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Original question:

LM5170-Q1: Calculation of the outer loop compenstation

LM5170-Q1: Gain and phase graph calculations

Ponsonby Rivers
Intellectual 620 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

After entering the appropriate data in the LM5170DESIGN-CALC spreadsheet found in snvc205, I have determined the Buck and Boost mode voltage loop compensation parameters as given in Section 11 and Section 12 of the spreadsheet respectively. This has allowed me to find the digital coefficients by using the Bilinear transformation which is required since we're using a microcontroller to perform the duties of the analog compensator.
(1) Is this the recommended/correct approach?
(2) The Buck and Boost mode voltage loop graphs shown in the LM5170DESIGN-CALC spreadsheet show the gain and phase margins. Can you share how this was determined as I'd like to understand the correlation between the information entered into the spreadsheet and the graphs. Where can I find the calculations which produces these graphs?
(3) The loads of our system will naturally vary. Would it be suggested to have coefficients at one particular condition or to have multiple coefficients for different conditions and select the most appropriate?

  • 0
    TI__Guru 68535 points

    Hi Ponsonby,

    Thank you for your interests in the LM5170.  (1) You approach sound right.  (2) There are two loops: the inner current loop and the output voltage loop.  The inner voltage loop calculation is described in the datasheet section 9.12.  The voltage loop is also a simple 1st order system if you put the loop crossover frequency at 1/10 or lower of the inner current loop crossover frequency.  The transfer function can be obtained from Figure 53:  when closing the voltage loop, the inner current loop is closed and the LM5170 is basically a current source.  Not that the boost loop may have the right half-plan zero, as predicted in conventional boost circuit. (3) the circuit can be made stable over the entire load range with one compensation. However, this circuit has a diode emulation mode and in DCM (light load) the gain does drop and you may change the compensation in your digital algorithm to better improve the loop response. You may have two to three additional coefficients to cover shallow and deep DCM conditions.

    Thanks,

    Youhao Xi, Applications Engineering