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Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
I'm looking at building a Dc/DC Boost converter for my Control Module so I can use a LiPO cell I have siting around form a partial dead pack and I have some issues with calculating the Resistor and Capacitor I need for compensation circuit using an analog devices ADP1610 since it was the least expensive DC/DC converter on digikey, and it requires very few parts and is capable of 1.2A, attached is the link to the data-sheet to help follow along, and maybe help me figure out what I did wrong in the calculations.
I already have a lot of the equations done and looking good so to help here's all I have so far:
Battery is 3.2-4.2V 4800mAH and 35C discharge
I would like 7V out at 1A
Feedback voltage divider consists of R1 47ohm and R2 10ohm
The duty cycle is .54
If using 700K the on time is 775nS, if using 1.23MHz on time is 441nS
An inductor between 1uH and 22uH is suggested
The inductors ripple current is .67A @ 700K, and .706 @ 1.23M
Peak inductor current is ~5.15A for both frequencies
Minimum duty cycle is .4
Now here is where I think I'm starting to get incorrect values, in the data sheet they suggest that Cout should be 10uF or larger, and the equations they give for Ripple Voltage and to determine Cout are a bit redundant so I just choose 10uF to start with the equations are on page 12 of the pdf.
Then on page 13 we have the start of where I'm getting hung up on, on page 13 left side we have the Right Half plane zero frequency easy enough to calculate but I don't know my Rload (exactly) the line below the equation, it's either the Load resistance (obviously) or the Vout divided by the load current, now I don't know for sure what my load current is but I presume its right around 600mA and I get a Fz of 665.5KHz @700K and 17.64KHz @ 1.23M.
Then there's the loop gain which I don't think I need to know since it looks like the y simplify the equation down to # 17 which would give us the value for the Compensation resistor which on page14 is suggested to be between 30K and 400K, but I can't seem to get my Rcomp calculation to get a value any where near that range.
The same issue can be said for the next equation which is for the Compensation Capacitor, which is suggested to be between 100pF and 1.2nF and I also get very high values for this, even if I double check with the table at the top of page 14 which has suggested values for "common" applications I still get an incorrect Ccomp but a close Rcomp value calculated.
However right now I have Rcomp at 14Kohm and Ccomp at 963pF for 700K, and 24.9Kohm for Rcomp, and 311pF for Ccomp at 1.23M.
I should also mention that it's suggested to have a frequency cutoff of 1/5 the Right half plane zero, or 1/15 the the switching frequency.
So might somebody help me understand either what I did wrong in my calculations for the cut off Frequency via the RC compensation circuit or if I messed up at all.
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Help with Right half plane zero compensation
