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LTC1174 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1174 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LTC1174 LTC1174-3.3/LTC1174-5 S APPLICATI I FOR ATIO Figure 6. LTC1174 Layout Diagram (See Board Layout Checklist) DESIGN EXAMPLE As a design example, assume VIN = 9V (nominal), VOUT = 5V, and IOUT = 350mA maximum. The LTC1174-5 is used for this application, with IPGM (pin 7) connected to VIN. The minmum value of L is determined by assuming the LTC1174-5 is operating in continuous mode. Figure 7. Continuous Inductor Current TIME IPEAK IV AVG CURRENT = IOUT = = 350mA IPEAK + IV 2 1174 F07 With IOUT = 350mA and IPEAK = 0.6A (IPGM = VIN), IV = 0.1A.The peak-to-peak ripple inductor current, IRIPPLE, is 0.5A and is also equal to: I VV L A RIPPLE OUT D PP =× + () − − 410 6 Solving for L in the above equation and with VD = 0.6V, L = 44.8 µH. The next higher standard value of L is 50µH (example: Coiltronics CTX50-4). The operating frequency, neglecting voltage across diode VD is: f V V kHz OUT IN ≈× − = 2 5 10 1 111 5 . With the value of L determined, the requirements for CIN and COUT are calculated. For CIN, its RMS current rating should be at least: I IV V V V A mA RMS OUT OUT IN OUT IN RMS = − () [] () = 12 174 / For COUT, the RMS current rating should be at least: I I A mA RMS PEAK RMS ≈ () = 2 300 Now allow VIN to drop to 6V. At this minimum input voltage the operating frequency will decrease. The new frequency is 42kHz. + 3 LTC1174 2 SW R1 8 7 6 1 5 4 L VOUT D 1174 F06 OUTPUT DIVIDER REQUIRED WITH ADJUSTABLE VERSION ONLY 0.1 µF LBOUT LBIN GND SHUTDOWN IPGM VIN R2 BOLD LINES INDICATE HIGH CURRENT PATH VIN CIN COUT VOUT (VFB) |
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