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SC4540EVB Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4540EVB Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 15 page 11 Applications Information (continued) SC4540 must withstand high voltage for an extended period of time. Soft-Start The soft-start mode reduces in-rush current by utilizing the external compensation network. As the error amplifier slowly charges the COMP node voltage, the duty cycle of the boost switch ramps from 0% to its final value when in regulation. The gradual increase of the duty cycle slowly charges the output capacitor and limits in-rush current during start up. Soft-start is implemented only when the power is cycled on the part. Thermal Shutdown A thermal shutdown mode is included for protection in the event the junction temperature exceeds 155°C. In thermal shutdown, the on-chip power switch is disabled. Switching and sinking resumes when the temperature drops by 20°C. Current Limit The power switch of the boost converter is protected by an internal current limit function. The switch is opened when the current exceeds the maximum switch current value. Inductor Selection The inductor value should be within the range of 22μH to 68μH. The DCR needs to be considered when selecting the inductor to ensure optimum efficiency. The largest inductor package that can be accommodated in the circuit area should be used since the DCR generally decreases with increasing package size. The saturation current of the inductor should be much higher than the peak current of the internal boost switch to ensure that the inductor never enters saturation during normal operation of the part. The equation to calculate the peak inductor current is: 2 I I I L IN L(Peak) where osc IN L f L D V I OUT IN V V 1 D IN OUT OUT IN V I V I D is the duty cycle for continuous operation. Efficiency (η) can be approximated by using the curves provided in the Typical Characteristics section. Table 1 lists inductors that have been proven to work with SC4540. Table 1 — Recommended Inductors Part Number Value (μH) DCR (Ω) Rated Current (A) Toler- ance Dimensions (L x W x H) (mm) Coilcraft LPS6235 56 0.28 1.1 ±20% 6.2 x 6.2 x 3.5 Coilcraft LPS4018-223ML 22 0.360 0.70 ±20% 3.9 x 3.9 x 1.7 Capacitor Selection The input capacitor should be at least 2.2μF. A larger capacitor will reduce the voltage ripple on the input. The output capacitor values can range from 0.22μF to 1μF. The compensation capacitor value should be 47nF. Capacitors of X5R type material or better can be used for any of the capacitors. See Table 2 for recommended capacitors. Table 2 — Recommended Capacitors Part Number Value (μF) Rated Voltage (V) Type Case Size Input Capacitor Murata GRM219R61E225KA12 2.2 25 X5R 0805 Output Capacitor Murata GRM21BR71H105KA12L 1.0 50 X7R 0805 Compensation Capacitor Taiyo Yuden EMK105BJ473KV-F 0.047 16 X7R 0402 |
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