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LTC3621 Datasheet(PDF) 15 Page - Analog Devices |
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LTC3621 Datasheet(HTML) 15 Page - Analog Devices |
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15 / 20 page  LTC3644/LTC3644-2 15 Rev. 0 For more information www.analog.com APPLICATIONS INFORMATION Design Example As a design example, consider using the LTC3644 in an application with the following specifications: SVIN = VIN1 = VIN2 = VIN3 = VIN4 = 10.8V to 13.2V VOUT1 = 5V, VOUT2 = 3.3V, VOUT3 = 2.5V, VOUT4 = 1.8V ILOAD1(MAX) = ILOAD2(MAX) = 400mA, ILOAD3(MAX) =1 A, ILOAD4(MAX) = 1.25A IOUT(MAX) = 1.25A IOUT(MIN) = 0 fSW = 1MHz Because efficiency is important at both high and low load current, Burst Mode operation will be utilized. To reduce input voltage and current ripple on the common inputsupply,thePHASEpinistiedtoINTVCCforanti-phase operation between channels 1,2 and channels 3,4. Given the internal oscillator of 1MHz, we can calculate the inductorsvalueforabout40%ripplecurrentatmaximumVIN: L1 = 5V 1MHz •0.5A ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 5V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 6.21µH L2 = 3.3V 1MHz •0.5A ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 3.3V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 4.95µH L3 = 2.5V 1MHz •0.5A ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 2.5V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 4.05µH L4 = 1.8V 1MHz •0.5A ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 1.8V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 3.11µH Using standard values of L1 = 6.8µH, L2 = 4.7µH, L3 = 3.3µH and L4 = 3.3µH for inductors results in maximum ripple currents of: ΔIL1= 5V 1MHz •6.8µH ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 5V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 0.46A ΔIL2 = 3.3V 1MHz • 4.7µH ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 3.3V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 0.53A ΔIL3 = 2.5V 1MHz •3.3µH ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 2.5V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 0.61A ΔIL4 = 1.8V 1MHz •3.3µH ⎛ ⎝⎜ ⎞ ⎠⎟ 1 − 1.8V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ = 0.47A COUT will be selected based on the ESR that is required to satisfy the output voltage ripple requirement and the bulk capacitance needed for loop stability. For this design, 47µF ceramic capacitors will be used. To prevent large voltage transients, CIN should be sized based on the maximum RMS current: IRMS ≅IOUT(MAX) VOUT VIN VIN VOUT −1 IRMS1=1.25A 5V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ 13.2V 5V −1= 0.606A IRMS2 =1.25A 3.3V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ 13.2 3.3V −1= 0.541A IRMS3 =1.25A 2.5V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ 13.2 2.5V −1= 0.490A IRMS4 =1.25A 1.8V 13.2V ⎛ ⎝⎜ ⎞ ⎠⎟ 13.2 1.8V −1= 0.429A IRMS =IRMS1+IRMS2 +IRMS3 +IRMS4 = 2.07A DecouplingtheVINXpinseachwith22µFceramiccapacitors is adequate for most applications. |
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