how to decide the err amp output dc level in dc-dc converter

[icpaopao]

err amp

:?::?:
When designing a dc-dc converter, usually a err amp is used to amplifier the voltage diffenrence between Vfb and Vref.
When I design a err amp, I don't know what the err amp output voltage should be.
And in some production, this node's voltage is called Vc.

So, the question is how to decide the Vc.
:?::?:

 

[kb81]

Re: how to decide the err amp output dc level in dc-dc conve

Ususally the output of your error amp will be compared to a ramp which you generate inside the chip. Suppose your ramp is between 0.5V and 1.5V then your error amp output dc level will be between those levels. The comparision between the error amp output and the ramp determines your duty cycle.

cheers
kb81

 

[icpaopao]

Thank you for reply.
I know the ramp's range, but I can't be sure the exact value.

 

[benariaa]

Re: how to decide the err amp output dc level in dc-dc conve

to get the Error amplifier output use the following stages:
1. calculate the desired Duty cycle
2. extrapulate from the Ramp the needed value

and

 

[icpaopao]

The err amp's output voltage is used to be the threshold of the PWM comparator, and the another input of the pwm comparator is the mixed voltage of the ramp and sensed inductor current.
So, the two parameters that are the ratio of the current sensing and the err amp's output voltage can both affect the duty cycle.

What makes me puzzeld is that I am not sure about the ratio of current sensing.

Added after 1 hours 26 minutes:

So, the two parameters should be decided.
I want to know how to design.

Thank you for help.

 

[leo_o2]

If your current sensing signal only has AC and no DC, then it will not influence the DC value of err amp output.

 

[icpaopao]

So, the Vfb and Vsense are both AC signals.
Only the err amp' s output voltage Vc and Vramp are regarded as DC value.
Then only know the ramp, and the duty cycle, I can decide the Vc.

Is it right?

 

[tekno1]

icpaopao

be carefull. current sensing signal adds to error amplifier signal to stop the cycle if current limit is reached. You need to calculate your current sensing output voltage at current limit value you have set and that voltage summed to scaled output voltage will compare against ramp value. (current sensing contribution is smaller than scaled voltage level, and onlt there for stability).

in short you need to calculate transfer function of your circuit. If you can not do that than (It is not good practice but) use values from spice run to determine gain etc. between points.

I hope this explanations sets you in right direction.

 

[icpaopao]

Thank you, tekno1.

Could you give me a detailed design process,please?

I am still not sure about setting the current sensing output voltage.