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LT3497EDDB Datasheet(PDF) 9 Page - Linear Integrated Systems |
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LT3497EDDB Datasheet(HTML) 9 Page - Linear Integrated Systems |
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9 / 20 page  LT3497 9 3497f APPLICATIONS INFORMATION DUTY CYCLE The duty cycle for a step-up converter is given by: D VV V VV V OUT D IN OUT D CESAT = + + – – where: VOUT = Output voltage VD = Schottky forward voltage drop VCESAT = Saturation voltage of the switch VIN = Input voltage The maximum duty cycle achievable for LT3497 is 88% when running at 2.3MHz switching frequency. Always ensure that the converter is not duty-cycle limited when powering the LEDs at a given frequency. INDUCTOR SELECTION A 15µH inductor is recommended for most LT3497 ap- plications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 2.3MHz and low DCR (copper wire resistance). Some inductors in this category with small size are listed in Table 1. The efficiency comparison of different inductors is shown in Figure 3. Table 1: Recommended Inductors PART L (µH) MAX DCR (Ω) CURRENT RATING (mA) VENDOR LQH32CN150K53 LQH2MCN150K02 LQH32CN100K53 LQH2MCN100K02 15 15 10 10 0.58 1.6 0.3 1.2 300 200 450 225 Murata www.murata.com SD3112-150 15 0.654 440 Cooper www.cooperet.com 1001AS-150M (TYPE D312C) 15 0.80 360 Toko www.toko.com CDRH2D11/HP 15 0.739 410 Sumida www.sumida.com CAPACITOR SELECTION The small size of ceramic capacitors make them ideal for LT3497 applications. Use only X5R and X7R types because they retain their capacitance over wider temperature ranges than other types such as Y5V or Z5U. A 1µF input capacitor and a 1µF output capacitor are sufficient for most applica- tions. Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for detailed information on their entire selection of ceramic parts. Table 2: Recommended Ceramic Capacitor Manufacturers Taiyo Yuden (800) 368-2496 www.t-yuden.com AVX (803) 448-9411 www.avxcorp.com Murata (714) 852-2001 www.murata.com OVERVOLTAGE PROTECTION The LT3497 has an internal open-circuit protection circuit for both converters. In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail open circuit, the converter VCAP voltage is clamped at 32V (typ). Figure 4a shows the transient response of the front page application step-up converter with LED1 disconnected. With LED1 disconnected, the converter starts switching at the peak inductor current limit. The converter output starts ramping up and finally gets clamped at 32V (typ). The converter will then switch at low inductor current to regulate the converter output at the clamp voltage. The VCAP and input current during output open circuit are shown in the Typical Performance Characteristics. Figure 3. Efficiency Comparison of Different Inductors LED CURRENT (mA) 0 65 70 80 15 µH MURATA LQH32CN150K53 15 µH MURATA LQH2MCN150K02 15 µH COOPER SD3112-150 15 µH TOKO 1001AS-150M TYPE D312C 15 µH SUMIDA CDRH2D11/HP 15 3497 F03 60 55 510 20 50 45 75 |
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