Patent Application
20080136382
The present invention relates to electronic circuitry and, in particular, to circuitry for voltage overshoot reduction of switch mode power supplies at the end of soft-start.
A high voltage, buck type switch mode regulator has a large voltage overshoot when it comes out of soft-start to normal operation after powerup. The overshoot is particulary high when the voltage regulator has higher input power supply and regulating to a lower voltage.
If the supply voltage is higher, and when pass switch turns ON, it builds up more energy in the external inductor, so that even when the pass switch is turned OFF, the ouput voltage overshoots because of the stored energy in the Inductor.
A conventional circuit diagram of a prior art switch mode regulator is shown in
The prior art circuit of
The other part of the buck regulator is another comparator 28 which compares the fed reference voltage to the feedback voltage derived from the feed back resistors 40 and 42 (feedback device) which monitors the output regulated voltage VOUT.
When the soft-start ramp voltage is equal to the bandgap voltage, the reference voltage switches from the ramp voltage to the bandgap voltage. This is when the problem arises. Even when the bandgap voltage is a few microvolts higher than the ramp voltage after the transition, the pass transistor 32 is allowed to turn ON, thereby increasing the current in inductor 36. Even when the pass transistor 32 is switched OFF, the stored energy in the inductor 36 makes the output regulated voltage over-shoot. This voltage over-shoot is particularly high when the input supply voltage VIN is high and the regulated output voltage VOUT is small.
A reference voltage generator for a switch mode regulator includes: a reference voltage source; a soft-start voltage generator; and a comparator having a directional offset for comparing an output of the reference voltage source with an output of the soft-start voltage generator. Because of the intentional directional offset, the pass switch does not turn ON during the transition from the ramp voltage to reference voltage, therefore reducing energy stored in the inductor. Thus the voltage overshoot is reduced during the end of the soft-start cycle.
In the drawings:
Large voltage overshoots can be avoided if the energy built up in the external inductor is small. In the preferred embodiment circuit of
In the prior art, the reference voltage is switched from the soft-start ramp voltage to the bandgap voltage, when the soft-start ramp voltage is equal to bandgap voltage. In preferred embodiment circuit of
Because of the intentional directional offset, the pass switch 32 does not turn ON during the transition from the ramp voltage to bandgap voltage, therefore there is no energy stored in the inductor 36. Thus the voltage overshoot is reduced during the end of the soft-start cycle.
The preferred embodiment provides the advantages of reduced voltage overshoot during power-up of a switch mode regulator at the end of soft-start and requires very little silicon area.
While this invention has been described with reference to an illustrative embodiment, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiment, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.
