Re: Power for Spartan 3

Peter Mendham wrote:
The values in the TI data
*** (figure 1) produce about 2.54 volts, 3.29 volts and of course,
1.2 volts.


I am indeed. I am probably being stupid, but using the values in all the TI example circuits (I have about 5 variations here) the Buck 2 potential divider uses values of 61K9 and 36K5 (or 319K and 365K). By equation 15 in the TI data*** these values give a Vout of 3.343V. Similarly, for the LDO the value 2.545V. These are 43mV and 45mV over spec respectively. Now, it is extremely likely that I am either worrying about nothing, or being very stupid. Is this over-voltage specified deliberately, or is it simply a product of using nearest-fit resistor values? Or have I done my calculations badly? If you can clarify that for me I would be very grateful.

The error on 3.3V is 1.3%, which will be nearest fit resistor values.
Normally, simplest design uses two resistors, and it is hard to nail
a value under 1% with available values.

If this is loosing you sleep, then move to a 3 resistor design, and
also be prepared to pay for resistors under 1% tolerance.

In a real design, you should measure/verify the voltage AT THE DEVICE,
which means a few mV high allows for some trace/choke IR drop.

Also check the dynamic power changes, and output impedance of your
regulators, as that is another error source.

-jg

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