and have broad commercial value. BUCK uses an inductor as an energy storage element, and uses two power tubes to periodically switch the inductor between two different charging states; using the volt-second balance theorem, under different duty cycles, BUCK can stabilize the output voltage at different values.

The control methods of BUCK are divided into hysteresis mode, voltage mode, current mode, COT and so on. Among them, the voltage mode and current mode can completely fix the working frequency. Both the voltage mode and the current mode BUCK compare the divided voltage of the output voltage with a fixed reference through a differential amplifier to obtain a feedback loop; the BUCK circuit automatically adjusts the duty cycle according to the feedback loop during operation, and finally obtains a stable output voltage. Generally speaking, the BUCK circuit will introduce discontinuous operation mode (DCM) to improve the working efficiency under light load. The working principle of DCM is to turn off the upper and lower power transistors at the same time when the inductor current crosses zero until the next cycle begins.

For BUCK, the transient jump means a change in the duty cycle. When the load jumps from light load to heavy load, the output voltage will drop. On the contrary, when the heavy load jumps to light load, the output voltage will rise, and there is a problem that the output voltage is too high. To put it simply, if the voltage difference between the input and output voltage of BUCK is small, the output voltage of BUCK may become higher than the input voltage during the load jump process, which may cause the loop to enter an out-of-control state, and conventional control will fail. . Previous solutions often avoid this situation from the design index, or reduce the output voltage overshoot value through transient enhancement, but so far, there is no circuit solution that directly corrects this situation.

Contents of the invention

Aiming at the problem that the loop is out of control when the above-mentioned BUCK converter has a low input-output pressure difference and a heavy load jumps to a light load, resulting in a large oscillation of the output voltage and inductor current, the present invention proposes an auxiliary control circuit for the input and output of the BUCK converter. When the voltage difference is small, by comparing and detecting the input voltage and output voltage of the BUCK converter, it is judged whether the system loop is in an abnormal state. If the output voltage is higher than the input voltage, the upper and lower tubes are turned off at the same time, and the BUCK converter stops working; There are two conditions to judge during normal operation, so as to ensure that the circuit can perform better when jumping from heavy load to light load or very light load.

Technical scheme of the present invention is:

An auxiliary control circuit for a low dropout BUCK converter, comprising

a first comparator, the first input end and the second input end of which are respectively connected to the input end and the output end of the BUCK converter;

A second comparator, the first input terminal and the second input terminal of which are respectively connected to the input terminal of the BUCK converter and the switch node, and the control signal thereof is an inversion signal of the output signal of the first comparator;

RS latch, its S input terminal is connected to the output terminal of the first comparator, and its R input terminal is connected to the output terminal of the second comparator;

The NOR gate, its two input terminals are respectively connected to the S input terminal and the Q output terminal of the RS latch, and its output terminal outputs a control signal to connect to the BUCK converter.

Specifically, the auxiliary control circuit is used for a BUCK converter of DCR sampling, and the BUCK converter of DCR sampling includes a DCR capacitor, and a voltage is further provided between the output terminal of the second comparator and the RS latch. The short pulse generating circuit is used to generate a short pulse signal, its input terminal is connected to the output terminal of the second comparator, and its output signal is used to control the DCR capacitor on the one hand, and is connected to the RS latch on the other hand the R input.

Specifically, a timing circuit is also provided between the first comparator and the RS latch, the timing circuit includes a shift register and an OR gate, the clock signal input end of the shift register is connected to a clock signal, Its enable terminal is connected to the undervoltage enable signal, its D input terminal is connected to the output terminal of the first comparator, and its first output terminal Q1 and second output terminal Q2 are connected to the two input terminals of the OR gate, so The output end of the OR gate is connected to the S input end of the RS latch.

The beneficial effects of the invention are: the auxiliary control circuit provided by the invention can prevent the BUCK converter from entering the failure state when the difference between the input voltage and the output voltage is small, and ensure the stability of the output voltage of the BUCK converter.

Description of drawings

FIG. 1 is a schematic structural diagram of an auxiliary control circuit for a low dropout BUCK converter provided by the present invention in an embodiment.

Fig. 2 is a schematic diagram of the structure of the current mode buck converter.

Fig. 3 is the timing diagram of the runaway instant of the BUCK converter.

Figure 4 is a runaway timing diagram of the BUCK converter.

Fig. 5 is a schematic diagram of current backflow of a BUCK converter.

FIG. 6 is a timing diagram of the BUCK converter corrected by the auxiliary control circuit proposed by the present invention.

Detailed ways

The present invention will be further elaborated below in conjunction with the accompanying drawings and specific embodiments.

The auxiliary control circuit proposed by the present invention is used to adjust the BUCK converter when the loop is out of control when the output voltage and the inductor current oscillate greatly when the output voltage and the inductor current are greatly oscillated, so that the output voltage of the BUCK converter is low. After the over-high loop is out of control, the BUCK system can stably return to the normal working state.

