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Anyone have experience with MICREL's KS8721B ethernet transceiver?

I'm a beginner and I need some help with MICREL's KS8721B Ethernet transceiver.

In the reference design and datasheet download from here:

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The KS8721B has 3 power supplies, one is the digital core 2.5V power supply, second is the analog 2.5V power supply and the third one is the analog PLL 2.5V power supply.

From the reference design obtained from the ftp, Micrel uses a LDO

together with a combination of ferrite beads and capacitors to produce these 3 power supplies.

My problem is i can only obtain my ICs through Farnell and Farnell does not have the LDO MIC5255 and the ferrite bead by Steward HI1206N101R-00. My question is, is it truly necessary to have these 3 types of voltages or can i simply replace it with a 3.3V supply (because i'm using a LM1117 to produce a 3.3V supply voltage to all my ICs)?

Reply to
Ant_Magma
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You want to replace the 2V5 by 3V3? Did you look at page 22 of the datasheet (rev 2.3) where they state the absolute maximum ratings? And did you read notes 1 & 2 on the same page?

There are many other ways to make 2V5, you don't need to use a MIC5255.

--DF

Reply to
Deefoo

Oh, sorry i neglected that. I saw the Vddio was capable of operating at

3.3V. Anyway, I mentioned 3.3V because all my other chips are using 3.3V, but it doesn't matter. I guess i can use a couple of resistors and build a voltage divider to get the 2.5V from my 3.3V source right?

The main question is that can i just ignore all those analog power supply, analog PLL etc and just give everyone a 2.5V (without those capacitors, ferrite beads etc)?

Reply to
Ant_Magma

To start tinkering, yes, probably. If your layout is not too bad. But you can use other beads that Farnell does sell. Just look up the datasheet of the specified bead and find one that corresponds. You should be looking for an impedance vs frequency curve.

A voltage divider is not a good idea to get to 2V5. You might try adding a series diode to the 3V3 line, that will get you down to about 2V7, that lies (just) within its ratings. Better is to add a seperate regulator that feeds on the source of your 3V3 regulator or find another LDO that works on 3V3. The TPS75225 (TI) works fine for me.

--DF

Reply to
Deefoo

I would like to simplify my designs with just a regulator providing the

2.5V since i'm still a beginner and anything more will only complicate the situation and give me a helluva time when i troubleshoot.
Reply to
Ant_Magma

Can you use the KS8721SL or CL? That has a built-in 2.5V regulator. Still needs the beads and caps. The PLL needs a very clean power supply, thus the beads.

--
Mark
Reply to
qrk

Reply to
Ant_Magma

Do it once right or do it twice wrong. So, Yes its necessary.

If the data sheet says to use it, then use it.

After your design is done, leave off those parts and see if it still works right. (right means does it still meet noise margins)

You can take off, its harder to add add-on.

If your concerned about a few parts, whats your boss going to say when you have to redesign the board.

donald

Reply to
Donald

Thx Donald. I suppose there's no way around it. I couldn't find any ferrite beads with the exact impedance freq curve so i ordered the one closest (Apparantly Steward also has stopped produce the -00 beads anymore)

Hopefully i can get it right.

Reply to
Ant_Magma

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