Control circuit and bulk DC/DC converter in constant on-time mode

[0001] 1. Field of the Invention

[0002] The present invention relates to a bulk DC/DC converter, and more particularly, to a bulk DC/DC converter in constant on-time mode.

[0003] 2. Description of the Prior Art

[0004] Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional bulk DC/DC converter 100. The bulk DC/DC converter 100 converts an input voltage source V_IN to be an output voltage source V_OUT, wherein the voltage V_OUT is lower than the voltage V_IN. As shown in FIG. 1, the DC/DC converter 100 comprises a control circuit 110, a switch set 120, an inductor L, an output capacitor C_OUT, and a voltage-dividing set 130. The switch set 120 comprises two switches Q₁ and Q₂. The voltage-dividing set 130 comprises two voltage-dividing resistors R_B1 and R_B2. The control circuit 110 comprises a comparator CMP₁, a pulse generator 111, and a drive circuit 112. The operation principles of the bulk DC/DC converter 100 are described as follows.

[0005] The control circuit 110 controls the operation of the DC/DC converter 100 by constant on-time manner. That is, when the control circuit 110 detects the output voltage V_OUT is lower than a predetermined value, the switch Q₁ is turned on for a constant period of time (constant on-time) by the control circuit 110 (while the switch Q₂ is turned off) for allowing the input voltage source V_IN conducting to the inductor L through the switch set 120.

[0006] During the operation of the DC/DC converter 100, the inductor L carries current I_L, and the current I_L flows into the equivalent serial resistor R_E of the output capacitor C_OUT so that the resistor R_E carries voltage V_L reflecting the current I_L. As shown in FIG. 1, the waveform of the voltage V_L is saw-toothed because the switch Q₁ is periodically turned on/off. The comparator CMP₁ receives the feedback voltage V_FB divided from the voltage V_L and the output voltage V_OUT by the resistors R_B1 and R_B2, and compares with a reference voltage V_REF1, so as to determine when to turn on the switch Q₁. More specifically, when the voltage (feedback voltage V_FB) on the negative input end of the comparator CMP₁ is lower than the voltage (reference voltage V_REF1) on the positive input end of the comparator CMP₁, which means the output voltage V_OUT is too low, and the switch Q₁ is needed to be turned on for allowing the input voltage source V_IN to charge the inductor L and the output capacitor C_OUT, the comparator CMP₁ controls the pulse generator 111 to generate a pulse signal P_ON. When the pulse generator 111 is triggered by the comparator CMP₁, the pulse generator 111 generates a pulse signal P_ON with a predetermined duration T_P and predetermined logic. The drive circuit 112 controls the switch set 120 according to the pulse signal P_ON. More particularly, when the drive circuit 112 receives the pulse signal P_ON, the switch Q₁ is driven to turn on for the predetermined duration T_P. In addition, except in the dead time both of the switches Q₁ and Q₂ are turned off, when the switch Q₁ is turned on, the switch Q₂ is turned off; when the switch Q₁ is turned off, the switch Q₂ is turned on. In this way, the control circuit 110 controls the DC/DC converter 100 to operate regularly in constant on-time mode.

[0007] However, not all kinds of capacitors definitely have equivalent serial resistors, and because of the improvement to the manufacture of capacitors, the equivalent serial resistances of the capacitors become smaller, or even do not exist. For example, the multi-layer ceramic capacitor (MLCC) is very similar to an ideal capacitor and therefore the equivalent serial resistor does not exist on the MLCC. Consequently, when the MLCC is utilized as the output capacitor C_OUT, the resistor R_E does not exist, and thus the information of the current I_L cannot be informed to the control circuit 110, causing the control circuit 110 unable to control the DC/DC converter 100 according to the voltage V_L effectively. The control circuit 110 is still able to operate by the feedback of the output voltage V_OUT. However, the phase of the output voltage V_OUT is far behind the phase of the voltage V_L because of the output capacitor C_OUT, which makes the control circuit 110 unable to react to the variation of the output voltage V_OUT in time. For this reason, the DC/DC converter 100 is not able to operate stably in constant on-time mode while utilizing the MLCC as the output capacitor, causing inconvenience.

[0008] The present invention provides a control circuit in constant on-time mode for controlling a DC/DC converter, comprising a comparing circuit comprising a comparator comprising a positive input end for receiving a control voltage; a negative input end for receiving a feedback voltage divided from an output voltage of the DC/DC converter; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal for controlling a switch set of the DC/DC converter.

[0009] The present invention further provides a bulk DC/DC converter in constant on-time mode, comprising a switch set coupled between an input voltage source and ground; an inductor coupled to the switch set for receiving the input voltage source through the switch set; an output end coupled to the inductor for outputting an output voltage; an output capacitor coupled between the capacitor and the ground; a voltage-dividing set coupled between the output end and the ground for generating a feedback voltage according to the output voltage; and a control circuit, comprising a comparing circuit comprising a comparator comprising a positive input end for receiving a control voltage; a negative input end coupled to the voltage-dividing set for receiving the feedback voltage; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal so as to transmit the input voltage source to the inductor through the switch set.

