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LTC2970CUFD Datasheet(PDF) 30 Page - Linear Technology |
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LTC2970CUFD Datasheet(HTML) 30 Page - Linear Technology |
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30 / 36 page ![]() LTC2970/LTC2970-1 30 29701fc APPLICATIO S I FOR ATIO 7-Step Procedure for Calculating Tracking Application Circuit Resistor Values, Counter Delay Values, and Terminal IDAC Codes The following 7-step procedure should be used to calculate the resistor values, tracking counter delays, and terminal IDAC codes for the Tracking Application Circuit shown in Figure 3. 1. Assume a value for R20 and solve for R21. VDCn,NOMistheoutputvoltageoftheDC/DCconverterwhen the LTC2970’s VOUTn pin is in a high impedance state. RR V V DC NOM DC NOM 21 20 1 0 =• , , (12) 2. Solve for R10 and R11. R Rn V V DC NOM FB 1 2 1 n n n = − ⎛ ⎝⎜ ⎞ ⎠⎟ , (13) 3. Solve for R40 and R41. For simplicity, this procedure assumes that R40 = R41. VDCn,MAX and VDCn,MIN are the maximum and minimum converter output margin voltages, respectively. The value of R40 = R41 is constrained by: RR V VV VV FB DC NOM DC MIN DC MAX D 40 41 =≥ • − () − n nn n ,, , C C NOM mV A n, () + ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ + 110 236 μ (14) Due to the forward drop of diodes D10 and D11 (0.8V max), the minimum value for R40 = R41 from expression (14) may result in small or even negative values of R30 and R31 in Step 4. If this is the case, assume a minimum allowable value for R3nB, and use the following expression to calculate the minimum value R40 = R41: RR V RB R RB R V FB 40 41 1 3 1 3 2 08 1 =≥ •+ + ⎛ ⎝⎜ ⎞ ⎠⎟ ++ n n n n n . 0 0 236 mV A μ (15) Note: Use the channel whose parameters yield the maxi- mum value for R40 = R41. 4. Solve for R30B and R31B. Solve for the upper limits of R30B and R31B and then determine which resistor value constrains the maximum value of the other resistor using Equation 17. RB RA V V mV V RR FB FB 3 4 236 0 8 10 1 1 1 n n n n n ≤ •− − − () •+ μ . 2 2n ⎛ ⎝⎜ ⎞ ⎠⎟ (16) RB R RB R 30 20 31 21 = (17) 5. Solve for R30A and R31A. R30A and R31A are constrained by: RA R R R VV V DC MAX DC NOM DC 3 2 1 2 1 n n n n nn ≤ + ⎛ ⎝⎜ ⎞ ⎠⎟ • − ,, n n n ,NOM RB ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − 3 (18) |
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