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LT3477IUF Datasheet(PDF) 7 Page - Linear Technology |
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LT3477IUF Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  7 LT3477 3477fb OPERATIO The LT3477 uses a fixed 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. The start of each oscillator cycle sets the SR latch and turns on power switch Q1. The signal at the noninverting input of the PWM comparator (A4 SLOPE) is proportional to the sum of the switch current and oscillator ramp. When SLOPE exceeds VC (the output of the feedback amplifier), the PWM comparator resets the latch and turns off the power switch. In this manner, the feedback amplifier and PWM comparators set the correct peak current level to keep the output in regulation. Ampli- fier A3 drives A4 inverting input. A3 has three inputs, one from the voltage feedback loop and the other two from the current feedback loop. Whichever feedback input is higher takes precedence, forcing the converter into either a constant-current or a constant-voltage mode. The LT3477 is designed to transition cleanly between the two modes of operation. Current sense amplifier IA1 senses the voltage between the ISP1 and ISN1 pins and provides a pre-gain to amplifier A1. When the voltage between ISP1 and ISN1 reaches 100mV, the output of IA1 provides VADJ to the inverting input of A1 and the converter is in constant- current mode. If the current sense voltage exceeds 100mV, the output of IA1 will increase causing the output of A3 to decrease, thus reducing the amount of current delivered to the output. In this manner the current sense voltage is regulated to 100mV. The current sense level is also pin adjustable by IADJ1. Forcing IADJ1 to less than 625mV will overwrite VADJ voltage that’s set internally, thus providing current level control. The second current sense amplifier, IA2, works the same as the first current sense amplifier IA1. Both current sense amplifiers provide rail-to-rail current sense operation. Similarly, for positive output voltage operation where FBP is tied to VREF, if the FBN pin increases above VREF, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). For negative output voltage operation where FBN is tied to GND, if the FBP pin decreases below GND level, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). The LT3477 also features a soft-start function. During start-up, 9 µA of current charges the external soft-start capacitor. The SS pin directly limits the rate of voltage rise on the VC pin, which in turn limits the peak switch current. The switch current is constantly monitored and not al- lowed to exceed the nominal value of 3A. If the switch current reaches 3A, the SR latch is reset regardless of the output of the PWM comparator. Current limit protects the power switch and external components. |
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