Propagation delay : overdrive question

[ljy4468]

Hi all

I've seen datasheet of some PWM controller.

and et. characteristic is like below.

I know what 'Propagation delay to outputs' means.
But test condition is overdrive=200mV.

What does that overdrive=200mV means????

Thanks.

 

[leehying]

May be Vgs - Vth, since using VDD to drive is not optimum, though we all do this.

ELE

 

[ljy4468]

So what does that mean in PWM controller, leehying??

 

[freescale_bharat]

I can tell this in the context of comparator. I think in PWM it should be the same..

Comparator has two inputs.. namely the reference voltage and data input
overdrive is voltage difference between the reference voltage to the rail voltage of input pulse. for example if the ref voltage is Vref and the input pulse is from 0V-Vdd, the overdirve voltage would be;
Vref-0 and Vdd-Vref. This overdrive decides the rise and fall time and propogation delay at the output.
If you draw the curve between overdive to propogation delay, there is optimum point where for particular value of overdrive the delay will be minimum.
I hope this is what the data sheet is also referring...

 

[ljy4468]

Thanks for your answer, freescale_bharat!

I didn't know meaning of 'overdrive voltage' in comparator.

So in the case of PWM, (one input is sawtooth, another is error amp's output(DC)),

overdrive voltage is difference between these two signals.
Isn't it?

You mean the value of EA output determines propagation delay, rise and fall time also?

Thanks,
Regards.

 

[freescale_bharat]

Yes... ljy4468,
EA output is DC Vref and sawtooth is pulse.
You will see something like the curve, I am trying to attach.... and at optimum overdrive, propogation delay will be minimum.

 

[liuyonggen_1]

Could you tell me what the propagation delay is ?
and how to determine the propagation delay? simulation?

 

[freescale_bharat]

The propogation delay is time taken for input to appear at output.
It may be clear like for Flip Flop or latch, what is TDQ (data to output delay) or TCQ (CLK to output delay)

you can place input with output waveforms together and 50% of Datain puse to 50% Dataout pulse will be propogation delay.