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Help me improve the start up time of bandgap

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naisare

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start up time of bandgap

Please help me to improve the start up time of bandgap..
If anybody of u have the start up circuit which is very fast, please upload here..
Bandgap circuit is the normal cascoded PMOS and cascoded NMOS circuit..
 

rfsystem

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Re: start up time of bandgap

Increase your startup helper current injection and switch it off after the bandgap reach near its full voltage.
 

jcpu

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Re: start up time of bandgap

Dear Sir:

Allow me for an additional question:
Generally, bandgap startup within a few usec.
How fast do we want to push? ~ 10nsec?

Isn't this highly related to the closed loop transient response,
and key characteristics as Gain, GBW, slewrate...
of the loop kernel, i.e., the opamp should play important role
in this response?
 

rfsystem

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Re: start up time of bandgap

That is true!

The PTAT loop which generate a voltage difference close to zero at the target operating point is dominating the speed. If the regulator output is driving the bandgap voltage and get its input from the PTAT voltage difference output it could be described as a frequency transfer function. There is depending on current density a parasitic pole. Higher current density conflict with higher DC accuracy and production spread. This pole define the loop bandwidth.

The startup could be a fast turnon current source with a similar current to the target PTAT. At initial startup this current charge up the compensation or dominat regulator pole cap. If the PTAT loop current exceed some limit this current could be switched off. So the startup time is to chargeup the compensation pole cap to the target value. If the startup and the PTAT current is in the same range the startup time is about the loop bandwidth.

The PTAT does not help at the beginning with low startup currents because the voltage difference is very small for low PTAT current initial. If there is an input voltage offset from the regulation amp the bandgap does not startup at all.

For 0.18-0.5um I observe between 200-1500ns startup time.
 

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