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LT1109ACS8 Datasheet(PDF) 5 Page - Linear Technology |
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LT1109ACS8 Datasheet(HTML) 5 Page - Linear Technology |
5 / 8 page 5 LT1109A The LT1109A is a micropower step-up DC/DC converter. It uses Burst Mode TM operation to achieve micropower operation yet still deliver more than 2W of output power from a 5V supply. Circuit operation can be best under- stood by referring to the LT1109A block diagram. With SHUTDOWN high, comparator A1 compares the feedback (FB) pin voltage with the 1.25V reference signal. When FB drops below 1.25V, A1 switches on the 120kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. When the FB voltage is sufficient to trip A1, the oscillator is turned off. A low signal on the shutdown pin gates off the oscillator, overriding A1. With SHUTDOWN low, quiescent current remains at 360 µA. Burst Mode TM is a trademark of Linear Technology Corporation OPERATIO S APPLICATI I FOR ATIO Inductor Selection A DC/DC converter operates by storing energy as mag- netic flux in an inductor core, and then switching this energy into the load. To operate as an efficient energy transfer element, the inductor must fulfill three require- ments: inductance value, saturation current and DC resis- tance. A fourth requirement is physical size. The inductors recommended with the LT1109A circuits are small, sur- face-mountable and are designed for switch-mode appli- cations. Avoid using RF chokes or air core units since they have very low peak current ratings. The LT1109A works best in situations where the input voltage does not vary much since the device has no internal switch current limit function. For situations where the input voltage varies, such as battery inputs, the LT1107 or LT1111 is suggested instead. Capacitor Selection The output capacitor should be chosen on the basis of its equivalent series resistance (ESR) and capacitance value. Low ESR tantalum surface-mountable capacitors such as those made by AVX are well-suited for DC/DC converter applications. Inexpensive aluminum electrolytics may have excessive ESR, resulting in high output ripple. These should be avoided. Diode Selection Speed, forward drop, and leakage current are the three main considerations in selecting a diode for LT1109A converters. General purpose rectifiers such as the 1N4001 are unsuitable for use in any switching regulator applica- tion. Although they are rated at 1A, the switching time of a 1N4001 is in the 10 µs to 50µs range. At best, efficiency will be severely compromised if this diode is used; at worst, the circuit may not work at all. The 1N5818 is an ideal choice for LT1109A circuits. Surface-mountable versions, such as the MBRS130T3, are available as well. Table 1. Inductor Manufacturers MANUFACTURER PART NUMBERS Coiltronics International Surface Mount 984 S.W. 13th Court OCTA-PAC TM Series Pompano Beach, FL 33069 305-781-8900 Sumida Electric Co., Ltd. CD54 637 E. Golf Road, Suite 209 CD105 Arlington Heights, IL 60005 Surface Mount 708-956-0666 Table 2. Capacitor Manufacturers MANUFACTURER PART NUMBERS AVX TPS Series Myrtle Beach, SC 29578 803-946-0690 Philips Components 49MC Series 2001 W. Blue Heron Blvd. P.O. Box 10330 Riviera Beach, FL 33404 407-881-3200 Sanyo Video Components OS-CON Series 1201 Sanyo Avenue San Diego, CA 92073 619-661-6322 OCTA-PAC TM is a trademark of Coiltronics International |
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