As shown in Figure 1, the present invention includes a first comparator, a second comparator, an RS latch, and a NOR gate. In order to make the input common mode range of the first comparator meet the requirements, a source-level comparator is generally used. The first comparator The first input end and the second input end of the comparator are respectively connected to the input end and the output end of the BUCK converter; the first input end and the second input end of the second comparator are respectively connected to the input end and the switch node of the BUCK converter, which The control signal is the inverted signal of the output signal of the first comparator; the S input end of the RS latch is connected to the output end of the first comparator, and its R input end is connected to the output end of the second comparator; The two input terminals are respectively connected to the S input terminal and the Q output terminal of the RS latch, and its output terminal outputs the control signal BUCK converter.

Figure 2 is a schematic diagram of the structure of the current mode buck converter, where V _IN is the power supply voltage, that is, the input voltage of the buck converter, V _O is the output voltage, V _hs is the driving voltage of the upper power tube, and V _ls is the lower power tube Drive voltage, V _SW is the switch node voltage. It is worth noting that the auxiliary control circuit proposed by the present invention is also applicable to the voltage model buck converter. The present invention will be described in detail below by taking DCR sampling (inductance parasitic series resistance sampling) as an example in the current mode BUCK converter.

Figure 3 shows the timing diagram of the buck converter at the moment of runaway. From figure 3, we can see the working state of the current mode buck converter in the runaway state. When the load jumps from heavy load to light load, the output The voltage V _O rises rapidly, and the inductor current I _L drops rapidly. At this time, the loop control mode of the BUCK converter will be converted from continuous control mode (CCM) to discontinuous control mode (DCM). Under the control of the normal discontinuous control mode DCM, when the inductor current I _L crosses zero in a certain period, the upper and lower power transistors in the BUCK converter will be turned off at the same time. The capacitor supplies power directly to the load. When the output capacitor directly supplies power to the load, the output voltage V _O will drop, and the inductor current _IL will remain near zero. However, if the BUCK converter works with a small input-output voltage difference, as shown in Figure 4, the output voltage V _O will exceed the input voltage V _IN by 0.7V or more during the transition from heavy load to light load. In the BUCK system, the change direction of the inductor current _IL is related to the voltage difference between its two ends, that is:

As shown in Figure 5, when both the upper and lower power transistors are turned off, the voltage at the left end of the inductor is the switch node voltage V _SW , and the voltage at the right end is the output voltage V _O . When the output voltage V _O is higher than the input voltage V _IN , the inductor current _IL will pour into the switch node, and the charge will be discharged through the parasitic body diode of the upper power transistor, so the voltage V _SW at the switch node will increase to the input voltage V _IN plus the power tube parasitic body diode forward voltage (about 0.7V), that is (V _IN +0.7V). At this time, if the output voltage V _O is higher than V _IN +0.7V, the discontinuous control mode DCM will appear out of control, because the inductor current _IL that would have been maintained at zero current will be reduced due to the voltage at the right end of the inductor being higher than the voltage at the left end Enhanced backfeeding, and getting bigger and bigger over time. Until the backflow current is too large, the charge on the output capacitor is quickly drawn away, and the output voltage V _O drops below V _IN +0.7V. After the output voltage V _O is lower than V _IN +0.7V, the reverse inductor current I _L starts to decrease, but there is still a certain current value, and the charge on the output capacitor is continuously drawn away, resulting in the output voltage V _O being too low, and the BUCK loop The circuit starts to pull up the output voltage V _O through feedback, but causes the output voltage V _O to rebound too high, and finally forms the long-term runaway waveform shown in Figure 4.

The auxiliary control circuit proposed in this embodiment can effectively eliminate the above out-of-control state. First of all, the auxiliary control circuit does not work when the BUCK converter is working normally. Except for the first comparator used to judge the output voltage V _O and the input voltage V _IN of the BUCK converter, other modules are in the off state, thereby reducing The overall power consumption of the auxiliary control circuit when the BUCK converter is working normally.

In order to prevent the auxiliary control circuit from being triggered by mistake, a timing circuit is added between the first comparator and the RS latch in this embodiment, and the timing of the number of cycles can be formed by a shift register or other structures. The timing circuit in this embodiment Including a shift register and an OR gate, the clock signal input end of the shift register is connected to the clock signal OSC_clk, its enable end is connected to the undervoltage enable signal UVLO, its D input end is connected to the output end of the first comparator, and its first output The terminal Q1 and the second output terminal Q2 are connected to the two input terminals of the OR gate, and the output terminal of the OR gate is connected to the S input terminal of the RS latch.

When the first comparator determines that the output voltage V _O of the BUCK converter is continuously higher than the input voltage V _IN for several switching periods, the auxiliary control circuit starts to work. The purpose of timing here is to ensure that the BUCK system will not accidentally trigger the auxiliary circuit when it is working normally, and to ensure the stability of the system. In fact, if only the function is required, the timing circuit can be removed.

