Datasheet LT3477 (Analog Devices) - 8

LT3477
operaTion
The LT3477 uses a fixed frequency, current mode control In this manner the current sense voltage is regulated to scheme to provide excellent line and load regulation. Op- 100mV. The current sense level is also pin adjustable by eration can be best understood by referring to the Block IADJ1. Forcing IADJ1 to less than 625mV will overwrite VADJ Diagram in Figure 1. The start of each oscillator cycle sets voltage that’s set internally, thus providing current level the SR latch and turns on power switch Q1. The signal at the control. The second current sense amplifier, IA2, works noninverting input of the PWM comparator (A4 SLOPE) is the same as the first current sense amplifier IA1. Both proportional to the sum of the switch current and oscillator current sense amplifiers provide rail-to-rail current sense ramp. When SLOPE exceeds VC (the output of the feedback operation. Similarly, for positive output voltage operation amplifier), the PWM comparator resets the latch and turns where FBP is tied to VREF, if the FBN pin increases above off the power switch. In this manner, the feedback amplifier VREF, the output of A3 will decrease to reduce the peak and PWM comparators set the correct peak current level current level and regulate the output (constant-voltage to keep the output in regulation. Amplifier A3 drives A4 mode). For negative output voltage operation where FBN inverting input. A3 has three inputs, one from the voltage is tied to GND, if the FBP pin decreases below GND level, feedback loop and the other two from the current feedback the output of A3 will decrease to reduce the peak current loop. Whichever feedback input is higher takes precedence, level and regulate the output (constant-voltage mode). forcing the converter into either a constant-current or a constant-voltage mode. The LT3477 is designed to transi- The LT3477 also features a soft-start function. During tion cleanly between the two modes of operation. Current start-up, 9µA of current charges the external soft-start sense amplifier IA1 senses the voltage between the I capacitor. The SS pin directly limits the rate of voltage rise SP1 and ISN1 pins and provides a pre-gain to amplifier A1. on the VC pin, which in turn limits the peak switch cur- When the voltage between ISP1 and ISN1 reaches 100mV, rent. The switch current is constantly monitored and not the output of IA1 provides VADJ to the inverting input of allowed to exceed the nominal value of 3A. If the switch A1 and the converter is in constant-current mode. If the current reaches 3A, the SR latch is reset regardless of the current sense voltage exceeds 100mV, the output of IA1 output of the PWM comparator. Current limit protects the will increase causing the output of A3 to decrease, thus power switch and external components. reducing the amount of current delivered to the output. 3477fd 8 For more information www.linear.com/LT3477 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Package Description Typical Application Related Parts