Decoupling capactior value calculation

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Hi,
I am using a GSM modem which requires a power of 3.2v min to 4.5V max.
In burst mode, it draws nearly 2A current for 2milli seconds, nomally it is 250mA.
I feel that it may drop the supply voltage to less than 3.2V. So, i am planning to keep decoupling capacitors. How to chose the value of capacitor? can u pls send the calculations.

Thanks in advance
 

C = I*Δt/ΔV . For a - say - max ΔV=1V you'd need C=4e-3F = 4000µF . 4700µF/6V is a std value.

If this takes too much space, you could use the smallest LiMnO3 accu cell, limiting the max. voltage to 3.9V (permitted long-time peak load voltage), e.g. by a simple Z-Diode in parallel.

A 5V supercap probably isn't appropriate because of its relatively high series resistance.
 

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