start-up circuit for ptat current

[zarric]

the circuit shows below

whether this R1 and C1 are a proper start-up circuit ?

if the circuit is ok ,then what's the proper value for R1 and C1

 

[PaloAlto]

R1 and C1 do not form a startup circuit. If all the nodes are zero, it will never startup

 

[erikl]

... If all the nodes are zero, it will never startup

Surely it will: When VDD is switched on (even if it rises slowly), C initially provides the necessary startUp current. The limiting R isn't necessary, IMHO, because the pmos limits the current anyway.

 

[frankaurora]

... If all the nodes are zero, it will never startup

Surely it will: When VDD is switched on (even if it rises slowly), C initially provides the necessary startUp current. The limiting R isn't necessary, IMHO, because the pmos limits the current anyway.

Could you explain it in detail?
Why C provides the startUp current?
Thanks.

 

[PaloAlto]

Surely it will: When VDD is switched on (even if it rises slowly), C initially provides the necessary startUp current. The limiting R isn't necessary, IMHO, because the pmos limits the current anyway.

Oops, you are totally right. If the node is zero, the P type transistors are on and if the node is higher than zero, the cap can provide the current. I have spoken too fast.

 

[frankliner]

I would not use this. The cap can store charge so relying on it always being low at startup is a bad idea.

 

[erikl]

I would not use this. The cap can store charge so relying on it always being low at startup is a bad idea.

Right: If you want to switch on the circuit several seconds after switching off, you should spend a (very) hi-ohmic resistor in parallel to the cap to discharge it.

 

[keith1200rs]

I have used junction capacitors as startup circuits in bipolar PTAT & bandgap designs and they work fine provided you ensure there is a discharge path for the capacitor. I cannot remember the exact mechanism in the designs I did but there was a distinct, predictable discharge path when the power was removed (possibly through one of the wells being forward biased when the power collapsed). The discharge could also be seen in simulations.

Keith.

 

[afujian]

what value should the Cap be?Is there any principle to set the value?

 

[keith1200rs]

I think the value depends on the possible leakage you need to overcome and the rate of rise of the power supply. Normally when designing such circuits I deliberately add in an excessive amount of leakage & check the circuit doesn't work. Then add the startup circuit with the slowest power supply rise you expect and make sure it then works.

Keith

 

[afujian]

I think the value depends on the possible leakage you need to overcome and the rate of rise of the power supply. Normally when designing such circuits I deliberately add in an excessive amount of leakage & check the circuit doesn't work. Then add the startup circuit with the slowest power supply rise you expect and make sure it then works.

Keith

Thanks a lot.

 

[keith1200rs]

Remember to check at the highest operating temperature.

Keith

 

[afujian]

Remember to check at the highest operating temperature.

Keith

Hi Keith,could you explain it in detail?Why we should pay special attention to the highest OP temperature?

 

[keith1200rs]

Because that is when leakage will be highest and so when the circuit is most likely to fail. However, do also check at low temperatures - on theory is that the higher leakage of the larger transistor will be sufficient to ensure starting, but that is not a method I have relied on, even though simulations may show it to be the case. It is a method which caused some controversial discussion, I think.

Keith