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LT3580MPMS8E Datasheet(PDF) 6 Page - Linear Technology |
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LT3580MPMS8E Datasheet(HTML) 6 Page - Linear Technology |
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6 / 28 page ![]() LT3580 6 3580fg BLOCK DIAGRAM The LT3580 uses a constant-frequency, current mode con- trol scheme to provide excellent line and load regulation. Refer to the Block Diagram which shows the LT3580 in a boost configuration. At the start of each oscillator cycle, the SR latch (SR1) is set, which turns on the power switch, Q1. The switch current flows through the internal current sense resistor generating a voltage proportional to the switch current. This voltage (amplified by A4) is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A3. When this voltage exceeds the level at the negative input of A3, the SR latch is reset, turning off the power switch. The level at the negative input of A3 (VC pin) is set by the error amplifier A1 (or A2) and is simply an amplified version of the difference between the feedback voltage (FB pin) and the reference voltage (1.215V or 5mV depending on the configuration). In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. The LT3580 has a novel FB pin architecture that can be used for either boost or inverting configurations. When configured as a boost converter, the FB pin is pulled up to the internal bias voltage of 1.215V by the RFB resistor connected from VOUT to FB. Comparator A2 becomes inactive and comparator A1 performs the inverting amplification from FB to VC. When the LT3580 is in an inverting configuration, the FB pin is pulled down to 5mV by the RFB resistor connected from VOUT to FB. Comparator A1 becomes inactive and comparator A2 performs the noninverting amplification from FB to VC. – + – + – + – + + – ∑ 7 5 3 1.215V REFERENCE ADJUSTABLE OSCILLATOR FREQUENCY FOLDBACK RAMP GENERATOR COMPARATOR DISCHARGE DETECT SS VC 275k Q2 SR2 R S 14.6k 14.6k Q SR1 A3 A4 A1 A2 SYNC ÷N RT SHDN FB 1.3V VC C1 SW 0.01Ω GND RT RFB DRIVER L1 D1 ILIMIT VIN VOUT CSS CC CIN RC VIN SOFT- START SYNC BLOCK UVLO R S Q 6 2 1 3580 BD 8 4 Q1 9 OPERATION |
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