# Datasheet duty cycle "@ 1khz"

Discussion in 'Electronic Design' started by Jon Slaughter, Jan 20, 2009.

1. ### Jon SlaughterGuest

What exactly is meant by @ 1khz? value should be somewhat independent of
frequency?

e.g., 100mA peak current at 1/10 duty @ 1khz. What ist he peak current
approximately if I run it at 10khz?

2. ### Bill SlomanGuest

The implication is that whatever gets warm when run at 100mA has an
appreciable thermal capacity, so it doesn't warm up too much if run at 100mA
for 100usec, provided that y9ou let ti cool off for the next 900usec.

It will warm up even less if run at 100mA for 10usec, but the extra heated
dissipated in the device wihile it is switching between on and off (and back
again) will probably wipe off any advanatge you might hope for.

Unfortunately, the heat generated while the current is high will increase
not only in proportion to the current but also with the voltage drop across
the relevant part of the device, which can rise very rapidly indeed with
increasing current when you push the current above specification.

3. ### PeterDGuest

Think about running that duty cycle at .01 hz, does that help?

4. ### Jon SlaughterGuest

Nope. Why not just run it at 0.0000001nHz?

Obviously as the duty cycle approaches 1 the peak current approaches the
average current. Also for very low frequencies the peak current approaches
the average current for a large time frame. (e.g., why not a duty cycle of
10^(-8) but a frequencfy of 10^(-1000000)

Ask your same question but with 10khz, then what? Because peak current is
independent of frequency, at least for all reasonable ranges, I would expect
no change. But what is a reasonable range? 10khz to 100hz? or what?

My question, again, is why they give 1khz? What if I want to run at another

5. ### Jon SlaughterGuest

So why not give a graph? Or are they saying 1khz is the optimal? i.e.,
ideally the peak current should be independent of frequency but it's
obviously not. But they give no clue as to it's relation.

6. ### TTmanGuest

Probably the same, for all practical purposes.

7. ### Bill SlomanGuest

Some people do. It takes more work to produce and makes the data
sheet longer
They shouldn't need to. The point is that the circuit would get too hot
if the 100mA was sustained for any extended period of time.

The circuit might get hot enough to burn out if you sustained the 100mA
for only one second, but the data sheet does tell you that if you sustain
the
100mA for 100usec and then leave it to cool off for 900usec, the circuit
will survive.

Presumably this is enough information for the people to whom they
expect to sell most examples of the circuit

Ring up the applications department, or send them an e-mail, if you

8. ### Guest

I'd have to see the datasheet, but this sounds like a limitation based
on electromigration. If you PWM faster than a millisecond, then the
average current in the metal path can be used in design for
electromigration limits, rather than the peak current.

A good example of this is the MAX7219. You will notice the digit
drivers are not sequential around the chip. This is to produce average
current in the metal paths, rather than peak. Think of it as the
firing order of a V-8, i.e. not sequential, but spread out for balance.

9. ### Guest

The reason that a frequency is given has to do with the thermal time
constant of the device. If the on time is equal to the thermal time
constant, then the temperature rise will reach will reach 63% of the
temperature rise at 100% duty cycle. One year on and on year off is a
50% duty cycle. So is 1nsec on and 1 nsec off. See the following
website for a nice write-u:. http://www.aosmd.com/pdfs/appNotes/character_mosfet.pdf.
Regards,
Jon

10. ### Jon SlaughterGuest

The reason that a frequency is given has to do with the thermal time
constant of the device. If the on time is equal to the thermal time
constant, then the temperature rise will reach will reach 63% of the
temperature rise at 100% duty cycle. One year on and on year off is a
50% duty cycle. So is 1nsec on and 1 nsec off. See the following
website for a nice write-u:.
http://www.aosmd.com/pdfs/appNotes/character_mosfet.pdf.
Regards,
Jon

------------

So given figure 3 it seems that higher frequency doesn't have much
benefit... at least for the mosfets under question.

Thanks,
Jon

11. ### Guest

I'm having a problem with the term normalized transient thermal
resistance, basically normalized to what? I think you are obsessing on
this too much. The 1ms rule is very common in process design rules.
Basically, if you cycle in 1ms or faster, treat the electromigration
rule as the DC average. If you really want to agonize over this, you

Unless I missed it, you didn't spec the part in question. If it is
power dissipation, if you meet the 1ms rule, use average power then
check to see if the junction temperature is in spec.

I never had a chip fail BU QA under the 1ms rule.

12. ### JosephKKGuest

The question you are asking makes clear just how badly you are missing
the point. It is a simple statement of test conditions; like @ 25
degrees C. It is intended as an aid in replicating the test
conditions in order to compare your own test data.