After the auxiliary control circuit starts to work normally, the auxiliary control circuit outputs a control signal connected to the BUCK converter, so that the upper and lower power transistors of the BUCK system are turned off simultaneously. In this example, the control signal of the current mode BUCK is pulled down to ensure that the upper and lower power tubes are continuously turned off. In practice, other methods can be used.

When the auxiliary control circuit starts to work normally, the second comparator also starts to work, and the comparator is used for judging that the current crosses zero. There are two judgment bases for the auxiliary control circuit to end the locked state of the upper and lower tubes of the BUCK converter: 1. The second comparator judges that the current has crossed zero; 2. The first comparator judges that the output voltage V _O falls back below the input voltage V _IN . That is to say, when the auxiliary control circuit is working, its output signal is always valid until one of the above two criteria is satisfied.

If the first criterion is satisfied, and the current mode BUCK circuit uses the DCR sampling scheme, then a short pulse needs to be generated before changing the output signal of the auxiliary control circuit, which is generated by the short pulse generating circuit in Figure 1. In this embodiment, the short pulse generating circuit is arranged between the output terminal of the second comparator and the RS latch, and is used to generate a short pulse signal, and its input terminal is connected to the output terminal of the second comparator, and its output signal on the one hand It is used to control the DCR capacitor, and on the other hand is connected to the R input of the RS latch.

This short pulse opens subsequent switches to bleed off the charge on the DCR sampling capacitor. The two plate voltages of the DCR sampling capacitor C _SENSE are the sampling voltage V _ISNS and the output voltage V _O . The purpose of doing this is to ensure the accuracy of DCR sampling, that is, when the inductor current I _L is zero, the voltage on the DCR sampling capacitor is also zero. After correcting the voltage on the sampling capacitor, the output signal of the auxiliary control circuit becomes invalid. This embodiment uses DCR sampling for illustration. If DCR sampling is not used, this step is unnecessary; if the second criterion is satisfied, the output signal of the auxiliary control circuit is directly changed, and the entire auxiliary control circuit stops working.

The timing diagram of the program proposed in this example is shown in Figure 6. After the load jumps, the output voltage V _O rises rapidly. When the output voltage V _O exceeds the input voltage V _IN , the auxiliary control circuit starts to work and turns off the upper and lower power tubes. At this time, the second comparator starts to work, and after a period of time, it detects that the switch node voltage V _SW drops below the input voltage V _IN , that is, the inductor current _IL crosses zero, then shuts down the work of this module, and outputs a recovery signal. When the load jumps, such as 6A to 2A, the light load at this time is relatively heavy, so before the second comparator detects the current zero crossing, the first comparator will detect that the output voltage V _O drops to the input voltage V Below _IN , the recovery signal is output in advance.

The key point in the present invention is to judge whether the output voltage is too high when the circuit is heavy-loaded or light-loaded by detecting the input and output signals of the BUCK circuit and the signal at the switch node, and use a series of methods to ensure that the output voltage is too high. After the high loop is out of control, the BUCK system can return to the normal working state stably.

Those skilled in the art can make various other specific modifications and combinations based on the technical revelations disclosed in the present invention without departing from the essence of the present invention, and these modifications and combinations are still within the protection scope of the present invention.

Claims (3)

1. a kind of auxiliary control circuit for low voltage difference BUCK converter, which is characterized in that including

First comparator, first input end and the second input terminal are separately connected input terminal and the output of the BUCK converter End；

Second comparator, first input end and the second input terminal are separately connected the input terminal and joint of the BUCK converter Point, control signal are the inversion signal of the first comparator output signal；

RS latch, S input terminal connect the output end of the first comparator, and R input connects second comparator Output end；

And nor gate, two input terminal be separately connected the RS latch S input terminal and Q output, output end it is defeated Control signal connects the BUCK converter out.

2. the auxiliary control circuit according to claim 1 for low voltage difference BUCK converter, which is characterized in that described auxiliary Help control circuit for the BUCK converter of DCR sampling, the BUCK converter of DCR sampling includes DCR capacitor, and described second It is additionally provided with short pulse generation circuit between the output end of comparator and the RS latch, for generating a short pulse signal, Its input terminal connects the output end of second comparator, on the one hand output signal is used to control the DCR capacitor, another party Face connects the R input of the RS latch.

3. the auxiliary control circuit according to claim 1 or 2 for low voltage difference BUCK converter, which is characterized in that institute State and be additionally provided with timing circuit between first comparator and the RS latch, the timing circuit include shift register and/or Door, the clock signal input terminal of the shift register connect clock signal, and enable end connects under-voltage enable signal, and D is defeated Enter the output end that end connects the first comparator, the first output end Q1 connects two described or door with second output terminal Q2 Input terminal, described or door output end connect the S input terminal of the RS latch.