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LTC3853 Datasheet(PDF) 29 Page - Linear Technology |
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LTC3853 Datasheet(HTML) 29 Page - Linear Technology |
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29 / 36 page  LTC3853 29 3853f standard values are 3.3μH, 3.3μH and 1.5μH. At the maximum input voltage (20V), the ripple will be: ΔI V L V V LMAX OUT OUT IN MAX () () • =− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ f 1 Channel 1 will have ~2.3A (46%) ripple, and both channel 2 and channel 3 will have ~1.75A (35%) ripple. The peak inductor current will be the maximum DC value plus one- half the ripple current, or 6.15A for channel 1 and 5.88A for channels 2 and 3. With ILIM high, the equivalent RSENSE resistor value can be calculated by using the minimum value for the maximum current sense threshold (65mV). R V I I SENSE EQUIV SENSE MIN LOAD MAX L NOM () () () ( = + Δ )) () .• 2 65 12 5 2 9 = + ⎛ ⎝⎜ ⎞ ⎠⎟ ≅ mV A I m LMAX Δ Ω The 1.2 factor adds margin for component variation and overcurrent headroom during full load transients. The equivalent RSENSE is the same for channels 1, 2 and 3. The Vishay IHLP2525CZER3R3M01 (30m Ω DCRMAX at 20°C) and IHLP2525CZER1R5M01 (15m Ω DCRMAX at 20°C,) are chosen. At 100°C, the estimated maximum DCR values are 39.6mV and 19.8mV. The divider ratios are: R R DCR at T m m D SENSE EQUIV MAX L MAX == = () () . 9 39 6 0 Ω Ω .. ; . . 23 9 19 8 045 and m m Ω Ω ≅ For each channel, 0.1μF is selected for C1. RR L DCR at C C H mF MAX 12 20 1 33 30 0 1 1 || () • . •. = ° = = μ μ Ω 1 100 15 15 0 1 1000 Ω Ω Ω . •. and H mF μ μ = APPLICATIONS INFORMATION Figure 13. High Efficiency Triple 5V/3.3V/1.2V Step-Down Converter TG1 TG2 BOOST1,2,3 SW1 SW2 BG1 BG2 SENSE1+ SENSE2+ SENSE1– SENSE2– VFB1 VFB2 ITH1,2,3 VIN INTVCC LTC3853 DRVCC12 MODE/PLLIN 20k 1% FREQ/PLLFLTR ILIM PGND PGOOD12 SGND EXTVCC TK/SS1,2,3 VFB3 SENSE3– SENSE3+ SW3 TG3 BG3 RUN1,2,3 PGOOD3 4.75k 1430Ω 20k 1% 3.16k 1nF 63.4k 1% 4.7μF 10k INTVCC VOUT1 5V 5A VOUT2 3.3V 5A 20k 1% 3853 F13 2.43k 4.75k VOUT3 1.2V 5A VIN 7V TO 20V CITH1,2,3 RITH1,2,3 0.1μF 330μF 10V M1, M2, M3: Si4816BDY CHANNEL 1: RITH1 = 15k, CITH1 = 1.5nF, CP1 = 220pF CHANNEL 2: RITH2 = 18k, CITH2 = 1.5nF, CP2 = 220pF CHANNEL 3: RITH3 = 10k, CITH3 = 1.5nF, CP3 = 330pF FOR CHANNEL 3, 0.068μF WAS SUBSTITUTED FOR 0.1μF AND 2.43k WAS SUBSTITUTED FOR 2.21k TO MEET MINIMUM 15mV RIPPLE AT SENSE INPUT REQUIREMENT CP1,2,3 22μF 50V CSS1,2,3 0.1μF 0.1μF 10pF 1430Ω 3.3μH 3.3μH 1.5μH CB1,2,3 0.1μF SW1,2,3 DB1,2,3 M2 M1 M3 0.068μF 1.82k + 330μF 6V + 330μF 6V + 10k 1% 105k 1% 10pF |
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