Help in power supply design

[datta]

hello friends,
I have a digital circuit consisting of square wave oscillator ,counters,flip flops etc....Now the thing is i want a 5 v supply for the whole logic block.Input voltage is 8-12 V.5V regulator is what i need.
I thought of designing a buck converter.So calculated the average current of the whole cmos circuit and found it to be around 3 mA.
Two questions:
1)Somebody help me out in designing a buck regulator for this much low current or is it wise to use a LDO.
2) My next question is only the average current is 3 mA.But the distribution of the current isn't going to be uniform.Only during switching the current will be there in the circuit that too for a very low time.Also that current will be much greater than 3 mA (Hope i am correct).Now how do i design a buck converter.
Thanks,
Datta

 

[eblc1388]

For 3mA load current just use a 78L05 and be done with it.

You cannot make a switching regulator smaller and cheaper than a 78L05.

 

[datta]

Hi ,
But the average current is only 3 mA.During switching ,the current will be much more than 3 mA right?That too for some short time say in nS.Won't that be a problem

 

[flat5]

For that low a current you could use a zener diode, resistor and a cap to handle the surge current.

 

[ericgibbs]

Hi ,
But the average current is only 3 mA.During switching ,the current will be much more than 3 mA right?That too for some short time say in nS.Won't that be a problem

hi,
The 7805 is capable of supply 1amp at 5Vdc.

Your circuit will only 'draw' the 3mA that it requires to operate, the 1amp rating will not harm your circuit.

They do make a 78L05 which is 100mA at 5Vdc.

I dont understand what you are saying about nSec.

 

[datta]

Hi,
But if you see the lm7805 datasheet, the output voltage will be given for test condition i out >5mA.So , one cannot say there will be regulation at low currents.
And as i said only the average current of the CMOS logic circuit is 3 mA. Switching current will be much higher.Hence i think i have to design a buck converter for a higher load.
And i preferred buck cos i thought power consumption will be low

 

[Roff]

Hi,
But if you see the lm7805 datasheet, the output voltage will be given for test condition i out >5mA.So , one cannot say there will be regulation at low currents.
And as i said only the average current of the CMOS logic circuit is 3 mA. Switching current will be much higher.Hence i think i have to design a buck converter for a higher load.
And i preferred buck cos i thought power consumption will be low

78L05 works with a load as low as 1mA. With (almost?) all DC power supplies, transient load currents are handled by a capacitor in parallel with the regulator output. In your case, 100nF will probably handle it. If you want to be safe, add a 100uF electrolytic cap in parallel with the 100nF.