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LT1108 Datasheet(PDF) 11 Page - Linear Technology |
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LT1108 Datasheet(HTML) 11 Page - Linear Technology |
11 / 12 page 11 LT1108 S APPLICATI I FOR ATIO Figure 8 details current limit circuitry. Sense transistor Q1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2’s collector current flows in Q1’s collector. This current passed through internal 80 Ω resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN sets the current limit. When sufficient switch current flows to develop a VBE across R1 + RLIM, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 2 µs. The current trip point becomes less accurate for switch-ON times less than 5 µs. Resistor values program- ming switch-ON time for 2 µs or less will cause spurious response in the switch circuitry although the device will still maintain output regulation. Figure 9. Setting Low Battery Detector Trip Point LT1108 • F09 R1 R2 1.245V REF SET GND IN V LT1108 47k 5V TO PROCESSOR R1 = VLB – 1.25V 35.1 µA VLB + – AO R3 = BATTERY TRIP POINT R2 = 33k R3 = 1.6M VBAT Table 1. Inductor Manufacturers MANUFACTURER PART NUMBERS Coiltronics International OCTA-PAC TM 984 S.W. 13th Court Series Pompano Beach, FL 33069 305-781-8900 Sumida Electric Co. USA CD54 708-956-0666 CDR74 CDR105 Table 2. Capacitor Manufacturers MANUFACTURER PART NUMBERS Sanyo Video Components OS-CON Series 1201 Sanyo Avenue San Diego, CA 92073 619-661-6322 Nichicon America Corporation PL Series 927 East State Parkway Schaumberg, IL 60173 708-843-7500 AVX Corporation TPS Series Myrtle Beach, SC 803-946-0690 Table 3. Transistor Manufacturers MANUFACTURER PART NUMBERS Zetex Inc. ZTX 749 (NPN) 87 Modular Avenue ZTX 849 (NPN) Commack, NY 11725 ZTX 949 (PNP) 516-543-7100 Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. USING THE GAIN BLOCK The gain block (GB) on the LT1108 can be used as an error amplifier, low battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 1.245V reference. The positive input comes out on the SET pin. Arrangement of the gain block as a low battery detector is straightforward. Figure 9 shows hookup. R1 and R2 need only be low enough in value so that the bias current of the SET input does not cause large errors. 33k for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to “snap” when the trip point is reached. Values in the 1M to 10M range are optimal. The addition however, of R3 will change the trip point. Figure 8. LT1108 Current Limit Circuitry LT1108 • F08 SW2 SW1 DRIVER OSCILLATOR VIN ILIM R1 80 Ω (INTERNAL) RLIM (EXTERNAL) Q1 Q2 Q3 |
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