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LT3471 Datasheet(PDF) 5 Page - Linear Technology |
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LT3471 Datasheet(HTML) 5 Page - Linear Technology |
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5 / 16 page  LT3471 5 3471fb BLOCK DIAGRAM Figure 1. Block Diagram OPERATION The LT3471 uses a constant frequency, current mode control scheme to provide excellent line and load regu- lation. Refer to the Block Diagram. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch, Q1 (Q2). 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 compara- tor A2 (A4). When this voltage exceeds the level at the negative input of A2 (A4), the SR latch is reset, turning off the power switch Q1 (Q2). The level at the negative input of A2 (A4) is set by the error amplifier A1 (A3) and is simply an amplified version of the difference between the negative feedback voltage and the positive feedback voltage, usually tied to the reference voltage VREG. 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. Similarly, if the error decreases, less current is delivered. Each switcher functions independently but they share the same oscillator and thus the switchers are always in phase. Enabling the part is done by taking either SHDN/SS pin above 1.8V. Disabling the part is done by grounding both SHDN/SS pins. The soft-start feature of the LT3471 allows for clean start-up conditions by limiting the amount of voltage rise at the output of comparator A1 and A2, which in turn limits the peak switching current. The soft-start feature for each switcher is enabled by slowly ramping that switcher’s SHDN/SS pin, using an RC network, for example. Typical resistor and capacitor values are 0.33μF and 4.7k, allowing for a start-up time on the order of milliseconds. The LT3471 has a current limit circuit not shown in the Block Diagram. The switch current is constantly monitored and not allowed to exceed the maximum switch current (typically 1.6A). If the switch – + RQ S 0.01Ω SW1 DRIVER 10 FB1N SHDN/SS1 1 9 FB1P 2 – + ∑ RAMP GENERATOR 1.00V REFERENCE LEVEL SHIFTER RC CC 1.2MHz OSCILLATOR GND GND Q1 A2 A1 VIN VREF 8 3 – + RQ S 0.01Ω SW2 DRIVER 6 11 FB2N SHDN/SS2 5 7 FB2P 4 – + ∑ RAMP GENERATOR LEVEL SHIFTER RC CC 3471 F01 Q2 A4 A3 |
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