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I tried to calculate the compensation network for a TPS92692 in buck-boost topology and I can't find the same value for the typical buck-boost led driver (page 41 of datasheet).

Section 9.2.2.2.12 Determining compensator parameters indicates:

A simple integral compensator provides a good starting point to achieve stable operation across the wide operating range. The modulator transfer function with the lowest frequency pole location is calculated based on maximum output voltage, VO(MAX), duty cycle, DMAX, LED dynamic resistance, rD(MAX), and minimum LED string current, ILED(MIN). (See Table 2 for more information.)

The data sheet also indicates the modulator's transfer function : $$ \frac{I_{\text{led}}}{V_{\text{comp}}} = G_0 \cdot \frac{(1 - \frac{s}{\omega_Z})}{(1 + \frac{s}{\omega_p})}$$

And the Small-Signal Model Parameters :

\begin{align} G_0 &= \frac{(1-D) \cdot V_O}{R_{\text{is}} \cdot (V_O + D \cdot r_D \cdot I_{\text{Led}})}\\ w_p &= \frac{V_O + (D \cdot r_D \cdot I_{\text{Led}})}{V_O \cdot r_D \cdot C_{\text{out}}}\\ w_z &= \frac{V_O \cdot (1-D)^2}{D \cdot L \cdot I_{\text{Led}}} \end{align}

Thus, by checking the value in Table 5 of the design parameters and calculations already carried out, I obtain :

  • VO(MAX) = 39.6 V
  • DMAX = 0.850
  • Rd(max) = 3.3 ohm
  • ILed(min) = 100 mA
  • Cout = 34.7 uF
  • Ris = 0.06 ohm
  • L = 33 uH

\begin{align} G_0 &= \frac{(1-0.850) \cdot 39.6}{0.06 \cdot (39.6 + 0.850 \cdot 3.3 \cdot 0.1)} =& 2.48\\ w_p &= \frac{39.4 + 0.850 \cdot 3.3 \cdot 0.1}{39.4 \cdot 3.3 \cdot 34.7 \cdot 10^{-6}} =& 8795.03\\ w_z &= \frac{39.4 \cdot (1-0.850)^2}{0.850 \cdot 33 \cdot 10^{-6} \cdot 0.1} =& 316043 \end{align} But for the same application, the data sheet indicates the following values:

$$ \frac{I_{\text{led}}}{V_{\text{comp}}} = G_0 \cdot \frac{(1 - \frac{s}{\omega_Z})}{(1 + \frac{s}{\omega_p})} = 2.48 \cdot \frac{(1 - \frac{s}{145 \times 10^3})}{(1 + \frac{s}{8.9 \times 10^3})} $$

I don't understand what's wrong with my approach. Is there another way of calculating that I didn't find in the data sheet?

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  • \$\begingroup\$ Just a quick 'sanity-check' before bothering with anything else. Your last equation is found on page 45, Eq. 89. I note \${\large +}\$ in the denominator. You show \${\large -}\$ there. Just a typo? Or something more? \$\endgroup\$
    – periblepsis
    Commented Apr 13 at 19:24
  • \$\begingroup\$ My mistake, it's a typo, I edited the question. \$\endgroup\$
    – msch
    Commented Apr 13 at 19:42
  • \$\begingroup\$ They show \$D\$ and not \$D_{\text{MAX}}\$ in the equations in Table 2. Is there is a reason you picked the value you did? (I'm not reading the text, so perhaps they clarify that point somewhere.) \$\endgroup\$
    – periblepsis
    Commented Apr 13 at 20:06
  • \$\begingroup\$ Yes, the clarification is quoted in the question. The choice of Dmax is intended to compensate for the low-frequency pole (same reasoning for other values). \$\endgroup\$
    – msch
    Commented Apr 13 at 20:33

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