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LM3488 External Slope Compensation for SEPIC circuit

Don Foong
Expert 4100 points
Other Parts Discussed in Thread: LM3488

Hi Team,

 

We have some questions regards to the slope compensation for the LM3488 circuit. The application is using SEPIC circuit. Hope you can help to advice, thanks in advance.

 

  1. With reference to LM3488 data sheet section 8.2.1.2.4 (Current Limit with External Slope Compensation), is this External Slope Compensation applicable to SEPIC circuit (or only to boost circuit) ?

2. Does equation (22) and (47) applicable to SEPIC circuit if external slope compensation is use ?

3. If Rsl is used, should the filtering capacitor Csn be placed on the Isense pin side or the Rsense side ? What is the recommended value for the Csn, 1nF ?

  • 0
    TI__Expert 6215 points

    Hello Don,

    The equations for control of a SEPIC can easily be derived from those for the control of a boost by making the following substitutions:

    1) Replace L with Lt = L2*L1/(L2 + L1)

    2) Replace Vo with Vos = Vo + Vin

    The reasons for these substitutions can be seen by assuming that Cs is large making it a DC voltage source with Vin across it.  The reason for Vin being across the capacitor is that if DCR is zero the average voltage across an inductor is zero.  This leaves Cs as the element that blocks Vin.  See attached figure.

    Using this information, the answers to your questions are:

    1) Slope compensation is needed for SEPIC converters with >50% DF (Vin < Vout).  In most cases, the LM3488’s built-in slope compensation should be sufficient – If you calculate Rsl, the minimum will be a negative number.  With the substitution from above, the equation for Rsl is now Rsl > (Rsen*(Vo-Vin)/(2*fs*Lt)-Vsl)/40uA.  The term (Vo-2*Vin) in the boost equation became (Vo-Vin) due to the substitution of Vos for Vo than canceling the Vo from the Vos term from the 2*Vin term.  

    2) Both datasheet equations 22 and 47 still apply but with the use of Lt in place of L and with Vos replacing Vo.  

    3) Place Rsl between the LM3488 and Csn.  See figure 23.

    Feel free to email me at Robert.blattner@ti.com if you wish to share confidential customer information for detailed calculations.  

    Regards, Robert

  • 0 in reply to Robert Blattner
    TI__Expert 4100 points
    Hi Robert,

    Thanks a lot for the detailed advice and the offer to further help by your email. I will advice customer accordingly and see if there is any further help needed.