basic question about reading datasheets current values

[panfilero]

Hello,

I want to design a small circuit, and want to power it with voltage
regulator which is connected to a 9V battery. I was looking at a
voltage regulator that can output 100mA of current... so I thought I
would look at all my individual ICs in my circuit and find out how
much current they need and make sure that everything will be ok....
but I got confused looking at the datasheets.

So, I have an LM555 timer chip... I pull up it's data sheet:

Power Dissapation = 600mW
(at this point, I know I will be using Vcc = 5V, so I'm thinking P/V
= I = 120mA... so that 100mA regulator wont do it)

Then I read on
Supply Current (Low Stable) @Vcc=5V, Icc=6mA (for high stable
Icc=5mA)...
Now this is really different than 120mA.... so I'm thinking.... maybe
my regulator will work.... I read on

Isource = 100mA, Isink=5mA
Hmmmmm.... if I can only send it 100mA tops.... am I too close to this
source level?

any help is appreciated, much thanks!

 

[panfilero]

Power dissipation is a total of what the chip consumes by itself
running itself,and power dissipated because of current you draw out
when high, or put in when the output is low.





The chip will source up to 100 mA max from the output when high, and
sink only up to 5 mA when low.

ok, so am I right then to think that the 100mA voltage reg. will not
be enough to even power this one 555 timer properly?

Does the sourcing and sinking not have anything to do with this? Do I
only need to be looking for power dissipation in order to know what
current needs to be made available... .and no need to look any further
in the datasheet after that?

Thanks

 

[BoldInventions]

It's true that the dissipation is a measure of how much power the
chip *can* dissipate safely without damage. Your circuit probably
will not be anywhere near that.

But I suggest you just build your circuit and power it with
batteries or a bench supply and simply measure it rather than spending
time trying to come up with a theoretical power number. Remember the
power used by the circuit can't be determined by summing up the
current specs for the individual IC's. You may have quite a bit of
power being dissipated in other components like resistors etc, and the
power that flows in to an IC may be dependent on how you are loading
the outputs too. Most likely you are going to be fine with 100mA
unless you are driving a big LED or something.

-Kevin

 

[Nobody]

ok, so am I right then to think that the 100mA voltage reg. will not
be enough to even power this one 555 timer properly?
No.

Does the sourcing and sinking not have anything to do with this? Do I
only need to be looking for power dissipation in order to know what
current needs to be made available... .and no need to look any further
in the datasheet after that?

It depends. For a chip which isn't being used to drive an external load,
then its power *consumption* will tell you how much current it's likely to
draw.

The 555 can drive a significant load (by IC standards) from its output
pin. The cited maximum power dissipation is essentially telling you how
large a load you can connect to it, not how much it will draw by itself
when driving little or no load.

A 500W PSU is capable of providing 500W; that doesn't mean that it will
actually use that much just by being turned on. The situation with the
555's power rating is the same.