A soft start circuit for PDM DC-DC switching power supply

Abstract

The invention discloses a soft starting circuit for a pulse width modulation DC-DC switch power, comprising a frequency splitting count module, a current threshold module, a current sampling module, a current limiting comparator, a two input or-gate, an oscillator and an RS trigger. The current in a power inductance is sampled when an external switch rectifier is conducted, and the current changes from small to big by stage by limiting the peak value of the current so as to control the step change of the duty ratio, thus leading the output voltage to increase by stage gradually, avoiding surge current in starting procedure and realizing the soft starting function. The invention has the advantages of integrating to the inside of large scale integrated circuit without high capacitance in the outside or a microcontroller to control, limiting the load current directly with good soft starting effect and being suitable for a voltage control mode and a current control mode; the circuit is simple correspondingly by adopting a current gating network; the soft starting time can be regulated automatically according to different loads.

Description

technical field [0001] The invention belongs to integrated circuit chip technology, and in particular relates to a soft start circuit of a pulse width modulation DC-DC switching power supply. Background technique [0002] During the start-up process of the DC-DC switching power supply, as the input voltage rises from the minimum value to the maximum value, the duty cycle of the PWM control signal of the switching tube changes from the maximum value and gradually decreases. Due to the presence of capacitors in the circuit, charging the capacitors is likely to generate a surge current. At this time, the current passing through the switch tube can reach a large level, which is easy to damage the circuit system. In order to avoid this situation, it is necessary to add a soft start circuit to the switching power supply chip. [0003] The soft-start circuit is used to control the duty cycle of the PWM pulse waveform from the minimum value to the value required for normal operatio...

AI Technical Summary

Problems solved by technology

This circuit makes up for the shortcomings of the first two circuits but also brings new deficiencies

First, this kind of circuit sampling voltage control is only suitable for voltage control mode, not for current control mode

Second, due to the use of a resistor divider network, when it is necessary to properly increase the soft-start rising stage to achieve a better soft-start effect, the circuit is too complicated

Third, the soft start time is determined by the digital counting part and the output voltage, and cannot be automatically adjusted according to different load currents

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