chargepump in RFID Tag

[shaikss]

Hello,

Normally array of caps are used for Charge pumps.
Can you please explain how the attached charge pump works?
Its not a series or cascade connection. I am confused in analysing this circuit.

Please help me.

Thanks,
Shaikss

 

[erikl]

I think it's just a variation of the Dickson charge pump: instead of cascading 2 voltage doublers, it uses 2 in parallel, stacking their voltages.

 

[shaikss]

I think it's just a variation of the Dickson charge pump: instead of cascading 2 voltage doublers, it uses 2 in parallel, stacking their voltages.

Is there any advantage going for parallel (voltage stacking) instead of cascading.
How does it works actually?

 

[erikl]

Is there any advantage going for parallel (voltage stacking) instead of cascading.

I'd think the decision (serial or parallel) depends on the source impedance. Try and simulate it!

 

[shaikss]

I'd think the decision (serial or parallel) depends on the source impedance. Try and simulate it!

Hi,

I had seen some strange behaviour when I simulated chargepump (dickson) constructed using MOSFETs.

What is the switching frequency to be used? How should we select Cap value?

I have chosen 500KHz as the clock frequency and cap is 1pf.
I tried b varying the clock frequency from 250KHz to 1GHz. This charge pump is used for low power RFID applications@ 865MHz.
I am aware that there are -ve implications when we go for higher frequency. but I experimented to see the behaviour.
Moreover, I changed the width of the MOSFET from 240nm to 1um.

Strangely, I saw that as the width is increased, output is decreased. I assumed that as width of MOSFET is increased, more will be the drain current and so output voltage must be higher. Surprisingly, I found the reverse, wherein, output voltage started decreasing as the width is increased. Why is it happening so?

I changed the capacitor value from 0.01pf to 1uf. Here also, I agree that cap should be small in order to see faster response time. But still, I experimented to see the results. To my surprise, I found that as cap is increased, response time is decreased. I don't understand why is it happening so.

Can you please explain the logic behind this?

Thanks!