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LTC2970CUFD Datasheet(PDF) 32 Page - Linear Technology |
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LTC2970CUFD Datasheet(HTML) 32 Page - Linear Technology |
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32 / 36 page  LTC2970/LTC2970-1 32 29701fc APPLICATIO S I FOR ATIO VV R R VmV DC MAX DC NOM FB 00 0 20 30 10 ,, >+ • − () →> + • − ()= VV k k VmV DC MAX 0 2 631 10 768 08 10 36 , . . .. Ω Ω 6 60V From Equation 7, the margining resolution will be less than: V R R RA k k k RES < •• = •• 20 30 40 276 256 10 768 665 μ Ω Ω Ω . . 2 276 256 933 μA = . mV/LSB Margining DC/DC Converter with TRIM Pin Design Example The output voltage of the DC/DC converter in Figure 2 needs to be margined ±10% about its nominal value. Assume that RTRIM = 10.22kΩ and VREF = 1.225V. 1. Solve for R30 using Equation 10: RR k TRIM DOWN DOWN 30 50 10 22 50 ≤ − ⎛ ⎝⎜ ⎞ ⎠⎟ = − • % % .• Δ Δ Ω 1 10 10 40 880 ⎛ ⎝⎜ ⎞ ⎠⎟ = , Ω Let R30 = 39.2kΩ. 2. Solve for R40 using Equations 11: R V A UP DOWN REF 40 1 236 1 10 10 ≥+ Δ Δ ⎛ ⎝⎜ ⎞ ⎠⎟ =+ ⎛ ⎝⎜ ⎞ % % • μ ⎠⎠⎟ = • . , 1 225 236 10 381 V A μ Ω Let R40 = 10.5kΩ. Tracking Application Circuit Design Example Consider the LTC2970-1 application circuit shown in Figure 3. Channel 0 is a 1.8V DC/DC converter while channel 1 is a 2.5V switching power supply. Both converters have a feedback node voltage of 0.8V and need to track on and off coincidentally. In addition, a margin range of +5% and –10% is required for each supply. 1. Assume a value for R20 and solve for R21. Let R20 = 5,970Ω. From Equation 12: RR V V V V DC NOM DC NOM 21 20 5 970 25 18 8 1 0 =• = • = , , , . . , Ω 2 292 Ω Let R21 = 8,250Ω (the nearest E192 Series resistor value). 2. Solve for R10 and R11. From Equation 13: R R V V V V DC NOM FB 10 20 1 5 970 18 08 0 0 = − ⎛ ⎝⎜ ⎞ ⎠⎟ = − , , . . Ω 1 1 4 776 ⎛ ⎝⎜ ⎞ ⎠⎟ = , Ω R R V V V V DC NOM FB 11 21 1 8 250 25 08 1 1 = − ⎛ ⎝⎜ ⎞ ⎠⎟ = − , , . . Ω 1 1 3 882 ⎛ ⎝⎜ ⎞ ⎠⎟ = , Ω Let R10 = 4,750Ω and R11 = 3,880Ω. 3. Solve for R40 and R41. Assume that R40 = R41. RR V VV VV FB DC NOM DC MIN DC MAX D 40 41 =≥ • − () − n nn n ,, , C C NOM mV A V n, . (. ) ( () + ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ + = • − 110 236 08 10 9 μ 1 105 1 110 236 10 212 .) , − + ⎛ ⎝⎜ ⎞ ⎠⎟ + = mV A μ Ω Let R40 = R41 = 10.5kΩ |
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