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LT3487 Datasheet(PDF) 9 Page - Linear Technology |
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LT3487 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LT3487 9 3487f Table 2. Recommended Ceramic Capacitor Manufacturers MANUFACTURER PHONE URL Taiyo Yuden (408) 573-4150 www.t-yuden.com Murata (814) 237-1431 www.murata.com Kemet (408) 986-0424 www.kemet.com Inrush Current The LT3487 uses internal Schottky diodes. When a sup- ply voltage is abruptly applied to the VIN pin, the voltage difference between VIN and VCAP generates inrush current flowing from the input through the inductor L1 and the internal Schottky diode DP to charge the boost output capacitor C4. For the inverting channel, there is a similar inrush current flowing from the input through the inductor L2 path, charging the flying capacitor C2 and returning through the internal Schottky diode DN. The maximum current the Schottky diodes in the LT3487 can sustain is 2A. The selection of inductor and capacitor values should ensure that the peak inrush current is below 2A. The peak inrush current can be calculated as follows: I V L eSIN P IN = ⎛ ⎝⎜ ⎞ ⎠⎟ –. • • • arctan – •arctan 06 ω α ω ω α ω ω α α ω ⎛ ⎝⎜ ⎞ ⎠⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ = + = r L LC r L 15 2 1 4 2 . • • – • where L is the inductance, r is the resistance of the inductor and C is the output capacitance. For low DCR inductors, which is usually the case for this application, the peak inrush current can be simplified as follows: I V L e P IN = ⎛ ⎝⎜ ⎞ ⎠⎟ –. • • –• 06 2 ω α ω π Table 3 gives inrush peak currents for some component selections. Note that inrush current is not a concern if the input voltage rises slowly. Table 3. Inrush Peak Current VIN (V) R ( Ω) L (μH) C (μF) IP (A) 5 0.18 10 4.7 1.44 5 0.235 15 2.2 1.06 3.6 0.18 10 4.7 0.979 3.6 0.245 10 4.7 0.958 3.6 0.345 15 2.2 0.704 External Diode Selection As stated previously, the LT3487 has internal Schottky diodes. The Schottky diode, DP, is sufficient for most step-up applications. However, for high current inverter applications, a properly selected external Schottky diode in parallel with DN can improve efficiency. For external diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes rated for higher current usually have lower forward voltage drops and larger capacitance, which can cause significant switching losses at a 2MHz switching frequency. Some recommended Schottky diodes are listed in Table 4. APPLICATIO S I FOR ATIO Table 4. Recommended Schottky Diodes PART NUMBER FORWARD CURRENT (mA) FORWARD VOLTAGE DROP (V) DIODE CAPACITANCE (pF at 10V) MANUFACTURER PMEG2010AEB 1000 0.51 7.5 Philips www.semiconductors. philips.com CMDSH2-3 200 0.49 15 Central Semiconductor www.centralsemi.com RSX051VA-30 500 0.35 30 ROHM www.rohm.com ZHCS400 400 0.425 18 Zetex www.zetex.com |
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