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LT1302 Datasheet(PDF) 6 Page - Linear Technology |
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LT1302 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LT1302/LT1302-5 BLOCK DIAGRA SM The LT1302’s operation can best be understood by examining the block diagram in Figure 2. The LT1302 operates in one of two modes, depending on load. With light loads, comparator CMP1 controls the output; with heavy loads, control is passed to error amplifier A1. Burst Mode operation consists of monitoring the FB pin voltage with hysteretic comparator CMP1. When the FB voltage, related to the output voltage by external attenu- ator R1 and R2, falls below the 1.24V reference voltage, the oscillator is enabled. Switch Q4 alternately turns on, causing current buildup in inductor L1, then turns off, allowing the built-up current to flow into output capaci- tor C3 via D1. As the output voltage increases, so does the FB voltage; when it exceeds the reference plus OPERATIO CMP1’s hysteresis (about 5mV) CMP1 turns the oscilla- tor off. In this mode, peak switch current is limited to approximately 1A by A2, Q2, and Q3. Q2’s current, set at 34 µA, flows through R5, causing A2’s negative input to be 25mV lower than VIN. This node must fall more than 36mV below VIN for A2 to trip and turn off the oscillator. The remaining 11mV is generated by Q3’s current flow- ing through R4. Emitter-area scaling sets Q3’s collector current to 0.625% of switch Q4’s current. When Q4’s current is 1A, Q3’s current is 6.25mA, creating an 11mV drop across R4 which, added to R5’s 25mV drop, is enough to trip A2. When the output load is increased to the point where the 1A peak current cannot support the output voltage, 1302 F03 – + – + – + VOS 15mV 36mV CMP1 OFF VIN 2 µA ENABLE A2 VIN SW PGND IT VC GND SHDN SHUTDOWN SENSE ERROR AMPLIFIER HYSTERETIC COMPARATOR DRIVER A1 4 3 5 8 7 2 1 1.24V REFERENCE 220kHz OSCILLATOR 300 Ω 3.6k Q3 R4 1.75 Ω R5 730 Ω VIN BIAS R1 315k R2 105k 6 Q4 160X A3 Q1 Q2 Q5 Figure 3. LT1302-5 Block Diagram |
Similar Part No. - LT1302_15 |
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Similar Description - LT1302_15 |
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