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LCQF Datasheet(PDF) 9 Page - Linear Technology |
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LCQF Datasheet(HTML) 9 Page - Linear Technology |
9 / 24 page LT3498 9 3498f LED CURRENT (mA) 0 65 70 80 15 60 55 510 20 50 45 75 3498 F02 15uH Murata LQH32CN150K53 15uH Murata LQH2MCN150K02 15uH Cooper SD3110-150 15uH Toko D312C 15uH Coilcraft DO3314-153ML The low noise boost of the LT3498 uses a novel control scheme to provide high efficiency over a wide range of output current. In addition, this technique keeps the switching frequency above the audio band over all load conditions. The operation of the part can be better understood by referring to the Block Diagram. The part senses the output voltage by monitoring the voltage on the FB2 pin. The user sets the desired output voltage by choosing the value of the external topside feedback resistor. The part incorporates a precision 182kΩ bottom-side feedback resistor. Assuming that output voltage adjustment is not used (CTRL2 pin is tied to 1.5V, or greater), the internal reference (VREF = 1.215V) sets the voltage to which FB2 will servo during regulation. The Switch Control block senses the output of the ampli- fier and adjusts the switching frequency, as well as other parameters to achieve regulation. During the start-up of the circuit, special precautions are taken to ensure that the inductor current remains under control. Because the switching frequency is never allowed to fall below approximately 50kHz, a minimum load must be present to prevent the output voltage from drifting too high. This minimum load is automatically generated within the part via the Shunt Control block. The level of this current is adaptable, removing itself when not needed to improve efficiency at higher load levels. The low-noise boost of the LT3498 also has an integrated Schottky diode and PMOS output disconnect switch. The PMOS switch is turned on when the part is enabled. When the part is in shutdown, the PMOS switch turns off, allowing the VOUT2 node to go to ground. This type of disconnect function is often required in power supplies. OPERATION— OLED DRIVER APPLICATIONS INFORMATION— LED DRIVER Inductor Selection A 15µH inductor is recommended for most applications for the LED driver of the LT3498. 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 small inductors in this category are listed in Table 1. The efficiency comparison of different inductors is shown in Figure 2. 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 SD3110-150 15 0.764 380 Cooper www.cooperet.com 1001AS-150M (TYPE D312C) 15 0.80 360 Toko www.toko.com D03314-153ML 15 0.86 680 Coilcraft www.coilcraft.com Figure 2. Efficiency Comparison of Different Inductors Capacitor Selection The small size of ceramic capacitors makes them ideal for LT3498 LED driver 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 4.7µF input capacitor and a 1µF output capacitor are sufficient for most applications. |
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