TLV62065: use 2.2uH instead of 1.0uH/1.2uH

Q: TLV62065: use 2.2uH instead of 1.0uH/1.2uH by 1098475

 
"We just finished the second revision of our Schematic. In this revision we implemented the TLV62065 several times.
 
All regulators will have a regulated 5V0 input voltage.
The output voltages are: 1V0, 1V4, 1V8 and 3V3.
 
What we will do now is to dimension the external components and the loop compensation. It would be nice if you could answer some questions on this design step.
 
1. Inductor
Although the datasheet suggest a value of 1uH to 1.2uH I would like to use a 2.2 uH inductor. The reason for this is that we don't have a small 1 uH inductor in our component range so far. The inductors we have start with 12 A and therefore are far too large.
I don't think the 2.2 uH inductor will make the regulator unstable. Can you agree to this?
Btw, we don't need the power save mode, so it is no problem to pull up the MODE pin to high. This might increase stability as well.
 
2. Output capacitor
We will use ceramic X5R or X7R output capacitors. Any value is available. I would either use 10 uF or 22 uF. Probably 10 uF is preferred, especially as we use a 2.2 uH inductor.
 
3. Loop compensation
We don't care for losses in the feedback divider, and therefore I would use resistors in the low kR range.
The feed forward capacitor (Cff) will be adapted accordingly. I have two question on the calculation of Cff:
a) It seems as if Cff is only dependent on R2. Is the value of R1 really irrelevant?
b) The formula for calculating Cff includes 35kHz. Is this frequency a constant for the TLV62065?"


A: Re: TLV62065: use 2.2uH instead of 1.0uH/1.2uH by 1090425

The TLV parts are lower cost, lower performance versions of their TPS equivalents.  So, there may be information on the TPS product page that also applies to the TLV version.  In this case, this app note, http://focus.ti.com/lit/an/slva441/slva441.pdf, is highly relevant.  It shows that the converter will be stable with 2.2uH inductors and an output cap of 10uF.  A 22uF output cap is unstable.  Keep in mind that this represents the entire capacitance on the output, not just what is shown on the TLV62065's page of the schematic.

Since this is a voltage mode converter, the MODE pin and load current have little effect on stability.

The app note and this one, http://focus.ti.com/lit/an/slva289/slva289.pdf, explain how to select the Cff.  This should be done independently for each of their output voltages.  They should also measure the stability/loop of their final circuit.

The 35kHz is the recommendation for the standard circuit with the recommended L and Cout.  Since you are using a different L and Cout, you need to select the Cff empirically, per SLVA289 above.  Smaller value resistors are just fine.  The Cff may be much larger than 22pF commonly used.