Datasheet LT3479 (Analog Devices) - 7

LT3479
OPERATION
The LT3479 uses a fi xed frequency, current mode control current reaches 3A, the SR latch is reset regardless of the scheme to provide excellent line and load regulation. Op- output of the PWM comparator. Current limit protects the eration can be best understood by referring to the Block power switch and external components. Diagram. The start of each oscillator cycle sets the SR latch Soft-start plays an important role in applications where the and turns on power switch Q1. The signal at the inverting switch will reach levels of 30V or higher. During startup, input of the PWM comparator (SLOPE) is proportional to an overshoot in the switch current together with the pres- the sum of the switch current and oscillator ramp. When ence of high switch voltage can overstress the switch. A SLOPE exceeds VC (the output of the feedback amplifi er), properly used soft-start feature will greatly improve the the PWM comparator resets the latch and turns off the robustness of such designs. power switch. In this manner, the feedback amplifi er and PWM comparators set the correct peak current level to In addition to soft-start, inrush current protection protects keep the output in regulation. the LT3479 against shorts and line transients. During such faults, the inductor current can momentarily exceed 3A and The LT3479 also features a soft-start function. During damage the switch. Through an internal 8.5mΩ resistor start-up, 10μA of current charges the external soft-start placed in series with the inductor, the inrush current protec- capacitor. The SS pin directly limits the rate of voltage rise tion comparator measures the inductor current. If it exceeds on the VC pin, which in turn limits the peak switch cur- 5A, a soft-start cycle is initiated. The LT3479 will remain in rent. The switch current is constantly monitored and not the soft-start condition until the fault has passed. allowed to exceed the nominal value of 3A. If the switch 3479fc 7