advertisement ThinSOT Switching Regulator Controls Inrush Current Design Note 365 Jesus Rosales Introduction Inrush current can be troublesome, especially when regulators are operated from batteries or a currentlimited source. In such cases, excessive inrush current can cause the input source to collapse and prevent the converter from ever starting up. Inrush current can also trip input line fuses or cause the output to overshoot. There are many ways to deal with this problem, but most of them are difficult to implement or have undesirable side effects, often making the cure worse than the cause. IIN 0.2A/DIV VOUT 5V/DIV 50μs/DIV Figure 2. Input Current and Output Voltage at Turn-On Without CSS Capacitor for Figure 1 Circuit A Simple Solution Figure 1 shows a solution to this problem using the LT®3467 in a 10V-16V input to 12V/300mA output SEPIC converter. The LT3467 ThinSOT™ switching regulator integrates a soft-start function with a built-in 42V, 1.1A switch. Figures 2 and 3 illustrate the effect that the LT3467’s soft-start feature has on inrush currents. IIN 0.2A/DIV VOUT 5V/DIV 2ms/DIV Figure 2 shows the start-up waveforms for the Figure 1 circuit with the SS feature disabled (by removing CSS from the SS pin). The inrush current in this case is limited only by the maximum duty cycle or maximum switch current of the LT3467. The top trace shows input current and the bottom trace shows output voltage. It takes only about 100μs for the output to go from 0V to 12V, requiring an input current of about 0.85A peak, thus exceeding the nominal steady-state level. VIN 10V TO 16V Figure 3 shows the same waveforms with a 0.1μF capacitor connected to the SS pin. Now, both input current and output voltage rampup in a controlled L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. 16V 1μF B0530W t CSS 0.1μF 6 1 VIN SW 4 SHDN LT3467 3 5 FB SS GND VOUT 12V 300mA 115k 10pF t L1B 10μH CLS62-100 13.3k 2 GND DN365 F01 Figure 1. 10VIN-16VIN to 12V at 300mA SEPIC Converter 06/05/365_conv DN365 F03 Figure 3. Input Current and Output Voltage at Turn-On with CSS Capacitor for Figure 1 Circuit L1 10μH CLS62-100 2.2μF 25V DN365 F02 4.7μF 16V fashion and settle comfortably into their respective steady-state levels. By using only one small capacitor between the SS pin and ground, inrush current has been eliminated. Figure 4 shows an efficiency curve for the SEPIC converter of Figure 1. Figure 5 shows a boost converter that provides 12V at 300mA from a 5V input source. Figures 6 and 7 show the input and output currents, with and without the soft-start feature enabled. Figure 6 shows that without soft-start, even though the output voltage follows the input by conducting through the inductor and diode, there is an inrush of current which exceeds 2A at the point where the converter begins to switch. Figure 7 shows the improved start-up results with soft-start enabled. Figure 8 shows the efficiency of the circuit. IIN 0.5A/DIV VOUT 5V/DIV 50μs/DIV Figure 6. Input Current and Output Voltage at Turn-On Without CSS Capacitor for Figure 5 Circuit IIN 0.5A/DIV VOUT 5V/DIV 82 2ms/DIV 81 EFFICIENCY (%) DN365 F06 DN365 F07 Figure 7. Input Current and Output Voltage at Turn-On with CSS Capacitor for Figure 5 Circuit 80 90 79 89 88 77 100 50 200 250 150 LOAD CURRENT (mA) EFFICIENCY (%) 78 300 87 86 85 DN365 F04 84 Figure 4. SEPIC Converter Efficiency for Figure 1 Circuit 83 82 10μH CR43-100 B0530W VIN 5V 2.2μF 4 5 6 1 VIN SW SHDN LT3467 SS CSS 0.1μF GND VOUT 12V 300mA 3 4.7μF 16V 13.3k 2 DN365 F07 Figure 5. 5VIN to 12V at 300mA BOOST Converter Data Sheet Download www.linear.com Linear Technology Corporation 100 150 250 200 LOAD CURRENT (mA) 300 DN365 F08 Figure 8. BOOST Converter Efficiency for Figure 5 Circuit 115k 10pF FB 50 Conclusion Countless hours can be spent looking for ways to control inrush current or just getting the output to come up. The LT3467 provides a simple solution. The addition of a single, tiny, low cost capacitor can make the difference between a smooth design cycle or a laborious one. For applications help, call (408) 432-1900 dn365f_conv LT/TP 0605 305K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005