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Peak switch current of MIC2619 PWM controller IC?

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opampsmoker

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Hi,
Do you know what is the peak switch current setting in the MIC2619 Boost controller?

The graph on page 4 suggests its ~600mA, but the datasheet on page 3 says its a minimum of 350mA.

We need to know the maximum of the switch current setting (maximum of the tolerance), and this is needed, as you know, so that we can size the inductor.

MIC2619 datasheet
 

KlausST

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Hi,
this is needed, as you know, so that we can size the inductor.
For inductor selection go through the chapter "inductor" on page 10.
There are the formulas for inductor current. Use them.

Klaus
 
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Hi,
Do you know what is the peak switch current setting in the MIC2619 Boost controller?

The graph on page 4 suggests its ~600mA, but the datasheet on page 3 says its a minimum of 350mA.

We need to know the maximum of the switch current setting (maximum of the tolerance), and this is needed, as you know, so that we can size the inductor.

MIC2619 datasheet

Yes, the switch current limit is needed in order to size the inductor. I believe 350mA is the minimum value for the current limit spread and 600mA is the typical value. Notice that the graphs are under the heading: Typical Characteristics.
 
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opampsmoker

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For inductor selection go through the chapter "inductor" on page 10.
Thanks,
I appreciate that......but i am sure you would agree those "datasheet method" things are terrible....its always best to work from first principles and do it like that.
At startup, the error amp is railed high, and the switch turns on until either
1...switch current limit is reached or
2....end of max on time.

as such, we need to know the switch current max limit.

I think a "message in a bottle" to microchip will be needed....chances of answer virtually nil
 

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Hi,

I think the datasheet is clear enough with this. And I agree with the inductor selection method given in the datasheet.

Why:
I think it´s not imortant to select the inductor for a short period of time during startup.
It´s sufficient to design it for the expectable steady state during normal operation.

So I see the forumla given in the chapter "Indutor" as valid.

And yes, one may expect the inductor to saturate at startup, and the current will be limite by 600mA worst case. But during this period of time the regulation loop will be saturated, too. Mabe this takes 10ms, maybe less.

What do you think is the worst case that can happen?
I see just a couple of overcurrent peaks, maybe increased EMI. No immedite failure, no long time failure.

Klaus
--- Updated ---

added:
I compare it with the inrush current of an incadescent lamp.
It may be 10 times (guessing) of the nominal current... for the first few milliseconds.

Klaus
 
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opampsmoker

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thanks, though i think incancesant lamps are possibly a lot tougher than FETs inside little DCDC modules.
 

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Hi,

the FET is overcurrent protected by the internal current limit. It won´t see above 600mA.

But the initial question was about inductor selection. If you want to be on the safe side, then use an inductor rated for 600mA.
But you may use an inductor with lower current rating ... while being safe for normal operting conditions.

Klaus
 

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