[0010] These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

[0013] Please refer to FIG. 2. FIG. 2 is a diagram illustrating a bulk DC/DC converter 200 of the present invention. The bulk DC/DC converter 200 converts an input voltage source V_IN to be an output voltage source V_OUT, wherein the voltage V_OUT is lower than the voltage V_IN. As shown in FIG. 2, the DC/DC converter 200 comprises a control circuit 210, a switch set 220, an inductor L, an output capacitor C_OUT, and a voltage-dividing set 230. The switch set 220 comprises two switches Q₁ and Q₂. The voltage-dividing set 230 comprises two voltage-dividing resistors R_B1 and R_B2. The control circuit 210 comprises a pulse generator 211, a drive circuit 212, a comparing circuit 213, and an amplifying circuit 214. The comparing circuit 213 comprises a comparator CMP₂, and a voltage adjusting circuit 215. The voltage adjusting circuit 215 comprises a voltage reference source V_REF2, a constant current source IS, a switch SW, a resistor R_X, and a capacitor C_X. The amplifying circuit 214 comprises an amplifier OP, a voltage reference source V_REF3, a resistor R₁, and a capacitor C₁. The switch SW comprises a first end 1, a second end 2, and a control end C. It is noticeable that the output capacitor C_OUT utilized in the DC/DC converter 200 is assumed to not have the equivalent serial resistor. The operation principles of the bulk DC/DC converter 200 are described as follows.

[0014] The control circuit 210 controls the operation of the DC/DC converter 200 by constant on-time manner. That is, when the control circuit 210 detects the output voltage V_OUT is lower than a predetermined value, the switch Q₁ is turned on for a constant period of time (constant on-time) by the control circuit 210 (while the switch Q₂ is turned off) for allowing the input voltage source V_IN conducting to the inductor L through the switch set 220.

[0015] Because the output capacitor C_OUT used in the DC/DC converter 200 is assumed to have no equivalent serial resistor, the information about the current carried by the inductor L is not able to feed back to the control circuit 210. Thus, the control circuit 210 is designed to control the DC/DC converter 200 to operate regularly without the equivalent serial resistor.

[0016] In the amplifying circuit 214, the amplifier OP receives the feedback voltage V_FB through the voltage-dividing set 230 dividing the output voltage V_OUT, and the received voltage V_FB is amplified through the resistor R₁ and the capacitor C₁ to be the amplified feedback voltage V_FBO. In addition, the voltage reference source V_REF3 provides a reference voltage V_REF3.

[0017] In the comparing circuit 213, the comparator CMP₂ compares the control voltage (V_C) on its positive input end and the amplified feedback voltage V_FBO on its negative input end, and accordingly outputs a comparing signal S_C. The voltage reference source V_REF2 provides a reference voltage V_REF2. Thus, the control voltage V_C equals to the reference voltage V_REF2 before being adjusted by the voltage adjusting circuit 215, and is smaller than the reference voltage V_REF2 after being adjusted by the voltage adjusting circuit 215. More specifically, when the amplified feedback voltage V_FBO is lower than the control voltage V_C, which means the output voltage V_OUT is too low, and the switch Q₁ is needed to be turned on for allowing the input voltage source V_IN to charge the inductor L and the output capacitor C_OUT, the comparator CMP₂ generates the comparing signal S_C to trigger the pulse generator 211 to generate the pulse signal P_ON. When the pulse generator 211 is triggered by the comparator CMP₁, the pulse generator 211 generates a pulse signal P_ON with a predetermined duration T_P and predetermined logic. The drive circuit 212 controls the switch set 220 according to the pulse signal P_ON. More particularly, when the drive circuit 212 receives the pulse signal P_ON, the switch Q₁ is driven to turn on for the predetermined duration T_P and the input voltage source V_IN conducts through the turned-on switch Q₁ to charge the inductor L and the output capacitor C_OUT for raising the output voltage V_OUT. Meanwhile, the pulse signal P_ON controls the switch SW to turn on for conducting the constant current source IS for lowering the control voltage V_C. After the predetermined duration T_P of the pulse signal P_ON, the switch SW is turned off so that the constant current source IS cannot discharge the resistor R_X and the capacitor C_X. Instead, the voltage reference source V_REF2 charges the resistor R_X and the capacitor C_X for raising the control voltage V_C. By repeatedly executing the above actions, the control voltage V_C can be of the saw-toothed waveform.

[0018] In other words, the inductor L starts to be charged at the beginning of the pulse signal P_ON and the current I_L increases, and the inductor L starts to be discharged at the end of the pulse signal P_ON and the current I_L decreases. Simply speaking, in the present invention, the pulse signal P_ON is used to simulate the variation of the current I_L on the inductor L and is fed back to the comparing circuit 213 to correspondingly adjust the control voltage V_C, i.e. the control voltage V_C varies in response to the variation of the current I_L. In this way, the control circuit 210 is capable of stably controlling the DC/DC converter 200 without the equivalent serial resistor R_E.

[0019] To sum up, the control circuit of the present invention, by utilizing the pulse signal to simulate the change of the current on the inductor, is still capable of stably controlling the bulk DC/DC converter when there is no equivalent serial resistor, providing great convenience.

[0020] Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.