ESR and CSR of Capacitor

Question

I'm trying to spec out an output capacitor for a TPS7201 voltage regulator (http://www.ti.com/lit/ds/symlink/tps7201.pdf). It needs to be selected carefully as it is used to stabilize the internal feedback loop. The datasheet suggests using a 10-15uF tantalum (with a max ESR of 1.3 ohms) and an external 0.5 ohm resistor in series with the cap.

First question: Is it be correct to say that the CSR = ESR + external resistor = 1.3 + 0.5 = 1.8 ohms? (EDIT: CSR in datasheet is "Compensated Series Resistance")

Now I would much rather use a ceramic cap, which from what I've read, has an ESR in the milliohms. In the TPS7201 datasheet, figure 30 illustrates the region of stability for a range of CSRs. It looks like a CSR of 0.1 to 2 ohms would be sufficient. So to get proper operation I would need add some additional external resistance to get to the necessary CSR.

Second question: If I use a ceramic cap instead of the tantalum, and if I add an external resistance of say 1 ohm (CSR then is 0.1 ohm for cap, and 1 ohm for external resistance, thus 1.1 ohms total, which is in stable region of operation according to figure 30), would this yield proper operation?

Does this make sense, or am I missing something fundamental?

Thanks

Answer 1

Is it be correct to say that the CSR = ESR + external resistor = 1.3 + 0.5 = 1.8 ohms?

According to a footnote in the datasheet, "CSR(compensation series resistance) refers to the total series resistance, including the equivalent series resistance (ESR) of the capacitor, any series resistance added externally, and PWB trace resistance to C_O."

In most cases, the trace resistance will be small enough that you can ignore it, and your formula would be correct enough.

Now I would much rather use a ceramic cap, which from what I've read, has an ESR in the milliohms.

Remember that a ceramic cap in the 10 - 15 uF range is going to be fairly large, which can cause reliability issues (due to mismatched CTE between the cap and board).

It looks like a CSR of 0.1 to 2 ohms would be sufficient.

Notice, comparing figs 28 and 29, that the required CSR changes depending on the capacitor value.

And your capacitor value will change depending on voltage and temperature. But 1-3 Ohms total resistance seems to be okay for any capacitance value.

If I use a ceramic cap instead of the tantalum, and if I add an external resistance of say 1 ohm would this yield proper operation?

Based on the above, this should work. Be sure that your capacitor will have 10 uF or so minimum value over all operating conditions.