Soft start circuit used for DC-DC converter

Abstract

The invention provides a soft start circuit used for a DC-DC converter, and belongs to the technical field of electronic circuits. In a soft start stage, the slope voltage VSS_OUT is compared with the division voltage VFB of output voltage, so output of the soft start is achieved. The slope voltage VSS_OUT is increased with a quite high slope rate k1, so establishing time of the soft start is shortened. When voltage on two ends of a first capacitor C1 reaches a threshold value voltage of a first NMOS tube M1, the rising slope rate of the slope voltage Vss_out is reduced to k2, thereby protected inductor current against overshooting. After the soft start is finished, i.e., the slope voltage VSS_OUT exceeds the reference voltage VREF, an additional charging branch is started, the grid electrode voltage VSS of a fourth NMOS tube M4 is quickly set to be close to the level of a power supply, the slope voltage VSS_OUT quits, and the reference voltage VREF is input in the positive input end of a comparer. According to the invention, slow rising of output is ensured at the soft start stage; the rising slope rate of the slope voltage Vss_out is quite high in the beginning; the establishing time of the soft start is shortened; and then the rising slope rate is reduced, so the occurrence of the surging current is inhibited.

Description

technical field [0001] The invention belongs to the technical field of electronic circuits, and in particular relates to a soft start circuit for a DC-DC converter. Background technique [0002] With the widespread application of handheld devices and portable electronic products, the demand for power management ICs continues to rise. DC-DC switching power supply is widely used due to the advantages of high conversion efficiency, large output current, small quiescent current, and wide output load range. The switching power supply works by converting the error signal into a duty cycle control signal to drive the switch. In the start-up phase, the error amplifier is in an unbalanced state, making the loop work at a 100% duty cycle, so after the power transistor is turned on, a large surge current will be generated to charge the capacitor. It is possible to damage the switch tube and other components, resulting in abnormalities in the system circuit system. In addition, in pr...

AI Technical Summary

Problems solved by technology

During the start-up phase, since the output voltage is much lower than the set value, V FB well below V REF , at this time the upper power tube will always be turned on with the maximum duty cycle (limited by the minimum off-time of the system) until V FB reach V REF , so a large surge current will be introduced into the output capacitor, resulting in a large overshoot of the output, which may damage the switch tube and other devices, resulting in abnormalities in the system circuit system

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