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LT3460 Datasheet(PDF) 5 Page - Linear Technology |
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LT3460 Datasheet(HTML) 5 Page - Linear Technology |
5 / 12 page LT3460/LT3460-1 5 3460fb BLOCK DIAGRAM Figure 1. Block Diagram, LT3460 – + – + – + VOUT R1 (EXTERNAL) R2 (EXTERNAL) RS (EXTERNAL) CS (EXTERNAL) FB SHUTDOWN SHDN RAMP GENERATOR 1.255V REFERENCE 1.3MHz OSCILLATOR R S Q A1 A2 COMPARATOR DRIVER RC CC SW Q1 0.1Ω GND VIN FB 3460 BD RS, CS OPTIONAL SOFT-START COMPONENTS OPERATION The LT3460/LT3460-1 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram in Figure 1. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 1.255V. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Feedback Loop Compensation The LT3460/LT3460-1 has an internal feedback compensa- tion network as shown in Figure 1 (RC and CC). However, because the small signal characteristics of a boost converter change with operation conditions, the internal compensa- tion network cannot satisfy all applications. A properly designed external feed forward capacitor from VOUT to FB (CF in Figure 2) will correct the loop compensation for most applications. The LT3460/LT3460-1 uses peak current mode control. The current feedback makes the inductor very similar to a current source in the medium frequency range. The power stage transfer function in the medium frequency range can be approximated as: GP(s) = K1 s• C2 , where C2 is the output capacitance, and K1 is a constant based on the operating point of the converter. In continuous current mode, K1 increases as the duty cycle decreases. The internal compensation network RC, CC can be ap- proximated as follows in medium frequency range: GC(s) =K2 • s•RC •CC + 1 s• CC The zero fZ = 1 2• •RC •CC is about 70kHz. |
Similar Part No. - LT3460_15 |
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Similar Description - LT3460_15 |
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