peak current of a cmos logic circuit

[datta]

Hi
i want to design a buck converter for supplying a digital cmos circuit consisting of counters and flip flops.For that i need to know the peak current of the circuit right.How to calculate for the peak current of a digital cmos circuit?
Thanks in advance

 

[kchriste]

That datasheets of the ICs will give you the max current draw of each for a given clock frequency. Supply bypass capacitors should be supplying any transient currents anyway.
For hobby circuits, it's far easier just to measure it and add a margin of safety.

 

[RadioRon]

As Kchriste explains, the buck converter needs only to worry about the average current, in theory, not the peak current. The peak current is supposed to be supplied by bypass capacitors, but if you have peak demands that last a longer amount of time, like many milliseconds for example, then it is possible that bypass capacitors can't supply enough, in which case your regulator may have to supply those longer peaks as well. However, CMOS counters and flip-flops and the like have very fast switching times and very short transient current demands, so the peaks will be very short. Bypass capacitors should be up to handling these.

 

[datta]

Thanks.
That is what i too thought.But the buck converter needs to replenish the bypass capacitor right.Bypass capacitor is connected directly to the Vcc(buck converter output).So once the bypass capacitor supplies the transient current of the cmos counter,buck converter needs to replenish the bypass capacitor.Wont that mean buck converter needs to supply the current equal to the amount of the peak current (transient current

 

[crutschow]

Thanks.
That is what i too thought.But the buck converter needs to replenish the bypass capacitor right.Bypass capacitor is connected directly to the Vcc(buck converter output).So once the bypass capacitor supplies the transient current of the cmos counter,buck converter needs to replenish the bypass capacitor.Wont that mean buck converter needs to supply the current equal to the amount of the peak current (transient current

As RadioRon stated it's the average current not the peak. Perhaps you don't understand average current. It would be the amount of the peak current times the transient time divided by the time between transients. Since the transient time is generally much shorter than the time between peaks, the average current is much less than the peak current.

 

[datta]

initially i selected a buck only with the average current....Now i am having doubt if i was right..as i said in my previous post,buck converter needs to recharge the bypass capacitor..What will be the value of that current that replenishes the bypass capacitor..wont it be of a higher value in the ranges of the peak transient current of the current..Hope i made myself clear...

 

[datta]

say the peak transient current is 50mA and the average current is 3mA for a counter...i have selected a buck for say 45 mA...i have put a bypass capacitor of 0.1uF...Now during transient say ,the bypass capacitor supplies the current of 50 mA..now bypass capacitor will loose charge and it is directly connected to buck converter output...Now what will be the current that flows through the bypass capacitor from the buck converter for recharging the capacitor...wont it be very high value which buck converter cannot supply...

 

[Roff]

Charge=current* time, or Q=I*T. Let's say you have a peak current of 50mA, and it flows for 5ns. The bypass capacitor has provided
Q=5e-2 Amps*5e-9 sec=2.5e-10 coulombs.
If the repetition rate of your pulse is 500nS, then the power supply has that amount of time to replace the charge.
Iavg=Q/T.
Iavg=2.5e-10 coulombs/500nS
Iavg=0.5mA
In this example, your power supply peak current may be more than 0.5mA, but it is not a requirement.

 

[crutschow]

say the peak transient current is 50mA and the average current is 3mA for a counter...i have selected a buck for say 45 mA...i have put a bypass capacitor of 0.1uF...Now during transient say ,the bypass capacitor supplies the current of 50 mA..now bypass capacitor will loose charge and it is directly connected to buck converter output...Now what will be the current that flows through the bypass capacitor from the buck converter for recharging the capacitor...wont it be very high value which buck converter cannot supply...

That doesn't happen because the converter can't respond to the fast transients. Most voltage regulators and converters have a relatively low frequency response (in the kHz region), so they just respond to regulate the low frequency voltage level changes and ignore the fast transients. They supply an average current to keep the cap charged to the average voltage level. The cap voltage will thus show a small high frequency sawtooth type ripple as the transient currents are charged and discharged but that's ignored by the regulator.

 

[Willbe]

It can be pretty high for input signals that have both output transistors partially on. This is not on any data sheet.