purpose of one-shot block in PWM controller

• Sep 16, 2010

• #1

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allennlowaton

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good day EDA fellows...
I got this PWM controller IC from a paper. From a figure below, there's a
one shot module after the oscillator. By the way, this oscillator utilizes a frequency spread technique. The oscillator produces a variable period with a fixed 50% duty cycle.
The paper said that the purpose of the one shot module is to rectify the duty cycle to a small value in order to avoid unexpected logic locking. I don't understand this statement. Please shed some light on this.

Below is the diagram of a one shot, I got from the other paper but the same author:

Below is the diagram of the PWM controller IC:

 

• Sep 17, 2010

• #2

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edwintsu

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It seems that two pics had less relations.
In 1st pic., One-shot block used in constant on-time arichtecture buck controller.
In 2nd pic.,the function of one-shot module is to rectify the duty cycle to a small value as you mentioned before. For <50% duty cycle, one-shot is draw into the chip.If no, maybe erro action appear.

 

• Sep 17, 2010

• #3

FvM

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In my understanding, the one-shot's purpose is to prevent the output from switching multiple times per pwm period.

 

• Sep 17, 2010

• #4

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allennlowaton

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thank you edwintsu...

FvM said:

In my understanding, the one-shot's purpose is to prevent the output from switching multiple times per pwm period.

Click to expand...

..

Can you expound it more please. I'm done with the oscillator, it produces a variable period(varied around its center freq.) but it has a fixed duty cycle. I'm confused with the purpose of the one shot.

 

• Sep 17, 2010

• #5

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dick_freebird

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It may be a way to get a sawtooth, the pulse discharges the
timing cap.

 

• Sep 17, 2010

• #6

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allennlowaton

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thank you di*k_freebird....
yes, a ramp will be produced at the node V2, but I'm having problem regarding the EN, where it come from and is it a pulse?
any idea?

 

• Sep 17, 2010

• #7

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dick_freebird

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EN is likely your SDb (-Shutdown) signal, jamming it high gives
the timing ramp amp a reference above zero for the timing
current I1, replica current I2 ramps the timing node up, every
time it crosses the upper timing reference V1 the comparator
kicks the one-shot and resets the ramp.

 

• Sep 17, 2010

• #8

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allennlowaton

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is it right to assume that the EN signal is an external one? can I say that in this application (LED driver), it comes from dimming?

 

• Sep 18, 2010

• #9

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edwintsu

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FvM said:

In my understanding, the one-shot's purpose is to prevent the output from switching multiple times per pwm period.

Click to expand...

My understanding is opposite, its purpose is to prevent no switching per PWM period.

 

• Sep 18, 2010

• #10

FvM

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My understanding is opposite

Click to expand...

May be right or wrong. I was referring to the well-known operation of industry standard current mode controllers. The logic functionality of the below controller hasn't been revealed in the discussion, so you have to guess about it. I expect it's more clear in the original paper. Usually, we have timing diagrams to clarify similar designs...

 

• Sep 18, 2010

• #11

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allennlowaton

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edwintsu said:

My understanding is opposite, its purpose is to prevent no switching per PWM period.

Click to expand...

..This makes sense also because, I had seen the one shot from a paper that deals with Pulse Skip Modulation.

 

• Aug 9, 2011

• #12

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plmkjn

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Can you share the paper, please?

 

• Aug 11, 2011

• #13

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Usually one-shot circuit used to set high-side switch on time.
In most case it is adjustable based on battery and output voltage.