| Current-mode PWM step-down DC/DC switching power supply transform high input voltage into stable low output voltage by controlling the switch tube open and closed time ratio with high efficiency at the same time, the general in more than 75%. DC/DC switching power supply with high integration, small size, low noise and high efficiency, has been widely used in the communication, computer and consumer electronics. As switching power supply stabilize output voltage by controlling the power MOS open time, so the loop frequency domain characteristics of switch power supply cannot be obtained by using Cadence simulation software directly.it brings to the chip design engineers and application engineers a lot of trouble and problems.Paper designs a control circuit for current mode PWM step-down DC/DC switching power supply. Based on current mode PWM step-down DC/DC switching power supply control principle, first of all, From the top of the point of view, analysis the inner current loop and outer voltage loop chip. Some analog circuit module design parameters are given, including the size of the compensation current loop of slope compensation, the transconductance and gain of error amplifier, the slope of the transfer function between current comparator and output of error amplifier. Secondly, From the bottom of the point of view, design some important modules circuits, including error amplifier, current comparator, and the transfer function between the error amplifier and current comparator. Again, according to the external voltage loop of the current mode switching power supply analyzed before, selceting the capacitor and resistor at the output pin of the error amplifier output pin make switch power supply voltage loop stability. Finally, simulating the whole the current mode of switching power supply chip transient simulation by Cadence and Hspice circuit simulation software, we can get stable output voltage. In the design process, the paper made the following innovative work:First, the use of Matlab mathematical tools for analyzing current mode switching power supply current and voltage loop obtain the loop frequency domain characteristics of switch power supply which cannot be obtained by circuit simulation software directly. In the frequency of the point of view, explain the reasons for the instability of current loop and choose the size of slope compensation, when the chip output voltage duty ratio is more than 50% of the time.Second, use an operational transconductance amplifier (OTA) as the error amplifier. Error amplifier work in a wide temperature range from -55℃ to 125℃ with high precision transconductance at the different corner. The output of the error amplifier has an external pin at the outside of the chip which is used to connect the compensation capacitance and resistance. The capacitance and resistance can compensate the voltage loop, to ensure the current mode DC/DC switching power supply chip adjust the high sensitivity to load voltage change.Third, on the circuit design view, the function from the output of error amplifier to the input of current comparator voltage of the comparator have high precision linear relationship. It improves the precision and stability of the output voltage of Buck circuit chip, for chip application engineer, it is easier to select current-mode switching power supply loop cross frequency, error amplifier output pin capacitance and resistance, the output load capacitance\Fourth, on the design of the current comparator, in addition to have the characteristics of high speed, high precision, it not only has wide input range (0V-16V), but also very high input voltage (the highest voltage can reach 38 v). |
PDF Full Text Request